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Contents1 Exchange of Materials 3

2 Transporting Substances Around the Body 9

3 Microbiology 16

End of Unit Questions 22

Copyright © 2009 Daniel HollowaySignificant contribution Nelson Thornes AQA Science [GCSE Biology]

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Active Transport

Two of the main ways in which diffused substances are transported across cells are osmosisand diffusion. Diffusion is the movement of a fluid across a concentration gradient useful to the cells; osmosis is the movement of water across a semi-permeable membrane. However, sometimes substances need to be transported against a concentration gradient or membrane, which is when active transport takes place.

By active transport, cells are able to move substances from an area of low concentration to an area of high concentration. This is what is meant by moving against the gradient. Because the substances are being transported against a gradient, energy is required for an active transport system to carry a molecule across the membrane and then return to its original position (see below diagram).

The energy required for active transport to take place comes from cellular respiration. The rate of active transport and rate of respiration in cells are closely linked. The process of respiration releases energy – so in other words, the more respiration happening, the more active transport is taking place. This is why cells involved in active transport (e.g. root hair cells and gut lining cells) usually have a lot of mitochondria to provide the energy needed from respiration.

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Active transport is an important process in plants. The uptake of mineral ions through the soil requires active transport because the ions are found in very dilute solutions, whereas the solution inside the plant cells is a lot stronger. This means the ions have to be taken in against the gradient (from dilute to concentrated). Glucose is moved out of the gut and kidney into your blood, even though that is against a gradient. Active transport is also used in marine birds and reptiles, because they consume large amounts of salt when they drink water, and as the kidneys cannot get rid of it all, they have salt glands which use active transport. Without the ability for the cells to do active transport, these marine animals would die, so active transport is essential to their lives.

Gaseous Exchange

We require a constant supply of oxygen to allow for respiration. Breathing in and out takes in oxygen as a supply for the cells and removes the waste carbon dioxide produced by the cells. The lungs (found in the thorax) are protected by the rib cage. The lungs are separated from the digestive organs, found in the abdomen, by the diaphragm.

When you breathe in…

your ribs move UP and OUT your diaphragm flattens air is pulled INTO the lungs

When you breathe out…

your ribs move DOWN and IN your diaphragm returns to its domed shape air is forced OUT OF the lungs

The lungs have been adapted especially for making gas exchange more efficient. They are made up of clusters of alveoli, which are tiny air sacs with large surface areas, and are kept moist. They also have a rich blood supply, which maintains a concentration gradient in both directions. Oxygen constantly being removed from the blood and carbon dioxide constantly entering the lungs means that gas exchange happens at the highest concentration gradients to make it rapid and effective.

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The diagram here shows an alveolus (singular of alveoli). It has been adapted to make gaseous exchange as efficient as possible in the following ways:

spherical shape gives large surface area moist surface makes diffusion easy as gases can dissolve thin walls make diffusion easy good bloody supply (explained before)

The following table shows the effectiveness of the gas exchange in the lungs:

Air Breathed In Air Breathed OutNitrogen, N Approx 80% Approx 80%Oxygen, O 20% 16%Carbon Dioxide, CO2 0.04% 4%

The Human Gut

The food we eat is broken down in the gut. It forms simple sugars, such as glucose, amino acids, fatty acids and glycerol. These products are of no use in the gut, and if they stayed there would simply be removed with faeces. This is why, via a combination of active transport and diffusion, the molecules from food enter the bloodstream. This is why food is broken down during the digestion process. After being broken down, the food molecules are small enough to pass through the walls of the small intestine and into the blood vessels. They can move this way because there is a very high concentration of food molecules in the gut, and a very low concentration in the blood, so the process here is diffusion. They move along a very steep concentration gradient.

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The lining of the small intestine is folded into thousands of tiny villi (above). These greatly increase the uptake of digested food by diffusion. This is because only a certain amount of digested food molecules can diffuse over a certain surface area at a given time – so the villi increase the surface area dramatically.

Diffusion is very rapid and efficient in the gut, because, as with the lungs, it has a rich blood supply – so digested food molecules are carried away the second it diffuses from one side to the other. Therefore, a steep concentration gradient is constantly maintained.

Material Exchange in Other Organisms

Fish have protective scales all over their bodies which prevent them from directly taking in oxygen from the water – so they have gills, made of very thin layers of tissue with a rich blood supply. Thegills are thin so there is less surface area for the gas to diffuse across. Fish do not need to worry about keeping the gills moist, considering the live in water.

Gills do not work in air, and “suffocate” out of water – because if they are not kept moistconstantly the gills stick together and there isn’t enough surface area for the fish to get enough oxygen from the air to survive.

Frogs begin their life cycles as tadpoles. All tadpoles begin with a set of gills to help them survive, seeing as they spend their entire lives in water – but when they become adult frogs, they can live on land and yet breathe in water at the same time. Tadpoles get their oxygen from diffusion through their gills, in the same way they lose carbon dioxide along the concentration gradient. But as the creature grows into an adult frog, the gills are reabsorbed back into the body – and the gills aren’t there anymore. We say the tadpole has undergone metamorphosis. An adult frog has moist, thick skin with a rich blood supply. The majority of its gaseous exchange takes place through the skin. However, gas exchange can be done through the mouth also, using the frog’s simple lungs system. In water, all respiration is done through the skin.

Insects’ muscles require a lot of oxygen because they are so active. But extremely little, or no, gaseous exchange can take place through the tough outer covering of an insect, so they, like us, have an internal respiratory system which supplies oxygen to all the cells which need it and removes carbon dioxide. Insects have many spiracles, which are tiny openings – they

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open when oxygen is needed and close when it is not. This also prevents water loss, much like plant stomata. Spiracles lead to a tube system which delivers the oxygen straight to the tissues where it is needed. Most of the gas exchange takes place in the tracheoles, tiny tubes which are freely permeable to gases. They are moist and constantly have air pumped in and out of them by the insect to maintain a steady concentration gradient.

Material Exchange in Plants

All plants require carbon dioxide and water for photosynthesis. The carbon dioxide is obtained via diffusion through the leaves. The flattened shape of the leaves increases the surface area for diffusion to take place across. Leaves are usually flat, too, so that the distance between the air and the photosynthesising cells is as short as possible.

A problem is that water is always being lost by evaporation. So allowing carbon dioxide in will also lose water vapour. However, the plant does not need carbon dioxide all the time, because at night there is no sunlight – so photosynthesis cannot take place. So they have openings known as stomata which can open and close at specific times to allow carbon dioxