Life processes 1 Movement Both animals and plants can move. Animals can move quickly and from place to place in search of food, shelter and favourable conditions; plants on the other hand are rooted to a spot and move much more slowly by growing and responding to external stimuli, such as light.
Reproduction Human egg surrounded by sperm All animals and plants can reproduce. They multiply in number by producing the next generation of offspring, thus carrying on their genes and ensuring the continuation of the species. Plants do this by producing seeds, which give rise to new plants of the same species. Reproduction can be of two types: 1. Sexual, involving two parents and the union of two gametes, and 2. Asexual, where one parent reproduces itself. Examples of this are strawberry plants or spider plants producing runners or offshoots.
Sensitivity The Venus flytrap responds to touch Sensitivity is an awareness of changes in the environment. Whereas animals respond more quickly to stimuli such as heat, light, touch and chemicals, plants appear less sensitive and respond more slowly - eg, to the direction of sunlight. Some plants such as the Venus flytrap respond to touch. Life processes 2 Growth Growth is an ongoing increase in size of the organism - as in growing from young to adult animals or from seedlings to mature plants.
Excretion Thousands of chemical reactions go on inside body cells producing both useful and waste substances which when allowed to accumulate can be harmful. Excretion is getting rid of metabolic waste, produced by the body, like urine and carbon dioxide. Getting rid of faeces or undigested food is not excretion but egestion.
Respiration
Mitochondrion inside a cell Energy contained in food is 'unlocked' or transferred to the organism by the process of respiration. Respiration takes place in the mitochondria of the cell. Energy is released in a controlled way in a series of reactions. There are two types of respiration, with or without oxygen: 1. Aerobic respiration uses oxygen, and releases a large amount of energy 2. Anaerobic respiration does not use oxygen and releases much less energy.
Nutrition Both animals and plants need food as a source of energy and growth. Plants make their own food by photosynthesis taking in simple substances like carbon dioxide and water and building them into complex carbohydrate molecules. Animals cannot make their own food and therefore rely on plants or other animals for their energy. Animals take in complex substances and break them down into simple, soluble molecules which can be used as a source of energy. Plant and animal cells All cells have these features in common: · a cell membrane which controls what passes into and out of the cell · cytoplasm where chemical processes (controlled by enzymes) take place, and · a nucleus containing genetic material which controls the cell's behaviour. In addition, plant cells have the following unique features: · a rigid cell wall made of cellulose · vacuoles containing cell sap, and · chloroplasts, containing chlorophyl which enables photosynthesis to take place. Make sure you can draw and label a generalised animal and a generalised plant cell, like this:
Although it is useful to think about the general features of plant and animal cells, all cells in living organisms display specialised features that make them suited to carrying out their very specific jobs - for example, root cells, stem cells and leaf cells within a plant. A complex organism can be compared to a factory production line with different people performing different tasks. Cells within an organism display a similar division of labour to make the whole organism work properly and at a high level of efficiency. Cell specialisation The following illustration shows a variety of animal and plant cells, with different shapes and structures. Each has a very specific job, which helps the organism as a whole to work efficiently. Specific functions of animal cells
generalised animal cell Example of animal cell Specific function Specialised features suited to the function Sperm cell(male gamete) To fertilise the egg cell (female gamete) The head, containing genetic information and a nucleus, has an enzyme to help penetrate the egg cell membrane.The middle section, immediately behind the head is packed with mitochondria for energy.The tail or flagellum moves the sperm to the egg. Nerve cell(motor neurone) Pass sensory impulses from receptor to an effector Dendrites to make connections with other neurones.Long axon or nerve fibre to carry the impulse to the target organ.End plate forms a synapse with an effector (a muscle or a gland). Epithelial cell Lining cells, eg inside the cheeks Flattened shape. Interlocking edges, cells fitting closely together to form a continuous lining. Ciliated epithelium Lining the nose and wind pipe Flattened shape. Interlocking edges, cells fitting closely together to form a continuous lining.Have very tiny hair-like extensions called cilia which move rhythmically to remove mucus, dust and germs. Red blood cell Contain haemoglobin to carry oxygen to the cells Thin outer membrane allows oxygen to diffuse through easily. Bi-concave shape increases the surface area to allow more oxygen to be absorbed efficiently.No nucleus means that the whole cell is full of haemoglobin which combines with and carries oxygen around the body.Bi-concave shape, i.e. thinner in the middle, the more flexible framework allows cells to squeeze through even the tiniest capillaries. White blood cell Fight disease; either engulf bacteria or make antibodies Irregular shape, some with many lobed nuclei. Can change shape to squeeze out of blood vessels and get to the site of infection.Some make antibodies. Others have cytoplasm which can flow making it possible for the cell to change shape, surround and engulf bacteria. Can increase in numbers to fight disease. Muscle cell(Skeletal muscle) Cause movement Elongated in shape.Have fibres which slide into each other and cause the muscle to contract or become shorter and then relax to its original length.Contain filaments of muscle proteins. Need energy to work therefore have a lot of mitochondria. Specific functions of plant cells
generalised plant cell Example of plant cell Specific function Specialised features suited to the function Root hair cell Take in water and mineral ions from the soil Located the root epidermis and in direct contact with the soil.Wall nearest the soil has a long 'finger-like' process projection with very thin walls into the soil.This projection massively increased surface area for more efficient uptake of water and ions. The thin walls make up-take of water easier. Palisade cell Carries out photosynthesis Packed with chloroplasts containing the light absorbing pigment cholophyll.Regular shaped, closely packed cells forming a continuous layer for efficient absorption of sunlight. Guard cells Predominate on lower surface of leaves; help to reduce water loss Surround pores in the outer layer of leaves.Predominate on the lower surface of the leaf to reduce water loss. Cell wall closest to the pore is thicker and less flexible. Have chloroplasts and carry out photosynthesis. Consequent changes in glucose concentration and osmotic potential allow water to enter and leave.When turgid, guard cells pull the thickened wall in, opening the stoma. Xylem Pipeline carrying water from the roots to the leaves Long and tube-like hollow vessels.Long and tube-like hollow vessels. Cells have no end walls, so form a 'pipeline' carrying water from leaves to root. Spirals and rings of lignin strengthen the walls, to withstand pressure of water. Phloem Sieve tubes carry food, away from the leaves Living cells with perforated end walls, hence the name sieve tubes.Have vertical strands of cytoplasm which carry glucose and other sugars dissolved in water to growing and storage areas of the plant. Pollen grains (male gametes) Fertilise the ovules (female gametes) Tiny grain with half the genetic information having been formed by meiosis.Have a hard protective outer coat to survive bad conditions. Shape and surface of outer coat is adapted to method of dispersal - eg smooth and sticky for insect dispersal, larger surface area for wind dispersal. Germinate on reaching the stigma of another flower of the same species. Cells and organisms Some organisms consist of a single cell. Unicells are one-cell living things which have microscopic structures or organelles inside them including chloroplasts, mitochondria and a nucleus. Multicellular organisms, such as ourselves, are made from very large numbers of specialised cells. Groups of cells of the same type are called tissues. Groups of tissues work together as organs to perform particular functions within an organism. For example, millions of human heart muscle cells are grouped together to form heart muscle tissue, which is in turn organised into an organ - the heart.
The heart in its turn forms part of a functional system - the circulatory system.
Diffusion, osmosis and active transport
Substances move in and out of cells by means of passive or active transport.
Examples of passive transport are diffusion and osmosis (the diffusion of water).
Important substances like oxygen, glucose, water and mineral salts are needed and must be allowed to enter the cells and metabolic waste products like carbon dioxide and toxins must be allowed to leave the cells.
The entry and exit of all substances from the cells is controlled by the cell membrane, which is partially permeable Diffusion Particles in liquids and gases have kinetic energy. They move about, at speed, in all directions. The particles move about randomly. In an area of high concentration, some of the particles collide with each other, lose energy and slow down. Others will escape from the concentrated area to places where there are fewer or no particles. Very few particles leave an area of low concentration to go to an area where the concentration is higher. The result is a concentration gradient, with particles diffusing from an area of high concentration to an area of low concentration. To view this piece of media, you will need an up-to-date version of the Macromedia Flash Plug-in. You can quickly download it from here Examples of diffusion Here are some examples of diffusion across concentration gradients: Place Particles move From To Gut Digested food products Gut cavity Blood in capillary of villus Leaf Oxygen Chloroplast Air spaces in mesophyll Lungs Oxygen Alveolar air space Blood circulating around the lungs Remember: particles continue to move from high to low concentration for as long as there is a concentration gradient. In the lungs, the blood will continue to take in oxygen from the alveolar air spaces, provided that there is more oxygen in the air spaces than in the blood. The oxygen diffuses across the alveolar walls into the blood. The circulation takes the oxygen-rich blood away and replaces it with blood low in oxygen. To view this piece of media, you will need an up-to-date version of the Macromedia Flash Plug-in. You can quickly download it from here In the exam you may be asked to give two examples of diffusion in organisms. Make sure you know an example from plants as well as from animals. Osmosis Some membranes in plant and animal cells allow certain particles to pass through them and not others. They are partially (or selectively) permeable. Osmosis is simply a special type of diffusion - diffusion of water molecules through a partially permeable membrane. To view this piece of media, you will need an up-to-date version of the Macromedia Flash Plug-in. You can quickly download it from here In the animation, more water molecules pass from the water into the dilute solution than pass back the other way, because there is a higher concentration of water molecules in the pure water than there is in the solution. This results in a net transfer of molecules down the concentration gradient from the water to the solution. Eventually the level on the more concentrated side of the membrane will rise, while that on the less concentrated side falls. When the concentration of water is the same on both sides of the membrane, the movement of water will be the same in both directions. At this point, the net exchange of water is zero and the system is in equilibrium. Osmosis is vitally important to plants. Plants gain water by osmosis through their roots, and it is osmosis that moves water into plant cells, making them turgid or stiff, and thus able to hold the plant upright. Active transport (Higher Tier) Active transport is the process by which dissolved molecules (solutes) move across a cell membrane from a lower to a higher concentration. In active transport, particles move against the concentration gradient - and therefore require an input of energy from the cell. Sometimes solutes are at a higher concentration inside the cell than outside, but because the organism needs these substances they still need to be absorbed. For this the organism cannot rely on diffusion or osmosis alone. Carrier proteins pick up specific molecules and take them through the cell membrane against the concentration gradient. To view this piece of media, you will need an up-to-date version of the Macromedia Flash Plug-in. You can quickly download it from here Examples of active transport In plants: Plants need mineral salts (eg nitrates) for making proteins and growth. Nitrates are at a higher concentration inside the root cells than they are when dissolved in the water around the soil particles. If the plant relied on diffusion alone, the vital nitrate salts would drain out of the cells into the soil. So energy is deployed by the cells to actively transport nitrates across the cell membrane into the root cells, against the concentration gradient. In humans: Active transport takes place during digestion of food in the ileum (small intestine). After food has been absorbed by the villi for some time, the concentration of food molecules inside the villi increases, making it impossible for more food to diffuse into the villi. So simple sugars, amino acids, minerals and vitamins are actively absorbed into the villi, from an area of lower to an area of higher concentration. Diffusion, osmosis and active transport compared Diffusion Osmosis Active Transport Random movement Random movement of water Selective movement From higher to lower concentration From higher to lower concentration From lower to higher concentration Along the concentration gradient Along the concentration gradient Against the concentration gradient No energy needed from the cell No energy needed from the cell Energy needed from the cell Remember: · Molecules move in all directions in diffusion and osmosis. · Osmosis is the diffusion of water. · Diffusion continues even after the particles have spread out equally. Even when a system is in equilibrium movement of particles continues, with as many particles leaving as are arriving in any one place. · In diffusion and osmosis molecules move by kinetic energy alone. · In active transport, metabolic energy (energy made by respiration in the cells) is required. Photosynthesis
Green plants are at the beginning of all food chains. Plants are known as 'producers' because only plants make their own food by building up carbohydrates, proteins and fats from simple inorganic chemicals.
Green plants need sunlight to produce food. They 'capture' the sun's light energy using the chlorophyll in their leaves and use it to make a sugar called glucose, which is either used in respiration or converted into starch and stored. How photosynthesis works Photosynthesis is the chemical change which happens in the leaves of green plants. It is the first step towards making food not just for plants, but ultimately for every animal on the planet as well. During this reaction, carbon dioxide and water are converted into glucose and oxygen. The reaction requires energy in the form of sunlight, and chlorophyll must also be present. The glucose produced in the photosynthesis reaction can be converted to sucrose and carried to other parts of the plant in phloem vessels. Glucose can also be converted into starch and stored (the starch can later be turned back into glucose and used in respiration). Oxygen is a 'waste' product of photosynthesis. Photosynthesis takes place in the mesophyll cells inside a green plant's leaves.
Make sure you know the parts labelled in this diagram.
As you can see there are two kinds of mesophyll cells - palisade mesophyll and spongy mesophyll. The mesophyll cells contain tiny bodies called chloroplasts which contain a green chemical called chlorophyll. Chlorophyll enables the light energy from sunlight to be converted into chemical energy for the photosynthesis reaction.
Palisade mesophyll cell Conditions needed for photosynthesis Photosynthesis needs: · chlorophyll · carbon dioxide (from the air) · water (from the soil) · sunlight energy (any light will do except green light) Photosynthesis produces: · glucose · oxygen (a waste product) Chlorophyll and light energy both need to be present for photosynthesis to take place, but they are not actually part of the reaction - they are not used up. So in the word equation for photosynthesis, remember to write them above the arrow, like this: Chemical equation for photosynthesis (Higher Tier)
You need to understand how the carbon dioxide and water molecules are broken down and reassembled to form the glucose molecules. Like all chemical equations the photosynthesis equation is balanced - that is, there are the same number of carbon, hydrogen and oxygen atoms on either side of the equation. Factors limiting photosynthesisThree factors limit photosynthesis from going any faster: Light level, carbon dioxide level, and temperature.· Without enough light a plant cannot photosynthesise very fast, even if there is plenty of water and carbon dioxide. Increasing the light intensity will make photosynthesis faster.· Sometimes photosynthesis is limited by the level of carbon dioxide. Even if there is plenty of light a plant cannot photosynthesise if it has run out of carbon dioxide.· Temperature can be a limiting factor too. If it gets too cold the rate of photosynthesis will slow right down; equally, plants cease to be able to photosynthesise if it gets too hot.If you plot the rate of photosynthesis against the levels of these three limiting factors you get graphs like the ones below. Maximising growthUnderstanding the factors that limit photosynthesis enables greenhouse farmers to maximise the conditions for plant growth. They often use paraffin lamps inside the greenhouse because burning paraffin produces carbon dioxide as well as heat, and so makes photosynthesis proceed faster. They may also use artificial light to enable photosynthesis to continue beyond daylight hours.
Photosynthesis and respiration To unlock the energy in the carbohydrate produced in photosynthesis, green plants need to respire, just as animals do. Respiration takes place in the plant's cells, using oxygen to produce energy and giving off carbon dioxide as a waste product. So in terms of the gas taken in and the gas given out, respiration is the opposite of photosynthesis. The result is that during the day when the plant is both respiring and photosynthesising there is a two-way traffic of oxygen and carbon dioxide both into and out of the plant. During the night when the plant is respiring but not photosynthesising, oxygen is being taken in but not given out - and carbon dioxide is being given out but not taken in. Luckily, plants use up more carbon dioxide in photosynthesis than they produce in respiration, and produce more oxygen while photosynthesising than they use up while respiring - otherwise there would not be enough oxygen in the atmosphere for us animals to breathe! Nutrition and digestion
A balanced diet contains carbohydrates, proteins, fats and fibre as well as vitamins and mineral salts. These substances must be present in the correct proportions. Digestion is the breakdown of carbohydrates, proteins and fats into small soluble substances which can be absorbed into our blood. Nutrition A mixture of different types of food in the correct amounts is needed to maintain health. The main food types are: Carbohydrates, found in potatoes, pasta, bread, bananas, sugar and rice. Carbohydrate is required by our bodies as a source of energy for other life processes. Sometimes referred to as starch, which is actually just one (very common) type of carbohydrate. Fats, found in cheese, butter, margarine and oils. Fats are needed to make cell membranes and to insulate our bodies. They also contain important fat-soluble vitamins. Protein, found in meat, fish, eggs and cheese. Proteins are required for growth and repair. Fibre, found in wholemeal bread, fruit, vegetables and pulses. The fibre or roughage in our diet is not digested - but is important because it allows the muscles in our intestines to move the material along by peristalsis. Digestive system: 1 The food we eat consists of large lumps of material. We must bite off small pieces and chew them up into even smaller ones before swallowing them. Once it gets to the stomach the food is further broken down by being pumelled by the stomach's muscular walls. This is physical digestion. But the substances which our body needs cannot be absorbed into our blood until they have been broken down further - converted into small soluble chemicals. This is done with the aid of enzymes and other chemicals in our gut, and is called chemical digestion. Food is moved through the digestive system by the contractions of two sets of muscles in the walls of the gut - one set running along the gut and the other set circling it. Their wave-like contractions create a squeeze moving down the gut. This movement is called peristalsis. Plug-in. You can quickly download it from here What food is converted to: · Carbohydrate is turned into glucose, which our bodies need to make energy. · Protein is turned into amino acids, required for cell growth and repair. · Fats and oils are turned into fatty acids and glycerol, needed to make cell membranes and to insulate our bodies. Fats also contain fat-soluble vitamins. · Vitamins and mineral salts do not have to be digested because they are already small enough to get into our blood. Digestive system: 2 In the exam you could be asked to label a diagram of the human digestive system, so make sure you know the names of all the parts! To view this piece of media, you will need an up-to-date version of the Macromedia Flash Plug-in. You can quickly download it from here
Mouth and stomach Chemical digestion starts in the mouth, as enzymes in the saliva start to break down starch. Food is then moved to the stomach, where chemical digestion continues with the help of the hydrochloric acid and protease enzymes in the gastric juices. Small intestine The food is next moved into the small intestine, where enzymes produced in the intestine wall and pancreas continue the process of chemical digestion. Bile, produced in the liver, helps to breakdown fats. The intestine is lined with tiny protuberances called villi, each in turn covered with even smaller microvilli. The villi have very thin walls and a plentiful blood supply to enable the products of digestion to be absorbed from the gut into the blood. There are many millions of them - providing a massive surface area to maximise the rate of absorption.
Large intestine Whatever indigestible food is left now moves to the large intestine, where any excess water is absorbed before it is excreted from the anus. Digestive enzymes The enzymes in our digestive system break down complex substances into simpler ones which can be absorbed. Enzymes work best at their optimum pH - so if the stomach, for example, does not have enough acid, its enzyme, pepsin, will not work properly. Amylase
The enzyme amylase is in the saliva, and starts to work as soon as we put food into our mouths. Amylase digests the long, complex starch molecules, cutting them up into the shorter, simpler molecules of the sugar maltose. The word equation is: starch maltose. Maltose however needs further digestion before it can be absorbed - as do the sugars sucrose and lactose. Another group of carbohydrase enzymes break these sugars down, turning them into the simple sugar glucose. The word equations are: maltose glucose sucrose glucose lactose glucose
Protease
Protease enzymes (also called pepsin) are secreted in the stomach and pancreas, and digest proteins. Proteins are long chains of amino acids, and protease enzymes cut them up into peptide - smaller chains of amino acids molecules - and eventually into individual amino acids, which are absorbed in the small intestine. The word equation for the protease reaction is: proteins amino acids Lipase
Lipase is secreted in the pancreas and the walls of the small intestine. It is the enzyme which digests lipids - ie, fats and oils. Lipids are complex molecules made up of fatty acids and glycerol. Lipase cuts lipids up into fatty acid molecules and glycerol molecules. The word equation for this reaction is: Lipids fatty acids + glycerol
You need to remember the main digestive enzymes and the food types they break down. Test yourself by dragging the correct enzyme onto each food type, and see them cut up the complex molecules into simpler ones... More about enzymes Enzymes are proteins. They are very important substances because they control the chemical reactions that happen in our bodies. They are known as biological catalysts - substances which speed up reactions but which do not get used up themselves. Enzyme names usually end in the letters -ase, as in amylase, protease and lipase. There are two main types of enzyme. Digestive enzymes are extracellular enzymes - they control reactions that take place outside cells. Those enzymes which control reactions inside cells are called (not surprisingly!) intracellular enzymes. Enzymes intervene in chemical reactions by locking onto one of the reactants and speeding up the reaction. The chemical which the enzyme locks onto is called the substrate, and the enzyme has a kind of chemical sensor, called an active site, which helps it to recognise the substrate. Just like a key only fits into a specific lock, each enzyme has its own specific substrate. Once the reaction is complete and the required product has been produced, the enzyme releases itself and moves on to the next reaction. To view this piece of media, you will need an up-to-date version of the Macromedia Flash Plug-in. You can quickly download it from here
Effect of temperature on enzymes Temperature is important in all reactions. As the temperature increases, so does the rate of reaction. This is because heat energy causes more collisions between the particles in the enzyme and particles in the substrate. However, very high temperatures damage or denature enzymes. If you plot the rate of an enzyme reaction against temperature, the rate gradually increases with temperature until it reaches around 37°C or body temperature. Then as the temperature continues to rise the rate of reaction falls rapidly as the heat energy begins to denature the enzyme.
The effect of pH on enzymes Different enzymes work best at different pH values. The optimum pH for an enzyme depends on its site of action. For example, enzymes in the stomach have an optimum pH of about 2 because the stomach is acid, but intestinal enzymes have an optimum pH of about 7.5. A graph plotting reaction rate against pH for any enzyme looks like this: Other substances that help digestion Hydrochloric acid The enzyme pepsin requires the presence of hydrochloric acid to create the right pH conditions for the enzyme to work effectively. Pepsin works best at pH2 (acidic) - which is also acidic enough to kill the bacteria taken in with food.
The hydrochloric acid is secreted, along with the pepsin, from tiny pits in the stomach lining. The stomach lining protects itself against digestion and corrosion by secreting sticky neutralising mucus.
Bile Fats and oils are broken down by the enzyme lipase; but before lipase can really get to work the lipids first need to be emulsified. This is done by bile, a substance secreted in the liver and stored in the gall bladder, which is added to food via the bile duct when food is passing through the duodenum. Bile is not an enzyme.
Bile works like detergent - it disperses fats into droplets (an emulsion) and so enlarges the surface area so that the enzyme lipase can get to work breaking down the fat much more efficiently. Bile salts also neutralise the stomach acid and help to create optimum pH conditions for digestive enzymes in the small intestine. Food tests We eat many complex foods which contain mixtures of carbohydrates, fats and proteins. Food tests enable you to find out what food types a food contains. For fats the test is simply to squash a sample of food onto a piece of paper and leave it to dry. A positive test for fat is a translucent stain around the food sample when you hold the paper up to the light. For the other food types, first prepare a sample of food for testing: 1. Crush some food in a pestle and mortar 2. Add a spatula-full to a boiling tube 3. Add 5cm3 of distilled water and stir 4. Bring to boil and simmer for 1 minute 5. Cool and add the test reagent You need to know the different tests for starch, sugars and protein.
Test for Reagent Positive test Carbohydrates - starch Dilute iodine Turns blue-black Carbohydrates - glucose Benedict's solution Orange/red precipitate Carbohydrates - sucrose A few drops of dilute HCl acid + a few drops of Benedict's solution Orange/red precipitate
Test for Reagent Positive test Protein Biuret test - add a few drops of dilute copper sulphate solution, followed by a few drops of sodium hydroxide Purple or violet precipitate Fat Rub a food sample onto a piece of paper. Leave to dry. Translucent stain round the sample when held up to the light
Inhaling and exhaling When we inhale... · the intercostal muscles contract, expanding the ribcage · the diaphragm contracts, pulling downwards. The net result is that ... · the thorax (top part of your body) increases in volume, lowering the pressure inside it and sucking air into the lungs. When we exhale ... · The intercostal muscles relax, allowing the ribcage to drop inwards and downwards, and ... · the diaphragm relaxes, moving back upwards. The result is that ... · the thorax decreases in volume, increasing the pressure inside it and forcing air out. The diagram shows the parts of the thorax (chest) involved in breathing.
As it is inhaled, the air travels down the trachea and through one of the two bronchi into one of the lungs. It then passes into the many branching bronchioles, finally arriving into some of the millions of tiny sacs called alveoli. This is where gas exchange takes place - oxygen passing out of the air into the blood , and carbon dioxide passing out of the blood into the air in the alveoli.
Gas exchangeGas exchange is the means by which we get oxygen from the air into our blood and carbon dioxide out of our blood into the air. Gas exchange happens by diffusion of gases across the very thin walls of the alveoli. In order to maximise the amount of diffusion taking place, the alveoli have a huge total surface area - between 70m2 and 90m2!The alveoli have a moist lining to help dissolve the gases, and are surrounded by many tiny capillaries so there is plenty of blood for the gases to pass into and out of. To view this piece of media, you will need an up-to-date version of the Macromedia Flash Plug-in. You can quickly download it from here As a result of gas exchange, the proportion of oxygen and carbon dioxide in the inhaled/exhaled air changes - with the air we breathe in containing a higher percentage of oxygen and a lower percentage of carbon dioxide than the air we breathe out. (Nitrogen, the main component of air, remains unaffected by breathing and stays at the same percentage.) The table shows the percentage change of oxygen and carbon dioxide between inhaled and exhaled air.Gas % in inhaled air % in exhaled air Oxygen 21 17 Carbon dioxide 0.04 4 Nitrogen 78 78 When we are taking exercise we breathe much faster than when we are resting. This is because when they are working hard the muscles need more energy, so they must respire faster. This means they need more oxygen and produce more carbon dioxide (the waste product of respiration) - so the breathing rate must go up.
Exam tips You should be able to: · Label a diagram of the thorax. · Describe how we inhale and exhale air (ventilate our lungs). · Explain what gas exchange is, and by what method it happens. · Say that the lung has a very large surface area to speed up gaseous exchange. · Explain why there is less oxygen in alveolar air than in atmospheric air. Respiration
Respiration takes place in all living things, all the time. It is the release of energy from glucose or other organic substances inside living cells. Every cell needs to respire in order to produce the energy it needs. How respiration makes energy Respiration is the release of energy from glucose or other organic substances. Energy is required for growth, repair, movement and other metabolic activities. There are two main types of respiration, aerobic and anaerobic. Aerobic respiration takes place in the presence of oxygen. The animation shows how glucose molecules react with oxygen molecules to form carbon dioxide and water molecules, with energy being released by the breaking of bonds in the glucose molecules. To view this piece of media, you will need an up-to-date version of the Macromedia Flash Plug-in. You can quickly download it from here The energy released from glucose in respiration is used to produce a chemical called adenosine triphosphate (ATP). ATP is where the energy released during respiration is stored for future use. Aerobic and anaerobic respiration In aerobic respiration glucose reacts with oxygen in the mitochondria of the cells to release energy. Carbon dioxide and water are by-products of the reaction. You need to learn the word and symbol equation for this:
To view this piece of media, you will need an up-to-date version of the Macromedia Flash Plug-in. You can quickly download it from here As you can see, a lot of energy is released in aerobic respiration - 2900 kj from one glucose and 6 oxygen molecules. Anaerobic respiration occurs when oxygen is not available. In anaerobic respiration the glucose is only partially broken down, and lactic acid is produced - together with a much smaller amount of energy. Here is the word and symbol equation for anaerobic respiration:
When anaerobic respiration occurs in yeast it is called fermentation. In this case ethanol (alcohol) is produced instead of lactic acid, and this reaction is used in the brewing of alcoholic drinks. The word equation is:
glucose ethanol + carbon dioxide + energy Oxygen debt During vigorous exercise the body needs a lot more energy. It gets this by breathing in deeper and faster and rushing the oxygen to the muscles in dilated blood vessels. This extra oxygen is then used to release more energy, needed to meet the higher level of demand. Soon a point is reached when the body cannot breathe any faster or harder, and aerobic respiration alone cannot meet the enhanced energy demands. So how do muscle cells get the extra energy they need? They get it by respiring anaerobically. But anaerobic respiration produces lactic acid, which accumulates in the muscles and causes muscle fatigue and cramps. To avoid damage to cells, lactic acid has to be broken down to carbon dioxide and water immediately the exercise has finished. This is an oxidisation reaction, and requires oxygen.
Running the 1500 metres will build up an oxygen debt This extra oxygen needed to neutralise the harmful effects of anaerobic respiration is called an oxygen debt. In order to get the extra oxygen to 'pay back' the debt, the body continues to breathe deeply for some time after vigorous activity has ceased. When all the lactic acid in the muscles is broken down the oxygen debt has been repaid and normal aerobic respiration resumes. One measure of a person's fitness is how quickly their breathing and pulse return to normal after exercise. This is because in a fit person aerobic respiration is more efficient, so they build up less of an oxygen debt while exercising, and need less extra oxygen to breakdown any lactic acid in their muscles resulting from anaerobic respiration. Exam tips You need to be able to: · Explain that respiration releases energy from glucose. · Explain the difference between aerobic and anaerobic respiration. · Write word equations for aerobic and anaerobic respiration. · Write a balanced chemical equation for aerobic respiration. · List the uses of energy in animals Habitats and populations
Biology is the study of living things; the study of living things within their environment is called ecology. It's important to study organisms in the context of their environment, because living things do not live independently of their surroundings or each other. They interact continually with their environment and with other living things. They are interdependent. Ecological termsIn order to get on top of this topic, you need to be clear about what each of the following terms mean:environment means all the conditions that surround any living organism - both the other living things and the non-living things or physical surroundings a habitat means a place where plants and animals live (eg a pond) a population means all the members of a single species that live in a habitat (eg minnows) a community of living things means all the populations of different organisms living together in a habitat an ecosystem refers to a community of animals, plants and micro-organisms, together with the habitat where they live
This pond ecosystem consists of a pond habitat, inhabited by populations of aquatic plants, waterside plants, micro-organisms (in the mud at the bottom of the pond), minnows and herons. The organisms together make up a community of living things.
Distribution of organisms Living organisms are not evenly distributed around the world, but are adapted to live in particular habitats. A number of factors affect the suitability of any habitat for the organisms that live there. The most important of these is climate, which determines the temperature range and rainfall levels in a given habitat. Warm temperatures and plentiful water make the tropical rainforest a rich environment which is home to a huge variety of species. Deserts on the other hand are sparsely populated with few species, because of the extremes of heat and cold and lack of water - conditions which few organisms are adapted to survive in.
We can group together ecosystems adapted to similar climate conditions into global ecological regions called biomes, each of which supports its own typical range of plant and animal species. The map above shows the major biomes of the world, while the pyramid below shows the relationship between biomes and climate.
Ecosystems An ecosystem consists of a community of organisms and the non-living factors that influence them. The living and non-living elements interact to support and maintain a balance between plant and animal communities within the ecosystem.
The biosphere as seen from space It is possible to think of the whole earth as an ecosystem. The biosphere includes all the earth's landmasses, oceans and atmosphere, and all the animals, plants, insects, birds and micro-organisms living in them. But because the biosphere is so large and all-encompassing and its relationships so complex, we normally study smaller ecosystems. As well as the vast global regions known as biomes there are two other categories of ecosystem:
Meso ecosystems are subdivisions of biomes, and occupy medium-sized habitats such as freshwater ponds, hedgerows, woodlands, sand dunes, and coral reefs . (Meso means 'middle-sized').
Micro ecosystems, also known as microclimates, are subdivisions of meso systems. They inhabit very small areas such as a water droplet, a single leaf, or the cool, damp space under a log - where the conditions may be quite different from the habitat as a whole but just right for the organisms that live there.
A coral reef is an example of a meso ecosystem Environmental influences Ecosystems are shaped by two types of environmental influence. Abiotic factors are the effect of the the material, non-living environment - things like: · light intensity · temperature range · rainfall levels and rate of water flow · water content of soil · humus content of soil · soil pH and nutrient levels · dissolved oxygen levels, and · pollutant concentrations
Biotic factors are the effect of all the living things in the environment - for example: · which producers (green plants) are present · which consumers and predators are present · the number and type of competitors for light, food, space, shelter, or mating opportunities · the pathogens and parasites which are present · the number and type of insect species present · the number and type of decomposers, and · the level of species diversity All ecosystems depend on two important processes: The flow of energy, and the recycling of nutrients. These processes are covered in the Revision bite on Energy and nutrient transfers. Example of an ecosystem The diagram shows a oak tree ecosystem. Note that different living things inhabit different zones or layers from the bottom to the top: · First there is the roots, soil and leaf-litter zone beneath the tree. Here decomposers such as bacteria, woodlice, and earthworms feed off last year's leaves and acorns, and fungi grow on its roots · Next is the trunk layer, which provides shelter or food to insects, caterpillars and larvae. · Finally comes the branches, leaves and canopy. In this zone bees gather pollen and nectar, fungi grow on the leaves, gall wasps and moths lay their eggs, and squirrels gather acorns. Small birds such as bluetits hunt the moth larvae; and sparrowhawks hunt the small birds.
An oak tree ecosystem. Each zone of the tree is home to a distinctive community of organisms Each of the organisms in this ecosystem has a particular way of fitting into the oak tree environment - they each occupy a niche within the ecosystem. For example the blue tits and the squirrels, though they both inhabit the same tree, do not directly compete for food: the squirrels feed on acorns, while the tits feed on moth larvae. The two species occupy different niches within the oak ecosystem. Adaptation The competition between organisms means that those who are best adapted are most likely to survive and reproduce. Natural selection means that over many generations organisms become progressively better adapted to their environment. (For more on natural selection, see the Revision bite on Evolution.) Adaptation in extreme habitats: three examples:
Camels are well adapted for survival in the desert. They have: · the ability to store a lot of water, and to lose very little via urination and sweating · the ability to tolerate body temperatures up to 42 degrees C · a large surface area / volume ratio - maximising heat loss · a hump which can store scarce food as fat without insulating the body · thick fur on the top of the body to provide shade, and thin fur on the rest of the body to allow easy heat loss, and · large, flat feet well-suited for walking on sand
Polar bears are well adapted for survival in the arctic. They have: · a thick layer of fur for insulation against the cold · a thick layer of blubber for further insulation, and also as a food store · a small surface area / volume ratio, to minimise heat loss · a greasy coat, which sheds water after swimming · large feet to spread the load on snow and ice, and · the ability to move fast on both land and water, while pursuing their prey
Cacti are well adapted for survival in the desert. They have: · spines instead of leaves, which minimise surface area and therefore evaporation, and also... · protect them from animals which might eat them · stems which can store water, and · widespread root systems, which can collect water from a large area Competition and co-operation Competition Habitats have finite amounts of the resources needed by living organisms, such as food, water and space, and all organisms strive to reproduce themselves and increase their numbers. Sooner or later the demand for these resources is going to exceed supply, and organisms have to compete with each other to get them. Plants typically compete with each other for: · light (for photosynthesis) · water, and · nutrients (minerals) Animals typically compete with each other for: · food · water · mates (so they can reproduce), and · living space It is this competition between organisms that enables natural selection to take place, by favouring chance mutations conferring a slight advantage in the race for limited resources. For more on this point take a look at the Revise bite on Evolution.
Co-operation Some organisms find that they are better able to survive and reproduce by living closely together with another organism of a different species. This type of interpendence between organisms is known as symbiosis. Some examples of different styles of symbiosis are:
Barnacles which attach to a whale or scallop shell. The barnacles get a home and transport, and the whale or scallop is not unduly affected. This type of symbiosis, where one organism benefits and the other suffers no harm, is called commensalism.
Lichens are formed by algae and fungi living together. Algae can photosynthesise and make food which is shared by the fungus. The fungus in turn shelters the algae from a harsh climate. This kind of mutually beneficial co-operative relationship is called mutualism.
A tapeworm lives inside another animal, attaching itself to the host's gut and absorbing its host's food. The host loses nutrition, and may develop weight loss, diarrhoea and vomiting. This kind of one-sided symbiosis is called parasitism. Usually parasites do not kill the host before they move on, as this would cut off their food supply. Interdependence of organisms All living things within an ecosystem are interdependent. A change in the size of one population affects all other organisms within the ecosystem. This is shown particularly clearly by the relationship between predator and prey populations. There is a continuous tussle between predators and their prey. Predator species need to be adapted for efficient hunting if they are to catch enough food to survive. Prey species on the other hand must be well adapted to escape their predators if enough of them are to survive for the species to continue. If the prey population in an ecosystem grows, predator numbers will respond to the increased food supply by increasing as well. Growing predator numbers will eventually reduce the food supply to the point where it can no longer sustain the predator population... and so on.
Case study 1: Ladybirds and aphids The animation shows this predator/prey dynamic between a population of ladybirds (predators) and a population of aphids (their prey). The graph showing the relationship between them will look fairly similar for any two populations of predators and prey.
Case study 2: the Canadian lynx and Snowshoe hare Another example of the predator / prey dynamic is the rise and fall in numbers of Canadian lynx and its favourite prey the Snowshoe hare. The two populations were estimated each year for some 75 years from the number of animals caught by fur traders. The lynx population was found to rise and fall in a 10-year cycle, with that of the hare following 2 years behind. No other cat is so dependent on a single prey species, which is why there is such a clear pattern of interdependence between the two populations.
Feeding relationships
A very important aspect of the interdependence of organisms in an ecosystem is their feeding relationship with each other. Food chains and food webs are simply ways of describing these relationships. Together with pyramids of number and biomass, they help us to understand the movement of biological material (biomass) and of energy through an ecosystem. Feeding relationships
Living organisms in an ecosystem depend on each other for food and essential nutrients such as carbon and nitrogen. This interdependence often takes the form of a feeding relationship - ie, they eat each other! Make sure you understand the meaning of the following terms: Producers ie, green plants. All food chains start with them, because they can make food by photosynthesis. Primary consumers feed primarily on plant material. They are herbivores - eg rabbits, caterpillars, cows, sheep, and deer. Secondary consumers feed primarily on animal material. They are carnivores - eg cats, dogs and lions. Omnivores eat both plants and animals - eg bears and humans. Predators kill for food. They are either secondary or tertiary consumers - eg polar bears, golden eagles. Prey are the organisms that predators feed on. Examples of predator and prey species are: fox and rabbit; blue tit and caterpillar; wolf and lamb. Scavengers feed on dead animals . They perform a useful cleaning-up function. Examples are crow, vulture, buzzard and hyena. Decomposers feed on dead and decaying organisms and on the undigested parts of plant and animal matter in the faeces. (They do not 'eat' the food like scavengers, as they have no mouth-parts. Instead they break down solid matter into liquids which they can absorb.) Examples: bacteria and some fungi.
Food chains A food chain shows who eats what in a particular habitat. For example: grass seed is eaten by a vole, which is eaten by a barn owl. The arrows between each item in the chain always point in the direction of energy flow - in other words, from the food to the feeder.
All food ultimately comes from green plants or producers. The other organisms in the food chain are consumers, because they all get their energy and biomass by consuming (eating) other organisms. All food chains are pretty short. There are never more than four steps, because a lot of energy is lost at each step, and after three steps most of the available energy has been expended. This also explains why the organisms at the top of food chains (eg owls) are very small in number compared with those lower down (eg grass plants). After 2 steps there is simply not enough available energy to support more than a few top predators. For more on how energy moves through food chains, have a look at Energy and nutrient transfers. Food webs In its natural habitat it is unusual for an animal to eat only one particular organism, so a more realistic way of showing feeding relationships is to draw a series of interconnecting food chains. This is called a food web. The food web below describes feeding relationships in a freshwater pond ecosystem. It allows you to follow the routes that biomass (and energy) take through the system.
Pondweed (a producer) is eaten by the mayfly nymphs, which are in turn fed upon by both the dragonfly nymphs and the brown trout. The brown trout make a meal of the dragonfly nymphs too. The other producer in this web is the microscopic algae. This is eaten by the freshwater shrimp. The shrimps are fed upon by dragon fly nymphs and brown trout (which also eat the dragonfly nymphs). Pyramids of number and biomass Food chains and webs show the flow of materials and energy in habitats, but they do not give you any idea of how many organisms there are in the habitat. To show this you need to draw a pyramid of numbers. Let's go back to the grass vole barn owl food chain. Suppose the numbers found in a particular habitat are as follows: 2000 grass plants 25 voles 1 barn owl The pyramid of numbers would be as shown below. The wide base represents the large number of grass plants. Above this is a narrower section representing the much smaller number of voles, while the top section is one unit wide and represents a single barn owl.
Pyramids of numbers will often be pyramid-shaped like this - but not always. Look at the next one.
If the producer is a large plant such as an oak tree, the second layer of the pyramid representing primary consumers (caterpillars in this example) will be much larger than the base. In this case it would make more sense to draw a pyramid of biomass, which shows not the numbers of organisms at each level, but the amount of biological material. A pyramid of biomass for the oak tree would look like this:
Pyramids of biomass are always pyramid-shaped. Exam tips Don't worry if you have never heard of the organisms used in a food chain and web question. There will always be enough information for you to work out the answer. When looking at a food web, check carefully which way the arrows are pointing. Remember the arrows represent a flow of materials or energy, so the arrow should go from the organism which is being eaten to the animal which is eating it. In questions asking you to suggest population changes in food webs, look carefully at the links between the organisms and try to work out what might happen. Energy and nutrient transfers
All living organisms require energy. The ultimate source of all this energy is the sun. Solar energy is trapped by plants, and then transferred from organism to organism in a food chain. At each stage in this transfer much of the energy is lost to the environment.
Living organisms also require nutrient elements - in particular carbon and nitrogen - which they take from the environment. If this were just a one-way process, ecosystems would soon run out of these nutrients; but in fact they are returned to the environment, with the help of bacteria, via nutrient cycles Energy transfer In every ecosystem, energy is transferred along food chains from one trophic level to the next. But not all the energy available to organisms at one trophic level can be absorbed by organisms at the next one: in fact the amount of available energy decreases dramatically at each level. Why? Some of the available energy goes into growth in biomass and the production of offspring: this energy does become available to the next trophic level. However most of the available energy is used up in other ways: · Some is used up at the first trophic level as a result of photosynthesis, which uses up lots of solar energy in making glucose. · Some is used up in respiration, and given off as heat · Some is lost, in the form of biological material and heat, through excretion. (This energy is actually transferred to the decomposer food chain: more about this later!) · Some is used for movement and transport. All the energy used in these ways returns to the environment, and is not available to the next trophic level. The animation shows how the level of available energy declines as it is transferred through a temperate forest foodchain You can see why it is that food chains almost never have more than four steps: so much energy is lost at each level that however much you start off with it is almost all gone by the fourth trophic level.
Calculating energy efficiency This bullock has eaten 100 kJ of stored energy in the form of grass, and excreted 63 kJ in the form of faeces, urine and gas. The energy stored in its body tissues is 4 kJ. So how much has been used up in respiration?
Energy balance in a bullock Now energy cannot just disappear - it must all be accounted for. So the total amount of energy used up by the bullock must equal the total taken in as food. Using this fact we can easily work out how much energy the bullock has used up in respiration. total energy input = 100 kJ total energy used = 63kJ + 4kJ + respiration energy used in respiration = 100 - 63 + 4 energy used in respiration = 33kJ We can also work out the energy efficiency at each trophic level by dividing the useful energy output by the total energy input. Multiplying this fraction by 100 gives you the percentage efficiency. "Useful" means "available to the next trophic level"; so the calculation for the bullock goes like this: useful energy output = 4kJ total energy input = 100kJ energy efficiency = or 0.04 x 100 to get % = 4%
Meat-eating and energy efficiency In fact the amount of energy available to someone eating this bullock would be even less than 4kJ, because many parts of the animal - eg brain, bones, hooves, large blood vessels and skin - are not eaten. Obviously eating beef is not a very efficient means of gaining energy! In fact it is far more efficient for humans to eat cereals such as wheat and barley themselves, rather than using them to feed cattle and then slaughter the cattle for beef. This is because the energy in the beef has already passed through two trophic levels by the time it gets to humans, and at each level the amount of available energy is reduced. When we eat cereals (or vegetables or fruit) the energy has only passed through one trophic level, and there has only been one set of energy losses instead of two. It follows that far more food can be produced from a given area of land if plants are fed to people, not to farm animals. It has been calculated that a meat-based diet requires approximately 7 times more land than a plant-based diet.
Carbon and the carbon cycle The element carbon is a basic constituent of all living organisms. Its atoms combine easily with other atoms to form a huge variety of molecules. Some of these (eg carbon dioxide, carbohydrates) have names which make it obvious they are carbon-based, while others (eg fat, proteins) you just have to remember. All cells - whether animal, plant or bacteria - contain carbon because they all contain proteins, fats and carbohydrates. Plant cell walls for example are made of cellulose, a carbohydrate. Living organisms need carbon in order to: · Make food. Green plants get their carbon from the carbon dioxide in the air, which enters the leaves and is used for photosynthesis. A product of photosynthesis is glucose - another carbon-based compound. · Make energy. In respiration glucose reacts with oxygen to produce energy (with carbon dioxide as a by-product). · Make new cells for growth and repair. Carbon compounds are essential cellular building-blocks.
The carbon cycle Carbon cycles through ecosystems, moving repeatedly from one organism to another, and between organisms and the environment. The carbon cycle is a key factor in maintaining the balance of an ecosystem. It works like this: 1. Plants photosynthesise, taking carbon in the form of carbon dioxide from the atmosphere and locking it into the carbohydrate glucose. (They also respire, giving out carbon dioxide; but they take in much more than they give out.) 2. Animals get their carbon from eating either plants (carbohydrates) or other animals (proteins and fats) which they then digest. They respire, giving off carbon dioxide to the environment. 3. Waste carbon-based material is excreted by animals, and is digested by decomposers - mainly microbes and fungi. The decomposers also respire, releasing carbon dioxide. 4. When animals die, their remains may be either eaten by scavengers (for example, crows) or digested by decomposers. Both scavengers and decomposers respire, giving off more carbon dioxide. 5. In certain conditions both animal and plant remains may become fossilised, eventually forming carbon-based fossil fuels (coal, oil and gas). 6. Both fossil fuels and plant material (wood) may later be combusted - releasing still more carbon dioxide to the environment. The animation should help you understand how the cycle works. The nitrogen cycle (Higher Tier) 79% of the air around us is nitrogen . Living things need nitrogen to make proteins, but they cannot get it directly from the air because nitrogen gas is too stable to react inside an organism to make new compounds. So nitrogen must be changed into a more reactive form to allow plants and animals to use it. Plants can take up and use nitrogen when it is in the form of nitrates or ammonium salts. Changing nitrogen into a more reactive substance is called nitrogen fixation. Nitrogen fixation happens in three different ways: 1. The energy in a lightning bolt can split the di-atomic nitrogen molecule in the air allowing each nitrogen atom to react with oxygen to form nitrogen oxides. These oxides are washed to the ground by the rain where they form nitrates. 2. The Haber process is used by industry to produce ammonia from nitrogen. Ammonia is used to make fertiliser for farmers to feed their crops. 3. Nitrogen-fixing bacteria found in the soil and in the root nodules of leguminous plants fix nitrogen into a usable form. Nitrogen compounds are returned to the soil by excretion and egestion from animals or when plants and animals die and decay. The nitrogen compounds returned in this way are changed back to nitrogen gas by denitrifying bacteria which live in the soil - thus completing the cycle.
The impact of humans
All living things have an effect on the environment, because organisms and their ecosystems are interdependent. But the impact of humans is greater than that of any other species, because
1 there are an awful lot of us, inhabiting the entire planet, and
2 distinctively human activities such as farming, fishing, and industry have a much bigger effect on the biosphere than the activities of other species. The threat to biodiversity The world is an astonishingly diverse place, inhabited by millions of different plant and animal species, with probably millions more that have not yet been discovered. The term biodiversity refers not only to the sheer number of different species, but to all the genetic variations within and between species - and all the differences between the many, many habitats and ecosystems that make up the earth's biosphere. Why is it important to maintain the huge variety of life? Some answers are that : · we have a moral responsibility to look after the planet and its resources, rather than simply use them up · biodiversity provides us with many direct benefits - eg clean air and water, food, medicines, fertile soil, and pollination for our crops · biodiversity represents the earth's total gene pool - a source of future variation which is vital for all species · reduction in biodiversity in ecosystems may reduce climatic stability, for example by upsetting the balance between intake and output of carbon dioxide
Impact of humans By far the biggest threat to the variety of life is posed by human exploitation of the environment. Our impact on the global environment is greater than that of any other species because of:
Our technologies: we use tools and techniques which can change the shape of the earth in a short space of time (eg clearing forests or changing the course of rivers)
Our population is increasing at a phenomenal rate. There are 6 billion of us now and by 2050 we are expected to number around 9 billion. Our sheer numbers will mean that even small activities multiplied 9 billion times will have a huge impact on the environment
Consumption and waste: we consume vast amounts of natural resources (eg water, fossil fuels) and produce vast amounts of waste (eg greenhouse gases). Both pose a threat to other forms of life - as well as to ourselves!
Human population growth Like all living things, humans exploit their surroundings for resources. Before the beginning of agriculture about 10,000 years ago, small groups of humans wandered across large areas hunting and gathering just enough to keep alive. Population numbers were kept low by the scarcity of food. The invention of agriculture began a population explosion which has accelerated enormously in the last 500 years. Unlike other species, humans can adapt to and survive in almost all habitats and climates, from the cold Arctic regions to dry, hot deserts. Currently around 6 billion and rising fast, human population growth now poses a threat to the global environment.
Human population growth over the last 10,000 years In the United States and many European countries, changing work patterns, universal education and health care, and family planning have all helped to halt population growth. But in developing countries, like those of India and Africa, population numbers continue to rise fast.
Growth-limiting factors In every ecosystem and every species, population sizes have limits beyond which they cannot safely expand. They are constrained by growth-limiting factors which not even humans can easily control. These are: · the availability of food and water · invasion of parasites, pathogens or disease · over-crowding (increasing competition for food, water and space) · severe or sudden climatic changes · pollution of air, soil and water If we do not take steps to control population growth ourselves, it is likely that one or more of these growth-limiting factors will eventually kick in to forcibly reduce our numbers. Urbanisation and industrialisation
Urbanisation means the growth of cities. Around 3 billion of us - half the world's population - now live in cities, and virtually all future population growth will take place in cities. Moreover our cities are getting bigger and bigger: by 2015 it is predicted that the world's 6 largest cities will each have more than 20 million inhabitants. Some of the effects of urbanisation are: · greatly increased pollution as a result of urban traffic, energy consumption and waste production · land taken out of food production, as more and more farmland is built on · loss of natural habitats, as cities and roads proliferate · fragmentation of rural communities and cultures, as more and more people leave to live in towns
Industrialisation Closely linked with population growth and the rise of cities has been a worldwide rapid development of industries such as manufacturing, mining, power generation and transport. These have a major impact on the environment, because: · they use up huge amounts of water and energy (usually non-renewable fossil-fuel energy), and · they are a major agent of climate change, because they release greenhouse gases into the atmosphere and speed up global warming Impact of farming: 1Farming is what makes possible the production of food surpluses and settled living. It also brings about big changes in the relationships between living things and in their habitats. Farming - especially modern, intensive farming - can damage the environment in many different ways.Effect of fertilisersFertilisers containing plant nutrients are sprayed onto fields to make plants grow faster and boost crop yields. When it rains the nutrients may get washed down from the fields and into rivers and lakes (this is called run-off). The result is eutrophication - which can kill almost everything living in the aquatic environment. It works like this:
Impact of farming: 2 Effect of pesticides Pesticides are chemicals used to kill insects, weeds and micro-organisms that might damage crops. However, pesticides damage other organisms apart from those they are intended to kill - for example, depriving insect-eating birds of food. Pesticides can also enter local food chains. Organisms that ingest them cannot break them down, so they persist in their bodies. (Substances that cannot be broken down are called persistent substances: the pesticide DDT is an example.) The pesticides may then build up at ever-higher levels until they become toxic to much larger organisms. Here's how it works:
Other impacts of farming Agriculture can impact on the environment in many other ways. For example: · farming takes up land, reducing habitats and wildlife · monocultures (large amounts of one type of food) provide lots of food for pests as well as humans · irrigation (watering of crops) may take too much water from rivers, depriving downstream habitats of water · clearing land for farming may result in soil erosion, damaging ecosystems and leaving land barren · Intensive livestock farming produces huge amount of faeces, which may pollute waterways Fishing and forestry Fish are an important part of the human diet, accounting for a worldwide average 15% of humans' protein intake. Most of these fish are caught wild, and if fish are caught at a faster rate than the remaining fish can reproduce, the stock of fish will obviously decline. Trying to harvest more fish than the sea can produce is an example of unsustainability. Since the 1960s North Sea cod have been overfished, as more and more (and bigger) fishing boats caught more and more cod. At first, catches continued to increase each year. But then - surprise, surprise! - they started to decline, as there were not enough breeding fish left to maintain the cod population. Today, North Sea cod are in danger of extinction.
Forestry Humans have now been cutting down trees for around 10,000 years - for wood to burn or build with, or to clear land for farming. Forestry is sustainable as long as forests are allowed to replace themselves, or are replanted after harvesting - but often this is not done. The result is that the area of forest is steadily shrinking - a process called deforestation - with profound consequences for ecosystems and biodiversity. Deforestation: · destroys forest habitats, endangering many forest-dwelling species · causes soil erosion, as the soil-stabilising effect of tree roots is removed. Barren land and flooding can result · causes atmospheric pollution (mostly carbon dioxide) as forests are cleared by burning trees · reduces the amount of photosynthesising vegetation, thus further increasing levels of carbon dioxide in the atmosphere The maps give an idea of how much of the earth's forest has been lost in the last 10,000 years.
Particularly worrying is the rapid destruction of tropical rainforest. This type of forest is often referred to as 'the earth's lungs' because it produces around 40% of the world's atmospheric oxygen. It is also home to an estimated 50% of all species on earth (once lost, the original habitats can never be replaced). Tropical rainforest is being cut down at the rate of 17 - 20 million hectares a year. Pollution Pollution is the addition of substances to the environment that may be harmful to living organisms. Atmospheric pollution Atmospheric pollution is caused either by the burning of wood and fossil fuels (coal, gas and oil); emissions from petrol or diesel engines; discharges from factories, power stations, and waste dumping; and from agricultural livestock. The pollutants involved include: · Smoke, which damages air quality and deposits soot on surfaces such as tree trunks and leaves · Carbon dioxide, the main greenhouse gas causing climate change · Methane, the second most important greenhouse gas causing climate change · Sulphur dioxide and nitrogen dioxide, which mix with rainwater to form acid rain. Acid rain corrodes buildings and damages trees and plants. · Carbon monoxide, which is poisonous to humans and animals (it reacts with haemoglobin and prevents it carrying oxygen) · Dust (produced by activities such as quarrying and mining) which may impair plants' ability to photosynthesise and people's ability to breathe
Water pollution Water pollution is caused by the discharge of harmful substances into the sea, rivers, lakes or the water table. Pollutants include: · Agrochemicals such as fertilisers and pesticides which damage aquatic ecosystems · Sewage, which when dumped into rivers or the sea can kill aquatic organisms and harm human health · Chemical contaminants, discharged by factories or through sewage systems, may contaminate soil or seabeds, and have unpredictable effects on wildlife - for example damage to reproductive organs through exposure to human sex hormones. · Slurry (a liquid manure made of animal droppings and urine) which if it drains into streams and rivers can harm aquatic wildlife · Oil - spilled from oil tankers and refineries - which destroys habitats, poisons water supplies, and kills aquatic wildlife and birds Other types of pollution Radiation pollution - caused by either deliberate or accidental discharge of nuclear waste - can contaminate the soil, water and air for years. Radiation can cause cancers like leukaemia. Radioactive waste cannot be safely burnt or buried or sunk, but has to be stored and transported in lead- or glass-lined containers which stop the radiation from passing out. Many radioactive compounds stay radioactive for thousands of years. Noise pollution, caused by aircraft and other engines, road or building works, can induce stress in both humans and wildlife. Litter - especially non-biodegradable litter such as plastic and glass - can seriously degrade human habitats, as well as injuring both humans and wildlife. Controlling pollution Humans do not have to pollute the environment as much as they do. Outputs of smoke and soot - once the main pollutants in industrial societies - have been massively reduced over the last 100 years. There are many things that could be done to reduce the impact of other types of human pollution as well. Type of pollution Source Control measures smoke and soot burning wood and fossil fuels smokeless zonessmokeless fuel carbon dioxidecarbon monoxide burning fossil fuels burn less fossil fuel (switch to clean energy sources)build cleaner engines (eg catalytic converters)reduce energy consumption methane burning fossil fuel, landfill waste and agricultural livestock burn less fossil fuelscut waste productioncut livestock numbers sulphur dioxidenitrogen dioxide fossil fuels burn less fossil fuelbuild cleaner enginesreduce energy consumption dust factories, quarrying etc efficient air filtersstricter laws against polluters agrochemicals: fertilisers and pesticides farms use organic fertilisers insteaduse biological pest controls instead oil tankers, refineries use less oilstricter laws against polluters
Green machines: wind turbines generate energy without pollution Global warming
As you can see from the graphs, the earth has got steadily warmer over the last 150 years, as levels of atmospheric carbon dioxide have got steadily higher. What is causing this? Global temperature is the result of a balance between heat received from the sun and heat radiated back into space. The earth's atmosphere forms an insulating layer, keeping some - but not all - of the sun's heat in, a bit like the glass roof of a greenhouse. Human activities such as burning fossil fuels for energy and transport are adding more and more carbon dioxide to the atmosphere. At the same time large-scale deforestation is reducing the number of trees and other green plants removing carbon dioxide for photosynthesis. Hence - rising carbon dioxide levels. Levels of atmospheric methane are also going up. Methane is produced by livestock farming, rotting plant material in marshes and paddy fields, and by drilling for oil and gas. Methane and carbon dioxide are greenhouse gases - they insulate the atmosphere so that it traps more of the sun's energy. This is the called the greenhouse effect, and it is the cause of global warming.
Results of global warming Rises of just a few degrees in world temperatures could have a dramatic impact on world climate: · global weather patterns may change, changing rainfall patterns, causing drought in some places and floods in others · melting of polar ice caps could raise sea levels, causing increased coastal erosion and flooding of low-lying land - including some major cities
Ozone destruction Chlorofluorocarbons or CFCs, used in fridges, airconditioners and aerosols, are another greenhouse gas. Though there are much lower levels of CFCs in the atmosphere than carbon dioxide and methane, they are thousands of times more effective at trapping heat. They also damage the outer ozone layer of the atmosphere by breaking apart ozone molecules. The ozone layer acts like a screen protecting us from the sun's harmful UV rays, and CFCs are causing holes to appear in the layer, allowing more UV rays to pass through, and increasing the risk of skin cancer.
My goodness Emma, what strange anonymous commenters you draw. Though I suppose beautiful women do make some people scurry for the comforts of biology 101. :)
I will never understand why Paltrow is in such denial about her boobs. She never, ever seems to want to do right by them. All they ask of her is a little support, but noooo.
I loved looking at this post. These pictures are great, so close up! Kate was so lovely, but yours is the only shot where I seem to be seeing some undergarment show-through.
Helen gets my vote. When you are young & gorgeous like Penelope, looking good is second nature. For Mirren, it is a carefully practiced art. :)
we just like to let you know, I really like the writing on your site. But I'm viewing it with my personal Google Chrome Browser and the display will not appear to become very appropriate. It's certainly not a big problem as I can certainly still view and look over the posts but simply thought I would allow you to understand about that. brilliant get the job done. Cheers.
Barraza looks terrible!
ReplyDeleteDiaz looks far more terrfying.
ReplyDeleteDon't like the look of Rinko any more
ReplyDeleteCruz was the best, but Kidman and Winslet were both stunning, too.
ReplyDeleteDiaz was worst.
Kate Winslet was my favorite. Kidman looked as great as ever. Cate Blanchett was great too (not in one of your pictures, but great).
ReplyDeleteThe babel girls were gross!
ReplyDeletePenelope = dreamy.
ReplyDeleteThis gown was pure red carpet fantasy, the ultimate Oscar frock.
I loved Reese Witherspoon and Helen Mirren. The best-dressed men would be Leo and Clive Owen (two of the hottest men in Hollywood).
ReplyDeleteI don't think that I can say it any better than I do here.
ReplyDeleteGwyn for me.
ReplyDeleteWhat a bold dress!
Penelope's dress is lovely =)
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Biology Year 10 Notes
ReplyDeleteLife processes 1
Movement
Both animals and plants can move. Animals can move quickly and from place to place in search of food, shelter and favourable conditions; plants on the other hand are rooted to a spot and move much more slowly by growing and responding to external stimuli, such as light.
Reproduction
Human egg surrounded by sperm
All animals and plants can reproduce. They multiply in number by producing the next generation of offspring, thus carrying on their genes and ensuring the continuation of the species. Plants do this by producing seeds, which give rise to new plants of the same species. Reproduction can be of two types:
1. Sexual, involving two parents and the union of two gametes, and
2. Asexual, where one parent reproduces itself. Examples of this are strawberry plants or spider plants producing runners or offshoots.
Sensitivity
The Venus flytrap responds to touch
Sensitivity is an awareness of changes in the environment. Whereas animals respond more quickly to stimuli such as heat, light, touch and chemicals, plants appear less sensitive and respond more slowly - eg, to the direction of sunlight. Some plants such as the Venus flytrap respond to touch.
Life processes 2
Growth
Growth is an ongoing increase in size of the organism - as in growing from young to adult animals or from seedlings to mature plants.
Excretion
Thousands of chemical reactions go on inside body cells producing both useful and waste substances which when allowed to accumulate can be harmful. Excretion is getting rid of metabolic waste, produced by the body, like urine and carbon dioxide. Getting rid of faeces or undigested food is not excretion but egestion.
Respiration
Mitochondrion inside a cell
Energy contained in food is 'unlocked' or transferred to the organism by the process of respiration. Respiration takes place in the mitochondria of the cell. Energy is released in a controlled way in a series of reactions. There are two types of respiration, with or without oxygen:
1. Aerobic respiration uses oxygen, and releases a large amount of energy
2. Anaerobic respiration does not use oxygen and releases much less energy.
Nutrition
Both animals and plants need food as a source of energy and growth. Plants make their own food by photosynthesis taking in simple substances like carbon dioxide and water and building them into complex carbohydrate molecules.
Animals cannot make their own food and therefore rely on plants or other animals for their energy. Animals take in complex substances and break them down into simple, soluble molecules which can be used as a source of energy.
Plant and animal cells
All cells have these features in common:
· a cell membrane which controls what passes into and out of the cell
· cytoplasm where chemical processes (controlled by enzymes) take place, and
· a nucleus containing genetic material which controls the cell's behaviour.
In addition, plant cells have the following unique features:
· a rigid cell wall made of cellulose
· vacuoles containing cell sap, and
· chloroplasts, containing chlorophyl which enables photosynthesis to take place.
Make sure you can draw and label a generalised animal and a generalised plant cell, like this:
Although it is useful to think about the general features of plant and animal cells, all cells in living organisms display specialised features that make them suited to carrying out their very specific jobs - for example, root cells, stem cells and leaf cells within a plant. A complex organism can be compared to a factory production line with different people performing different tasks. Cells within an organism display a similar division of labour to make the whole organism work properly and at a high level of efficiency.
Cell specialisation
The following illustration shows a variety of animal and plant cells, with different shapes and structures. Each has a very specific job, which helps the organism as a whole to work efficiently.
Specific functions of animal cells
generalised animal cell
Example of animal cell Specific function Specialised features suited to the function
Sperm cell(male gamete) To fertilise the egg cell (female gamete) The head, containing genetic information and a nucleus, has an enzyme to help penetrate the egg cell membrane.The middle section, immediately behind the head is packed with mitochondria for energy.The tail or flagellum moves the sperm to the egg.
Nerve cell(motor neurone) Pass sensory impulses from receptor to an effector Dendrites to make connections with other neurones.Long axon or nerve fibre to carry the impulse to the target organ.End plate forms a synapse with an effector (a muscle or a gland).
Epithelial cell Lining cells, eg inside the cheeks Flattened shape. Interlocking edges, cells fitting closely together to form a continuous lining.
Ciliated epithelium Lining the nose and wind pipe Flattened shape. Interlocking edges, cells fitting closely together to form a continuous lining.Have very tiny hair-like extensions called cilia which move rhythmically to remove mucus, dust and germs.
Red blood cell Contain haemoglobin to carry oxygen to the cells Thin outer membrane allows oxygen to diffuse through easily. Bi-concave shape increases the surface area to allow more oxygen to be absorbed efficiently.No nucleus means that the whole cell is full of haemoglobin which combines with and carries oxygen around the body.Bi-concave shape, i.e. thinner in the middle, the more flexible framework allows cells to squeeze through even the tiniest capillaries.
White blood cell Fight disease; either engulf bacteria or make antibodies Irregular shape, some with many lobed nuclei. Can change shape to squeeze out of blood vessels and get to the site of infection.Some make antibodies. Others have cytoplasm which can flow making it possible for the cell to change shape, surround and engulf bacteria. Can increase in numbers to fight disease.
Muscle cell(Skeletal muscle) Cause movement Elongated in shape.Have fibres which slide into each other and cause the muscle to contract or become shorter and then relax to its original length.Contain filaments of muscle proteins. Need energy to work therefore have a lot of mitochondria.
Specific functions of plant cells
generalised plant cell
Example of plant cell Specific function Specialised features suited to the function
Root hair cell Take in water and mineral ions from the soil Located the root epidermis and in direct contact with the soil.Wall nearest the soil has a long 'finger-like' process projection with very thin walls into the soil.This projection massively increased surface area for more efficient uptake of water and ions. The thin walls make up-take of water easier.
Palisade cell Carries out photosynthesis Packed with chloroplasts containing the light absorbing pigment cholophyll.Regular shaped, closely packed cells forming a continuous layer for efficient absorption of sunlight.
Guard cells Predominate on lower surface of leaves; help to reduce water loss Surround pores in the outer layer of leaves.Predominate on the lower surface of the leaf to reduce water loss. Cell wall closest to the pore is thicker and less flexible. Have chloroplasts and carry out photosynthesis. Consequent changes in glucose concentration and osmotic potential allow water to enter and leave.When turgid, guard cells pull the thickened wall in, opening the stoma.
Xylem Pipeline carrying water from the roots to the leaves Long and tube-like hollow vessels.Long and tube-like hollow vessels. Cells have no end walls, so form a 'pipeline' carrying water from leaves to root. Spirals and rings of lignin strengthen the walls, to withstand pressure of water.
Phloem Sieve tubes carry food, away from the leaves Living cells with perforated end walls, hence the name sieve tubes.Have vertical strands of cytoplasm which carry glucose and other sugars dissolved in water to growing and storage areas of the plant.
Pollen grains (male gametes) Fertilise the ovules (female gametes) Tiny grain with half the genetic information having been formed by meiosis.Have a hard protective outer coat to survive bad conditions. Shape and surface of outer coat is adapted to method of dispersal - eg smooth and sticky for insect dispersal, larger surface area for wind dispersal. Germinate on reaching the stigma of another flower of the same species.
Cells and organisms
Some organisms consist of a single cell. Unicells are one-cell living things which have microscopic structures or organelles inside them including chloroplasts, mitochondria and a nucleus.
Multicellular organisms, such as ourselves, are made from very large numbers of specialised cells.
Groups of cells of the same type are called tissues. Groups of tissues work together as organs to perform particular functions within an organism. For example, millions of human heart muscle cells are grouped together to form heart muscle tissue, which is in turn organised into an organ - the heart.
The heart in its turn forms part of a functional system - the circulatory system.
Diffusion, osmosis and active transport
Substances move in and out of cells by means of passive or active transport.
Examples of passive transport are diffusion and osmosis (the diffusion of water).
Important substances like oxygen, glucose, water and mineral salts are needed and must be allowed to enter the cells and metabolic waste products like carbon dioxide and toxins must be allowed to leave the cells.
The entry and exit of all substances from the cells is controlled by the cell membrane, which is partially permeable
Diffusion
Particles in liquids and gases have kinetic energy. They move about, at speed, in all directions.
The particles move about randomly. In an area of high concentration, some of the particles collide with each other, lose energy and slow down. Others will escape from the concentrated area to places where there are fewer or no particles. Very few particles leave an area of low concentration to go to an area where the concentration is higher.
The result is a concentration gradient, with particles diffusing from an area of high concentration to an area of low concentration.
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Examples of diffusion
Here are some examples of diffusion across concentration gradients:
Place Particles move From To
Gut Digested food products Gut cavity Blood in capillary of villus
Leaf Oxygen Chloroplast Air spaces in mesophyll
Lungs Oxygen Alveolar air space Blood circulating around the lungs
Remember: particles continue to move from high to low concentration for as long as there is a concentration gradient.
In the lungs, the blood will continue to take in oxygen from the alveolar air spaces, provided that there is more oxygen in the air spaces than in the blood. The oxygen diffuses across the alveolar walls into the blood. The circulation takes the oxygen-rich blood away and replaces it with blood low in oxygen.
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In the exam you may be asked to give two examples of diffusion in organisms. Make sure you know an example from plants as well as from animals.
Osmosis
Some membranes in plant and animal cells allow certain particles to pass through them and not others. They are partially (or selectively) permeable. Osmosis is simply a special type of diffusion - diffusion of water molecules through a partially permeable membrane.
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In the animation, more water molecules pass from the water into the dilute solution than pass back the other way, because there is a higher concentration of water molecules in the pure water than there is in the solution. This results in a net transfer of molecules down the concentration gradient from the water to the solution. Eventually the level on the more concentrated side of the membrane will rise, while that on the less concentrated side falls.
When the concentration of water is the same on both sides of the membrane, the movement of water will be the same in both directions. At this point, the net exchange of water is zero and the system is in equilibrium.
Osmosis is vitally important to plants. Plants gain water by osmosis through their roots, and it is osmosis that moves water into plant cells, making them turgid or stiff, and thus able to hold the plant upright.
Active transport (Higher Tier)
Active transport is the process by which dissolved molecules (solutes) move across a cell membrane from a lower to a higher concentration. In active transport, particles move against the concentration gradient - and therefore require an input of energy from the cell.
Sometimes solutes are at a higher concentration inside the cell than outside, but because the organism needs these substances they still need to be absorbed. For this the organism cannot rely on diffusion or osmosis alone. Carrier proteins pick up specific molecules and take them through the cell membrane against the concentration gradient.
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Examples of active transport
In plants:
Plants need mineral salts (eg nitrates) for making proteins and growth. Nitrates are at a higher concentration inside the root cells than they are when dissolved in the water around the soil particles. If the plant relied on diffusion alone, the vital nitrate salts would drain out of the cells into the soil. So energy is deployed by the cells to actively transport nitrates across the cell membrane into the root cells, against the concentration gradient.
In humans:
Active transport takes place during digestion of food in the ileum (small intestine). After food has been absorbed by the villi for some time, the concentration of food molecules inside the villi increases, making it impossible for more food to diffuse into the villi. So simple sugars, amino acids, minerals and vitamins are actively absorbed into the villi, from an area of lower to an area of higher concentration.
Diffusion, osmosis and active transport compared
Diffusion Osmosis Active Transport
Random movement Random movement of water Selective movement
From higher to lower concentration From higher to lower concentration From lower to higher concentration
Along the concentration gradient Along the concentration gradient Against the concentration gradient
No energy needed from the cell No energy needed from the cell Energy needed from the cell
Remember:
· Molecules move in all directions in diffusion and osmosis.
· Osmosis is the diffusion of water.
· Diffusion continues even after the particles have spread out equally. Even when a system is in equilibrium movement of particles continues, with as many particles leaving as are arriving in any one place.
· In diffusion and osmosis molecules move by kinetic energy alone.
· In active transport, metabolic energy (energy made by respiration in the cells) is required.
Photosynthesis
Green plants are at the beginning of all food chains. Plants are known as 'producers' because only plants make their own food by building up carbohydrates, proteins and fats from simple inorganic chemicals.
Green plants need sunlight to produce food. They 'capture' the sun's light energy using the chlorophyll in their leaves and use it to make a sugar called glucose, which is either used in respiration or converted into starch and stored.
How photosynthesis works
Photosynthesis is the chemical change which happens in the leaves of green plants. It is the first step towards making food not just for plants, but ultimately for every animal on the planet as well. During this reaction, carbon dioxide and water are converted into glucose and oxygen. The reaction requires energy in the form of sunlight, and chlorophyll must also be present.
The glucose produced in the photosynthesis reaction can be converted to sucrose and carried to other parts of the plant in phloem vessels. Glucose can also be converted into starch and stored (the starch can later be turned back into glucose and used in respiration). Oxygen is a 'waste' product of photosynthesis.
Photosynthesis takes place in the mesophyll cells inside a green plant's leaves.
Make sure you know the parts labelled in this diagram.
As you can see there are two kinds of mesophyll cells - palisade mesophyll and spongy mesophyll. The mesophyll cells contain tiny bodies called chloroplasts which contain a green chemical called chlorophyll. Chlorophyll enables the light energy from sunlight to be converted into chemical energy for the photosynthesis reaction.
Palisade mesophyll cell
Conditions needed for photosynthesis
Photosynthesis needs:
· chlorophyll
· carbon dioxide (from the air)
· water (from the soil)
· sunlight energy (any light will do except green light)
Photosynthesis produces:
· glucose
· oxygen (a waste product)
Chlorophyll and light energy both need to be present for photosynthesis to take place, but they are not actually part of the reaction - they are not used up. So in the word equation for photosynthesis, remember to write them above the arrow, like this:
Chemical equation for photosynthesis (Higher Tier)
You need to understand how the carbon dioxide and water molecules are broken down and reassembled to form the glucose molecules. Like all chemical equations the photosynthesis equation is balanced - that is, there are the same number of carbon, hydrogen and oxygen atoms on either side of the equation.
Factors limiting photosynthesisThree factors limit photosynthesis from going any faster: Light level, carbon dioxide level, and temperature.· Without enough light a plant cannot photosynthesise very fast, even if there is plenty of water and carbon dioxide. Increasing the light intensity will make photosynthesis faster.· Sometimes photosynthesis is limited by the level of carbon dioxide. Even if there is plenty of light a plant cannot photosynthesise if it has run out of carbon dioxide.· Temperature can be a limiting factor too. If it gets too cold the rate of photosynthesis will slow right down; equally, plants cease to be able to photosynthesise if it gets too hot.If you plot the rate of photosynthesis against the levels of these three limiting factors you get graphs like the ones below. Maximising growthUnderstanding the factors that limit photosynthesis enables greenhouse farmers to maximise the conditions for plant growth. They often use paraffin lamps inside the greenhouse because burning paraffin produces carbon dioxide as well as heat, and so makes photosynthesis proceed faster. They may also use artificial light to enable photosynthesis to continue beyond daylight hours.
Photosynthesis and respiration
To unlock the energy in the carbohydrate produced in photosynthesis, green plants need to respire, just as animals do. Respiration takes place in the plant's cells, using oxygen to produce energy and giving off carbon dioxide as a waste product. So in terms of the gas taken in and the gas given out, respiration is the opposite of photosynthesis.
The result is that during the day when the plant is both respiring and photosynthesising there is a two-way traffic of oxygen and carbon dioxide both into and out of the plant. During the night when the plant is respiring but not photosynthesising, oxygen is being taken in but not given out - and carbon dioxide is being given out but not taken in.
Luckily, plants use up more carbon dioxide in photosynthesis than they produce in respiration, and produce more oxygen while photosynthesising than they use up while respiring - otherwise there would not be enough oxygen in the atmosphere for us animals to breathe!
Nutrition and digestion
A balanced diet contains carbohydrates, proteins, fats and fibre as well as vitamins and mineral salts. These substances must be present in the correct proportions.
Digestion is the breakdown of carbohydrates, proteins and fats into small soluble substances which can be absorbed into our blood.
Nutrition
A mixture of different types of food in the correct amounts is needed to maintain health. The main food types are:
Carbohydrates, found in potatoes, pasta, bread, bananas, sugar and rice. Carbohydrate is required by our bodies as a source of energy for other life processes. Sometimes referred to as starch, which is actually just one (very common) type of carbohydrate.
Fats, found in cheese, butter, margarine and oils. Fats are needed to make cell membranes and to insulate our bodies. They also contain important fat-soluble vitamins.
Protein, found in meat, fish, eggs and cheese. Proteins are required for growth and repair.
Fibre, found in wholemeal bread, fruit, vegetables and pulses. The fibre or roughage in our diet is not digested - but is important because it allows the muscles in our intestines to move the material along by peristalsis.
Digestive system: 1
The food we eat consists of large lumps of material. We must bite off small pieces and chew them up into even smaller ones before swallowing them. Once it gets to the stomach the food is further broken down by being pumelled by the stomach's muscular walls. This is physical digestion.
But the substances which our body needs cannot be absorbed into our blood until they have been broken down further - converted into small soluble chemicals. This is done with the aid of enzymes and other chemicals in our gut, and is called chemical digestion.
Food is moved through the digestive system by the contractions of two sets of muscles in the walls of the gut - one set running along the gut and the other set circling it. Their wave-like contractions create a squeeze moving down the gut. This movement is called peristalsis.
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What food is converted to:
· Carbohydrate is turned into glucose, which our bodies need to make energy.
· Protein is turned into amino acids, required for cell growth and repair.
· Fats and oils are turned into fatty acids and glycerol, needed to make cell membranes and to insulate our bodies. Fats also contain fat-soluble vitamins.
· Vitamins and mineral salts do not have to be digested because they are already small enough to get into our blood.
Digestive system: 2
In the exam you could be asked to label a diagram of the human digestive system, so make sure you know the names of all the parts!
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Mouth and stomach
Chemical digestion starts in the mouth, as enzymes in the saliva start to break down starch. Food is then moved to the stomach, where chemical digestion continues with the help of the hydrochloric acid and protease enzymes in the gastric juices.
Small intestine
The food is next moved into the small intestine, where enzymes produced in the intestine wall and pancreas continue the process of chemical digestion. Bile, produced in the liver, helps to breakdown fats.
The intestine is lined with tiny protuberances called villi, each in turn covered with even smaller microvilli. The villi have very thin walls and a plentiful blood supply to enable the products of digestion to be absorbed from the gut into the blood. There are many millions of them - providing a massive surface area to maximise the rate of absorption.
Large intestine
Whatever indigestible food is left now moves to the large intestine, where any excess water is absorbed before it is excreted from the anus.
Digestive enzymes
The enzymes in our digestive system break down complex substances into simpler ones which can be absorbed. Enzymes work best at their optimum pH - so if the stomach, for example, does not have enough acid, its enzyme, pepsin, will not work properly.
Amylase
The enzyme amylase is in the saliva, and starts to work as soon as we put food into our mouths. Amylase digests the long, complex starch molecules, cutting them up into the shorter, simpler molecules of the sugar maltose. The word equation is:
starch maltose.
Maltose however needs further digestion before it can be absorbed - as do the sugars sucrose and lactose. Another group of carbohydrase enzymes break these sugars down, turning them into the simple sugar glucose. The word equations are:
maltose glucose
sucrose glucose
lactose glucose
Protease
Protease enzymes (also called pepsin) are secreted in the stomach and pancreas, and digest proteins. Proteins are long chains of amino acids, and protease enzymes cut them up into peptide - smaller chains of amino acids molecules - and eventually into individual amino acids, which are absorbed in the small intestine. The word equation for the protease reaction is:
proteins amino acids
Lipase
Lipase is secreted in the pancreas and the walls of the small intestine. It is the enzyme which digests lipids - ie, fats and oils. Lipids are complex molecules made up of fatty acids and glycerol. Lipase cuts lipids up into fatty acid molecules and glycerol molecules. The word equation for this reaction is:
Lipids fatty acids + glycerol
You need to remember the main digestive enzymes and the food types they break down. Test yourself by dragging the correct enzyme onto each food type, and see them cut up the complex molecules into simpler ones...
More about enzymes
Enzymes are proteins. They are very important substances because they control the chemical reactions that happen in our bodies. They are known as biological catalysts - substances which speed up reactions but which do not get used up themselves. Enzyme names usually end in the letters -ase, as in amylase, protease and lipase.
There are two main types of enzyme. Digestive enzymes are extracellular enzymes - they control reactions that take place outside cells. Those enzymes which control reactions inside cells are called (not surprisingly!) intracellular enzymes.
Enzymes intervene in chemical reactions by locking onto one of the reactants and speeding up the reaction. The chemical which the enzyme locks onto is called the substrate, and the enzyme has a kind of chemical sensor, called an active site, which helps it to recognise the substrate. Just like a key only fits into a specific lock, each enzyme has its own specific substrate. Once the reaction is complete and the required product has been produced, the enzyme releases itself and moves on to the next reaction.
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Effect of temperature on enzymes
Temperature is important in all reactions. As the temperature increases, so does the rate of reaction. This is because heat energy causes more collisions between the particles in the enzyme and particles in the substrate. However, very high temperatures damage or denature enzymes.
If you plot the rate of an enzyme reaction against temperature, the rate gradually increases with temperature until it reaches around 37°C or body temperature. Then as the temperature continues to rise the rate of reaction falls rapidly as the heat energy begins to denature the enzyme.
The effect of pH on enzymes
Different enzymes work best at different pH values. The optimum pH for an enzyme depends on its site of action. For example, enzymes in the stomach have an optimum pH of about 2 because the stomach is acid, but intestinal enzymes have an optimum pH of about 7.5.
A graph plotting reaction rate against pH for any enzyme looks like this:
Other substances that help digestion
Hydrochloric acid
The enzyme pepsin requires the presence of hydrochloric acid to create the right pH conditions for the enzyme to work effectively. Pepsin works best at pH2 (acidic) - which is also acidic enough to kill the bacteria taken in with food.
The hydrochloric acid is secreted, along with the pepsin, from tiny pits in the stomach lining. The stomach lining protects itself against digestion and corrosion by secreting sticky neutralising mucus.
Bile
Fats and oils are broken down by the enzyme lipase; but before lipase can really get to work the lipids first need to be emulsified. This is done by bile, a substance secreted in the liver and stored in the gall bladder, which is added to food via the bile duct when food is passing through the duodenum. Bile is not an enzyme.
Bile works like detergent - it disperses fats into droplets (an emulsion) and so enlarges the surface area so that the enzyme lipase can get to work breaking down the fat much more efficiently. Bile salts also neutralise the stomach acid and help to create optimum pH conditions for digestive enzymes in the small intestine.
Food tests
We eat many complex foods which contain mixtures of carbohydrates, fats and proteins. Food tests enable you to find out what food types a food contains.
For fats the test is simply to squash a sample of food onto a piece of paper and leave it to dry. A positive test for fat is a translucent stain around the food sample when you hold the paper up to the light.
For the other food types, first prepare a sample of food for testing:
1. Crush some food in a pestle and mortar
2. Add a spatula-full to a boiling tube
3. Add 5cm3 of distilled water and stir
4. Bring to boil and simmer for 1 minute
5. Cool and add the test reagent
You need to know the different tests for starch, sugars and protein.
Test for Reagent Positive test
Carbohydrates - starch Dilute iodine Turns blue-black
Carbohydrates - glucose Benedict's solution Orange/red precipitate
Carbohydrates - sucrose A few drops of dilute HCl acid + a few drops of Benedict's solution Orange/red precipitate
Test for Reagent Positive test
Protein Biuret test - add a few drops of dilute copper sulphate solution, followed by a few drops of sodium hydroxide Purple or violet precipitate
Fat Rub a food sample onto a piece of paper. Leave to dry. Translucent stain round the sample when held up to the light
Inhaling and exhaling
When we inhale...
· the intercostal muscles contract, expanding the ribcage
· the diaphragm contracts, pulling downwards. The net result is that ...
· the thorax (top part of your body) increases in volume, lowering the pressure inside it and sucking air into the lungs.
When we exhale ...
· The intercostal muscles relax, allowing the ribcage to drop inwards and downwards, and ...
· the diaphragm relaxes, moving back upwards. The result is that ...
· the thorax decreases in volume, increasing the pressure inside it and forcing air out.
The diagram shows the parts of the thorax (chest) involved in breathing.
As it is inhaled, the air travels down the trachea and through one of the two bronchi into one of the lungs. It then passes into the many branching bronchioles, finally arriving into some of the millions of tiny sacs called alveoli.
This is where gas exchange takes place - oxygen passing out of the air into the blood , and carbon dioxide passing out of the blood into the air in the alveoli.
Gas exchangeGas exchange is the means by which we get oxygen from the air into our blood and carbon dioxide out of our blood into the air. Gas exchange happens by diffusion of gases across the very thin walls of the alveoli. In order to maximise the amount of diffusion taking place, the alveoli have a huge total surface area - between 70m2 and 90m2!The alveoli have a moist lining to help dissolve the gases, and are surrounded by many tiny capillaries so there is plenty of blood for the gases to pass into and out of. To view this piece of media, you will need an up-to-date version of the Macromedia Flash Plug-in. You can quickly download it from here As a result of gas exchange, the proportion of oxygen and carbon dioxide in the inhaled/exhaled air changes - with the air we breathe in containing a higher percentage of oxygen and a lower percentage of carbon dioxide than the air we breathe out. (Nitrogen, the main component of air, remains unaffected by breathing and stays at the same percentage.) The table shows the percentage change of oxygen and carbon dioxide between inhaled and exhaled air.Gas % in inhaled air % in exhaled air
Oxygen 21 17
Carbon dioxide 0.04 4
Nitrogen 78 78
When we are taking exercise we breathe much faster than when we are resting. This is because when they are working hard the muscles need more energy, so they must respire faster. This means they need more oxygen and produce more carbon dioxide (the waste product of respiration) - so the breathing rate must go up.
Exam tips
You should be able to:
· Label a diagram of the thorax.
· Describe how we inhale and exhale air (ventilate our lungs).
· Explain what gas exchange is, and by what method it happens.
· Say that the lung has a very large surface area to speed up gaseous exchange.
· Explain why there is less oxygen in alveolar air than in atmospheric air.
Respiration
Respiration takes place in all living things, all the time. It is the release of energy from glucose or other organic substances inside living cells. Every cell needs to respire in order to produce the energy it needs.
How respiration makes energy
Respiration is the release of energy from glucose or other organic substances. Energy is required for growth, repair, movement and other metabolic activities.
There are two main types of respiration, aerobic and anaerobic. Aerobic respiration takes place in the presence of oxygen. The animation shows how glucose molecules react with oxygen molecules to form carbon dioxide and water molecules, with energy being released by the breaking of bonds in the glucose molecules.
To view this piece of media, you will need an up-to-date version of the Macromedia Flash Plug-in. You can quickly download it from here
The energy released from glucose in respiration is used to produce a chemical called adenosine triphosphate (ATP). ATP is where the energy released during respiration is stored for future use.
Aerobic and anaerobic respiration
In aerobic respiration glucose reacts with oxygen in the mitochondria of the cells to release energy. Carbon dioxide and water are by-products of the reaction. You need to learn the word and symbol equation for this:
To view this piece of media, you will need an up-to-date version of the Macromedia Flash Plug-in. You can quickly download it from here
As you can see, a lot of energy is released in aerobic respiration - 2900 kj from one glucose and 6 oxygen molecules.
Anaerobic respiration occurs when oxygen is not available. In anaerobic respiration the glucose is only partially broken down, and lactic acid is produced - together with a much smaller amount of energy. Here is the word and symbol equation for anaerobic respiration:
When anaerobic respiration occurs in yeast it is called fermentation. In this case ethanol (alcohol) is produced instead of lactic acid, and this reaction is used in the brewing of alcoholic drinks. The word equation is:
glucose ethanol + carbon dioxide + energy
Oxygen debt
During vigorous exercise the body needs a lot more energy. It gets this by breathing in deeper and faster and rushing the oxygen to the muscles in dilated blood vessels. This extra oxygen is then used to release more energy, needed to meet the higher level of demand. Soon a point is reached when the body cannot breathe any faster or harder, and aerobic respiration alone cannot meet the enhanced energy demands. So how do muscle cells get the extra energy they need? They get it by respiring anaerobically.
But anaerobic respiration produces lactic acid, which accumulates in the muscles and causes muscle fatigue and cramps. To avoid damage to cells, lactic acid has to be broken down to carbon dioxide and water immediately the exercise has finished. This is an oxidisation reaction, and requires oxygen.
Running the 1500 metres will build up an oxygen debt
This extra oxygen needed to neutralise the harmful effects of anaerobic respiration is called an oxygen debt. In order to get the extra oxygen to 'pay back' the debt, the body continues to breathe deeply for some time after vigorous activity has ceased. When all the lactic acid in the muscles is broken down the oxygen debt has been repaid and normal aerobic respiration resumes.
One measure of a person's fitness is how quickly their breathing and pulse return to normal after exercise. This is because in a fit person aerobic respiration is more efficient, so they build up less of an oxygen debt while exercising, and need less extra oxygen to breakdown any lactic acid in their muscles resulting from anaerobic respiration.
Exam tips
You need to be able to:
· Explain that respiration releases energy from glucose.
· Explain the difference between aerobic and anaerobic respiration.
· Write word equations for aerobic and anaerobic respiration.
· Write a balanced chemical equation for aerobic respiration.
· List the uses of energy in animals
Habitats and populations
Biology is the study of living things; the study of living things within their environment is called ecology. It's important to study organisms in the context of their environment, because living things do not live independently of their surroundings or each other. They interact continually with their environment and with other living things. They are interdependent.
Ecological termsIn order to get on top of this topic, you need to be clear about what each of the following terms mean:environment means all the conditions that surround any living organism - both the other living things and the non-living things or physical surroundings
a habitat means a place where plants and animals live (eg a pond)
a population means all the members of a single species that live in a habitat (eg minnows)
a community of living things means all the populations of different organisms living together in a habitat
an ecosystem refers to a community of animals, plants and micro-organisms, together with the habitat where they live
This pond ecosystem consists of a pond habitat, inhabited by populations of aquatic plants, waterside plants, micro-organisms (in the mud at the bottom of the pond), minnows and herons. The organisms together make up a community of living things.
Distribution of organisms
Living organisms are not evenly distributed around the world, but are adapted to live in particular habitats.
A number of factors affect the suitability of any habitat for the organisms that live there. The most important of these is climate, which determines the temperature range and rainfall levels in a given habitat. Warm temperatures and plentiful water make the tropical rainforest a rich environment which is home to a huge variety of species. Deserts on the other hand are sparsely populated with few species, because of the extremes of heat and cold and lack of water - conditions which few organisms are adapted to survive in.
We can group together ecosystems adapted to similar climate conditions into global ecological regions called biomes, each of which supports its own typical range of plant and animal species. The map above shows the major biomes of the world, while the pyramid below shows the relationship between biomes and climate.
Ecosystems
An ecosystem consists of a community of organisms and the non-living factors that influence them. The living and non-living elements interact to support and maintain a balance between plant and animal communities within the ecosystem.
The biosphere as seen from space
It is possible to think of the whole earth as an ecosystem. The biosphere includes all the earth's landmasses, oceans and atmosphere, and all the animals, plants, insects, birds and micro-organisms living in them. But because the biosphere is so large and all-encompassing and its relationships so complex, we normally study smaller ecosystems.
As well as the vast global regions known as biomes there are two other categories of ecosystem:
Meso ecosystems are subdivisions of biomes, and occupy medium-sized habitats such as freshwater ponds, hedgerows, woodlands, sand dunes, and coral reefs . (Meso means 'middle-sized').
Micro ecosystems, also known as microclimates, are subdivisions of meso systems. They inhabit very small areas such as a water droplet, a single leaf, or the cool, damp space under a log - where the conditions may be quite different from the habitat as a whole but just right for the organisms that live there.
A coral reef is an example of a meso ecosystem
Environmental influences
Ecosystems are shaped by two types of environmental influence.
Abiotic factors are the effect of the the material, non-living environment - things like:
· light intensity
· temperature range
· rainfall levels and rate of water flow
· water content of soil
· humus content of soil
· soil pH and nutrient levels
· dissolved oxygen levels, and
· pollutant concentrations
Biotic factors are the effect of all the living things in the environment - for example:
· which producers (green plants) are present
· which consumers and predators are present
· the number and type of competitors for light, food, space, shelter, or mating opportunities
· the pathogens and parasites which are present
· the number and type of insect species present
· the number and type of decomposers, and
· the level of species diversity
All ecosystems depend on two important processes: The flow of energy, and the recycling of nutrients. These processes are covered in the Revision bite on Energy and nutrient transfers.
Example of an ecosystem
The diagram shows a oak tree ecosystem. Note that different living things inhabit different zones or layers from the bottom to the top:
· First there is the roots, soil and leaf-litter zone beneath the tree. Here decomposers such as bacteria, woodlice, and earthworms feed off last year's leaves and acorns, and fungi grow on its roots
· Next is the trunk layer, which provides shelter or food to insects, caterpillars and larvae.
· Finally comes the branches, leaves and canopy. In this zone bees gather pollen and nectar, fungi grow on the leaves, gall wasps and moths lay their eggs, and squirrels gather acorns. Small birds such as bluetits hunt the moth larvae; and sparrowhawks hunt the small birds.
An oak tree ecosystem. Each zone of the tree is home to a distinctive community of organisms
Each of the organisms in this ecosystem has a particular way of fitting into the oak tree environment - they each occupy a niche within the ecosystem. For example the blue tits and the squirrels, though they both inhabit the same tree, do not directly compete for food: the squirrels feed on acorns, while the tits feed on moth larvae. The two species occupy different niches within the oak ecosystem.
Adaptation
The competition between organisms means that those who are best adapted are most likely to survive and reproduce. Natural selection means that over many generations organisms become progressively better adapted to their environment. (For more on natural selection, see the Revision bite on Evolution.)
Adaptation in extreme habitats: three examples:
Camels are well adapted for survival in the desert. They have:
· the ability to store a lot of water, and to lose very little via urination and sweating
· the ability to tolerate body temperatures up to 42 degrees C
· a large surface area / volume ratio - maximising heat loss
· a hump which can store scarce food as fat without insulating the body
· thick fur on the top of the body to provide shade, and thin fur on the rest of the body to allow easy heat loss, and
· large, flat feet well-suited for walking on sand
Polar bears are well adapted for survival in the arctic. They have:
· a thick layer of fur for insulation against the cold
· a thick layer of blubber for further insulation, and also as a food store
· a small surface area / volume ratio, to minimise heat loss
· a greasy coat, which sheds water after swimming
· large feet to spread the load on snow and ice, and
· the ability to move fast on both land and water, while pursuing their prey
Cacti are well adapted for survival in the desert. They have:
· spines instead of leaves, which minimise surface area and therefore evaporation, and also...
· protect them from animals which might eat them
· stems which can store water, and
· widespread root systems, which can collect water from a large area
Competition and co-operation
Competition
Habitats have finite amounts of the resources needed by living organisms, such as food, water and space, and all organisms strive to reproduce themselves and increase their numbers. Sooner or later the demand for these resources is going to exceed supply, and organisms have to compete with each other to get them.
Plants typically compete with each other for:
· light (for photosynthesis)
· water, and
· nutrients (minerals)
Animals typically compete with each other for:
· food
· water
· mates (so they can reproduce), and
· living space
It is this competition between organisms that enables natural selection to take place, by favouring chance mutations conferring a slight advantage in the race for limited resources. For more on this point take a look at the Revise bite on Evolution.
Co-operation
Some organisms find that they are better able to survive and reproduce by living closely together with another organism of a different species. This type of interpendence between organisms is known as symbiosis.
Some examples of different styles of symbiosis are:
Barnacles which attach to a whale or scallop shell. The barnacles get a home and transport, and the whale or scallop is not unduly affected. This type of symbiosis, where one organism benefits and the other suffers no harm, is called commensalism.
Lichens are formed by algae and fungi living together. Algae can photosynthesise and make food which is shared by the fungus. The fungus in turn shelters the algae from a harsh climate. This kind of mutually beneficial co-operative relationship is called mutualism.
A tapeworm lives inside another animal, attaching itself to the host's gut and absorbing its host's food. The host loses nutrition, and may develop weight loss, diarrhoea and vomiting. This kind of one-sided symbiosis is called parasitism. Usually parasites do not kill the host before they move on, as this would cut off their food supply.
Interdependence of organisms
All living things within an ecosystem are interdependent. A change in the size of one population affects all other organisms within the ecosystem. This is shown particularly clearly by the relationship between predator and prey populations.
There is a continuous tussle between predators and their prey. Predator species need to be adapted for efficient hunting if they are to catch enough food to survive. Prey species on the other hand must be well adapted to escape their predators if enough of them are to survive for the species to continue.
If the prey population in an ecosystem grows, predator numbers will respond to the increased food supply by increasing as well. Growing predator numbers will eventually reduce the food supply to the point where it can no longer sustain the predator population... and so on.
Case study 1: Ladybirds and aphids
The animation shows this predator/prey dynamic between a population of ladybirds (predators) and a population of aphids (their prey). The graph showing the relationship between them will look fairly similar for any two populations of predators and prey.
Case study 2: the Canadian lynx and Snowshoe hare
Another example of the predator / prey dynamic is the rise and fall in numbers of Canadian lynx and its favourite prey the Snowshoe hare. The two populations were estimated each year for some 75 years from the number of animals caught by fur traders. The lynx population was found to rise and fall in a 10-year cycle, with that of the hare following 2 years behind. No other cat is so dependent on a single prey species, which is why there is such a clear pattern of interdependence between the two populations.
Feeding relationships
A very important aspect of the interdependence of organisms in an ecosystem is their feeding relationship with each other. Food chains and food webs are simply ways of describing these relationships. Together with pyramids of number and biomass, they help us to understand the movement of biological material (biomass) and of energy through an ecosystem.
Feeding relationships
Living organisms in an ecosystem depend on each other for food and essential nutrients such as carbon and nitrogen. This interdependence often takes the form of a feeding relationship - ie, they eat each other!
Make sure you understand the meaning of the following terms:
Producers ie, green plants. All food chains start with them, because they can make food by photosynthesis.
Primary consumers feed primarily on plant material. They are herbivores - eg rabbits, caterpillars, cows, sheep, and deer.
Secondary consumers feed primarily on animal material. They are carnivores - eg cats, dogs and lions.
Omnivores eat both plants and animals - eg bears and humans.
Predators kill for food. They are either secondary or tertiary consumers - eg polar bears, golden eagles.
Prey are the organisms that predators feed on. Examples of predator and prey species are: fox and rabbit; blue tit and caterpillar; wolf and lamb.
Scavengers feed on dead animals . They perform a useful cleaning-up function. Examples are crow, vulture, buzzard and hyena.
Decomposers feed on dead and decaying organisms and on the undigested parts of plant and animal matter in the faeces. (They do not 'eat' the food like scavengers, as they have no mouth-parts. Instead they break down solid matter into liquids which they can absorb.) Examples: bacteria and some fungi.
Food chains
A food chain shows who eats what in a particular habitat. For example: grass seed is eaten by a vole, which is eaten by a barn owl. The arrows between each item in the chain always point in the direction of energy flow - in other words, from the food to the feeder.
All food ultimately comes from green plants or producers. The other organisms in the food chain are consumers, because they all get their energy and biomass by consuming (eating) other organisms.
All food chains are pretty short. There are never more than four steps, because a lot of energy is lost at each step, and after three steps most of the available energy has been expended. This also explains why the organisms at the top of food chains (eg owls) are very small in number compared with those lower down (eg grass plants). After 2 steps there is simply not enough available energy to support more than a few top predators.
For more on how energy moves through food chains, have a look at Energy and nutrient transfers.
Food webs
In its natural habitat it is unusual for an animal to eat only one particular organism, so a more realistic way of showing feeding relationships is to draw a series of interconnecting food chains. This is called a food web.
The food web below describes feeding relationships in a freshwater pond ecosystem. It allows you to follow the routes that biomass (and energy) take through the system.
Pondweed (a producer) is eaten by the mayfly nymphs, which are in turn fed upon by both the dragonfly nymphs and the brown trout. The brown trout make a meal of the dragonfly nymphs too.
The other producer in this web is the microscopic algae. This is eaten by the freshwater shrimp. The shrimps are fed upon by dragon fly nymphs and brown trout (which also eat the dragonfly nymphs).
Pyramids of number and biomass
Food chains and webs show the flow of materials and energy in habitats, but they do not give you any idea of how many organisms there are in the habitat. To show this you need to draw a pyramid of numbers.
Let's go back to the grass vole barn owl food chain. Suppose the numbers found in a particular habitat are as follows:
2000 grass plants
25 voles
1 barn owl
The pyramid of numbers would be as shown below. The wide base represents the large number of grass plants. Above this is a narrower section representing the much smaller number of voles, while the top section is one unit wide and represents a single barn owl.
Pyramids of numbers will often be pyramid-shaped like this - but not always. Look at the next one.
If the producer is a large plant such as an oak tree, the second layer of the pyramid representing primary consumers (caterpillars in this example) will be much larger than the base. In this case it would make more sense to draw a pyramid of biomass, which shows not the numbers of organisms at each level, but the amount of biological material.
A pyramid of biomass for the oak tree would look like this:
Pyramids of biomass are always pyramid-shaped.
Exam tips
Don't worry if you have never heard of the organisms used in a food chain and web question. There will always be enough information for you to work out the answer.
When looking at a food web, check carefully which way the arrows are pointing. Remember the arrows represent a flow of materials or energy, so the arrow should go from the organism which is being eaten to the animal which is eating it.
In questions asking you to suggest population changes in food webs, look carefully at the links between the organisms and try to work out what might happen.
Energy and nutrient transfers
All living organisms require energy. The ultimate source of all this energy is the sun. Solar energy is trapped by plants, and then transferred from organism to organism in a food chain. At each stage in this transfer much of the energy is lost to the environment.
Living organisms also require nutrient elements - in particular carbon and nitrogen - which they take from the environment. If this were just a one-way process, ecosystems would soon run out of these nutrients; but in fact they are returned to the environment, with the help of bacteria, via nutrient cycles
Energy transfer
In every ecosystem, energy is transferred along food chains from one trophic level to the next. But not all the energy available to organisms at one trophic level can be absorbed by organisms at the next one: in fact the amount of available energy decreases dramatically at each level. Why?
Some of the available energy goes into growth in biomass and the production of offspring: this energy does become available to the next trophic level. However most of the available energy is used up in other ways:
· Some is used up at the first trophic level as a result of photosynthesis, which uses up lots of solar energy in making glucose.
· Some is used up in respiration, and given off as heat
· Some is lost, in the form of biological material and heat, through excretion. (This energy is actually transferred to the decomposer food chain: more about this later!)
· Some is used for movement and transport.
All the energy used in these ways returns to the environment, and is not available to the next trophic level. The animation shows how the level of available energy declines as it is transferred through a temperate forest foodchain
You can see why it is that food chains almost never have more than four steps: so much energy is lost at each level that however much you start off with it is almost all gone by the fourth trophic level.
Calculating energy efficiency
This bullock has eaten 100 kJ of stored energy in the form of grass, and excreted 63 kJ in the form of faeces, urine and gas. The energy stored in its body tissues is 4 kJ. So how much has been used up in respiration?
Energy balance in a bullock
Now energy cannot just disappear - it must all be accounted for. So the total amount of energy used up by the bullock must equal the total taken in as food. Using this fact we can easily work out how much energy the bullock has used up in respiration.
total energy input = 100 kJ
total energy used = 63kJ + 4kJ + respiration
energy used in respiration = 100 - 63 + 4
energy used in respiration = 33kJ
We can also work out the energy efficiency at each trophic level by dividing the useful energy output by the total energy input. Multiplying this fraction by 100 gives you the percentage efficiency. "Useful" means "available to the next trophic level"; so the calculation for the bullock goes like this:
useful energy output = 4kJ
total energy input = 100kJ
energy efficiency = or 0.04
x 100 to get % = 4%
Meat-eating and energy efficiency
In fact the amount of energy available to someone eating this bullock would be even less than 4kJ, because many parts of the animal - eg brain, bones, hooves, large blood vessels and skin - are not eaten. Obviously eating beef is not a very efficient means of gaining energy!
In fact it is far more efficient for humans to eat cereals such as wheat and barley themselves, rather than using them to feed cattle and then slaughter the cattle for beef.
This is because the energy in the beef has already passed through two trophic levels by the time it gets to humans, and at each level the amount of available energy is reduced. When we eat cereals (or vegetables or fruit) the energy has only passed through one trophic level, and there has only been one set of energy losses instead of two.
It follows that far more food can be produced from a given area of land if plants are fed to people, not to farm animals. It has been calculated that a meat-based diet requires approximately 7 times more land than a plant-based diet.
Carbon and the carbon cycle
The element carbon is a basic constituent of all living organisms. Its atoms combine easily with other atoms to form a huge variety of molecules. Some of these (eg carbon dioxide, carbohydrates) have names which make it obvious they are carbon-based, while others (eg fat, proteins) you just have to remember.
All cells - whether animal, plant or bacteria - contain carbon because they all contain proteins, fats and carbohydrates. Plant cell walls for example are made of cellulose, a carbohydrate.
Living organisms need carbon in order to:
· Make food. Green plants get their carbon from the carbon dioxide in the air, which enters the leaves and is used for photosynthesis. A product of photosynthesis is glucose - another carbon-based compound.
· Make energy. In respiration glucose reacts with oxygen to produce energy (with carbon dioxide as a by-product).
· Make new cells for growth and repair. Carbon compounds are essential cellular building-blocks.
The carbon cycle
Carbon cycles through ecosystems, moving repeatedly from one organism to another, and between organisms and the environment. The carbon cycle is a key factor in maintaining the balance of an ecosystem. It works like this:
1. Plants photosynthesise, taking carbon in the form of carbon dioxide from the atmosphere and locking it into the carbohydrate glucose. (They also respire, giving out carbon dioxide; but they take in much more than they give out.)
2. Animals get their carbon from eating either plants (carbohydrates) or other animals (proteins and fats) which they then digest. They respire, giving off carbon dioxide to the environment.
3. Waste carbon-based material is excreted by animals, and is digested by decomposers - mainly microbes and fungi. The decomposers also respire, releasing carbon dioxide.
4. When animals die, their remains may be either eaten by scavengers (for example, crows) or digested by decomposers. Both scavengers and decomposers respire, giving off more carbon dioxide.
5. In certain conditions both animal and plant remains may become fossilised, eventually forming carbon-based fossil fuels (coal, oil and gas).
6. Both fossil fuels and plant material (wood) may later be combusted - releasing still more carbon dioxide to the environment.
The animation should help you understand how the cycle works.
The nitrogen cycle (Higher Tier)
79% of the air around us is nitrogen . Living things need nitrogen to make proteins, but they cannot get it directly from the air because nitrogen gas is too stable to react inside an organism to make new compounds.
So nitrogen must be changed into a more reactive form to allow plants and animals to use it. Plants can take up and use nitrogen when it is in the form of nitrates or ammonium salts. Changing nitrogen into a more reactive substance is called nitrogen fixation.
Nitrogen fixation happens in three different ways:
1. The energy in a lightning bolt can split the di-atomic nitrogen molecule in the air allowing each nitrogen atom to react with oxygen to form nitrogen oxides. These oxides are washed to the ground by the rain where they form nitrates.
2. The Haber process is used by industry to produce ammonia from nitrogen. Ammonia is used to make fertiliser for farmers to feed their crops.
3. Nitrogen-fixing bacteria found in the soil and in the root nodules of leguminous plants fix nitrogen into a usable form.
Nitrogen compounds are returned to the soil by excretion and egestion from animals or when plants and animals die and decay. The nitrogen compounds returned in this way are changed back to nitrogen gas by denitrifying bacteria which live in the soil - thus completing the cycle.
The impact of humans
All living things have an effect on the environment, because organisms and their ecosystems are interdependent. But the impact of humans is greater than that of any other species, because
1 there are an awful lot of us, inhabiting the entire planet, and
2 distinctively human activities such as farming, fishing, and industry have a much bigger effect on the biosphere than the activities of other species.
The threat to biodiversity
The world is an astonishingly diverse place, inhabited by millions of different plant and animal species, with probably millions more that have not yet been discovered.
The term biodiversity refers not only to the sheer number of different species, but to all the genetic variations within and between species - and all the differences between the many, many habitats and ecosystems that make up the earth's biosphere.
Why is it important to maintain the huge variety of life? Some answers are that :
· we have a moral responsibility to look after the planet and its resources, rather than simply use them up
· biodiversity provides us with many direct benefits - eg clean air and water, food, medicines, fertile soil, and pollination for our crops
· biodiversity represents the earth's total gene pool - a source of future variation which is vital for all species
· reduction in biodiversity in ecosystems may reduce climatic stability, for example by upsetting the balance between intake and output of carbon dioxide
Impact of humans
By far the biggest threat to the variety of life is posed by human exploitation of the environment. Our impact on the global environment is greater than that of any other species because of:
Our technologies: we use tools and techniques which can change the shape of the earth in a short space of time (eg clearing forests or changing the course of rivers)
Our population is increasing at a phenomenal rate. There are 6 billion of us now and by 2050 we are expected to number around 9 billion. Our sheer numbers will mean that even small activities multiplied 9 billion times will have a huge impact on the environment
Consumption and waste: we consume vast amounts of natural resources (eg water, fossil fuels) and produce vast amounts of waste (eg greenhouse gases). Both pose a threat to other forms of life - as well as to ourselves!
Human population growth
Like all living things, humans exploit their surroundings for resources. Before the beginning of agriculture about 10,000 years ago, small groups of humans wandered across large areas hunting and gathering just enough to keep alive. Population numbers were kept low by the scarcity of food.
The invention of agriculture began a population explosion which has accelerated enormously in the last 500 years. Unlike other species, humans can adapt to and survive in almost all habitats and climates, from the cold Arctic regions to dry, hot deserts. Currently around 6 billion and rising fast, human population growth now poses a threat to the global environment.
Human population growth over the last 10,000 years
In the United States and many European countries, changing work patterns, universal education and health care, and family planning have all helped to halt population growth. But in developing countries, like those of India and Africa, population numbers continue to rise fast.
Growth-limiting factors
In every ecosystem and every species, population sizes have limits beyond which they cannot safely expand. They are constrained by growth-limiting factors which not even humans can easily control. These are:
· the availability of food and water
· invasion of parasites, pathogens or disease
· over-crowding (increasing competition for food, water and space)
· severe or sudden climatic changes
· pollution of air, soil and water
If we do not take steps to control population growth ourselves, it is likely that one or more of these growth-limiting factors will eventually kick in to forcibly reduce our numbers.
Urbanisation and industrialisation
Urbanisation means the growth of cities. Around 3 billion of us - half the world's population - now live in cities, and virtually all future population growth will take place in cities. Moreover our cities are getting bigger and bigger: by 2015 it is predicted that the world's 6 largest cities will each have more than 20 million inhabitants.
Some of the effects of urbanisation are:
· greatly increased pollution as a result of urban traffic, energy consumption and waste production
· land taken out of food production, as more and more farmland is built on
· loss of natural habitats, as cities and roads proliferate
· fragmentation of rural communities and cultures, as more and more people leave to live in towns
Industrialisation
Closely linked with population growth and the rise of cities has been a worldwide rapid development of industries such as manufacturing, mining, power generation and transport. These have a major impact on the environment, because:
· they use up huge amounts of water and energy (usually non-renewable fossil-fuel energy), and
· they are a major agent of climate change, because they release greenhouse gases into the atmosphere and speed up global warming
Impact of farming: 1Farming is what makes possible the production of food surpluses and settled living. It also brings about big changes in the relationships between living things and in their habitats. Farming - especially modern, intensive farming - can damage the environment in many different ways.Effect of fertilisersFertilisers containing plant nutrients are sprayed onto fields to make plants grow faster and boost crop yields. When it rains the nutrients may get washed down from the fields and into rivers and lakes (this is called run-off). The result is eutrophication - which can kill almost everything living in the aquatic environment. It works like this:
Impact of farming: 2
Effect of pesticides
Pesticides are chemicals used to kill insects, weeds and micro-organisms that might damage crops. However, pesticides damage other organisms apart from those they are intended to kill - for example, depriving insect-eating birds of food.
Pesticides can also enter local food chains. Organisms that ingest them cannot break them down, so they persist in their bodies. (Substances that cannot be broken down are called persistent substances: the pesticide DDT is an example.) The pesticides may then build up at ever-higher levels until they become toxic to much larger organisms. Here's how it works:
Other impacts of farming
Agriculture can impact on the environment in many other ways. For example:
· farming takes up land, reducing habitats and wildlife
· monocultures (large amounts of one type of food) provide lots of food for pests as well as humans
· irrigation (watering of crops) may take too much water from rivers, depriving downstream habitats of water
· clearing land for farming may result in soil erosion, damaging ecosystems and leaving land barren
· Intensive livestock farming produces huge amount of faeces, which may pollute waterways
Fishing and forestry
Fish are an important part of the human diet, accounting for a worldwide average 15% of humans' protein intake. Most of these fish are caught wild, and if fish are caught at a faster rate than the remaining fish can reproduce, the stock of fish will obviously decline. Trying to harvest more fish than the sea can produce is an example of unsustainability.
Since the 1960s North Sea cod have been overfished, as more and more (and bigger) fishing boats caught more and more cod. At first, catches continued to increase each year. But then - surprise, surprise! - they started to decline, as there were not enough breeding fish left to maintain the cod population. Today, North Sea cod are in danger of extinction.
Forestry
Humans have now been cutting down trees for around 10,000 years - for wood to burn or build with, or to clear land for farming.
Forestry is sustainable as long as forests are allowed to replace themselves, or are replanted after harvesting - but often this is not done. The result is that the area of forest is steadily shrinking - a process called deforestation - with profound consequences for ecosystems and biodiversity. Deforestation:
· destroys forest habitats, endangering many forest-dwelling species
· causes soil erosion, as the soil-stabilising effect of tree roots is removed. Barren land and flooding can result
· causes atmospheric pollution (mostly carbon dioxide) as forests are cleared by burning trees
· reduces the amount of photosynthesising vegetation, thus further increasing levels of carbon dioxide in the atmosphere
The maps give an idea of how much of the earth's forest has been lost in the last 10,000 years.
Particularly worrying is the rapid destruction of tropical rainforest. This type of forest is often referred to as 'the earth's lungs' because it produces around 40% of the world's atmospheric oxygen. It is also home to an estimated 50% of all species on earth (once lost, the original habitats can never be replaced). Tropical rainforest is being cut down at the rate of 17 - 20 million hectares a year.
Pollution
Pollution is the addition of substances to the environment that may be harmful to living organisms.
Atmospheric pollution
Atmospheric pollution is caused either by the burning of wood and fossil fuels (coal, gas and oil); emissions from petrol or diesel engines; discharges from factories, power stations, and waste dumping; and from agricultural livestock. The pollutants involved include:
· Smoke, which damages air quality and deposits soot on surfaces such as tree trunks and leaves
· Carbon dioxide, the main greenhouse gas causing climate change
· Methane, the second most important greenhouse gas causing climate change
· Sulphur dioxide and nitrogen dioxide, which mix with rainwater to form acid rain. Acid rain corrodes buildings and damages trees and plants.
· Carbon monoxide, which is poisonous to humans and animals (it reacts with haemoglobin and prevents it carrying oxygen)
· Dust (produced by activities such as quarrying and mining) which may impair plants' ability to photosynthesise and people's ability to breathe
Water pollution
Water pollution is caused by the discharge of harmful substances into the sea, rivers, lakes or the water table. Pollutants include:
· Agrochemicals such as fertilisers and pesticides which damage aquatic ecosystems
· Sewage, which when dumped into rivers or the sea can kill aquatic organisms and harm human health
· Chemical contaminants, discharged by factories or through sewage systems, may contaminate soil or seabeds, and have unpredictable effects on wildlife - for example damage to reproductive organs through exposure to human sex hormones.
· Slurry (a liquid manure made of animal droppings and urine) which if it drains into streams and rivers can harm aquatic wildlife
· Oil - spilled from oil tankers and refineries - which destroys habitats, poisons water supplies, and kills aquatic wildlife and birds
Other types of pollution
Radiation pollution - caused by either deliberate or accidental discharge of nuclear waste - can contaminate the soil, water and air for years. Radiation can cause cancers like leukaemia. Radioactive waste cannot be safely burnt or buried or sunk, but has to be stored and transported in lead- or glass-lined containers which stop the radiation from passing out. Many radioactive compounds stay radioactive for thousands of years.
Noise pollution, caused by aircraft and other engines, road or building works, can induce stress in both humans and wildlife.
Litter - especially non-biodegradable litter such as plastic and glass - can seriously degrade human habitats, as well as injuring both humans and wildlife.
Controlling pollution
Humans do not have to pollute the environment as much as they do. Outputs of smoke and soot - once the main pollutants in industrial societies - have been massively reduced over the last 100 years. There are many things that could be done to reduce the impact of other types of human pollution as well.
Type of pollution Source Control measures
smoke and soot burning wood and fossil fuels smokeless zonessmokeless fuel
carbon dioxidecarbon monoxide burning fossil fuels burn less fossil fuel (switch to clean energy sources)build cleaner engines (eg catalytic converters)reduce energy consumption
methane burning fossil fuel, landfill waste and agricultural livestock burn less fossil fuelscut waste productioncut livestock numbers
sulphur dioxidenitrogen dioxide fossil fuels burn less fossil fuelbuild cleaner enginesreduce energy consumption
dust factories, quarrying etc efficient air filtersstricter laws against polluters
agrochemicals: fertilisers and pesticides farms use organic fertilisers insteaduse biological pest controls instead
oil tankers, refineries use less oilstricter laws against polluters
Green machines: wind turbines generate energy without pollution
Global warming
As you can see from the graphs, the earth has got steadily warmer over the last 150 years, as levels of atmospheric carbon dioxide have got steadily higher. What is causing this?
Global temperature is the result of a balance between heat received from the sun and heat radiated back into space. The earth's atmosphere forms an insulating layer, keeping some - but not all - of the sun's heat in, a bit like the glass roof of a greenhouse.
Human activities such as burning fossil fuels for energy and transport are adding more and more carbon dioxide to the atmosphere. At the same time large-scale deforestation is reducing the number of trees and other green plants removing carbon dioxide for photosynthesis. Hence - rising carbon dioxide levels.
Levels of atmospheric methane are also going up. Methane is produced by livestock farming, rotting plant material in marshes and paddy fields, and by drilling for oil and gas. Methane and carbon dioxide are greenhouse gases - they insulate the atmosphere so that it traps more of the sun's energy.
This is the called the greenhouse effect, and it is the cause of global warming.
Results of global warming
Rises of just a few degrees in world temperatures could have a dramatic impact on world climate:
· global weather patterns may change, changing rainfall patterns, causing drought in some places and floods in others
· melting of polar ice caps could raise sea levels, causing increased coastal erosion and flooding of low-lying land - including some major cities
Ozone destruction
Chlorofluorocarbons or CFCs, used in fridges, airconditioners and aerosols, are another greenhouse gas. Though there are much lower levels of CFCs in the atmosphere than carbon dioxide and methane, they are thousands of times more effective at trapping heat.
They also damage the outer ozone layer of the atmosphere by breaking apart ozone molecules. The ozone layer acts like a screen protecting us from the sun's harmful UV rays, and CFCs are causing holes to appear in the layer, allowing more UV rays to pass through, and increasing the risk of skin cancer.
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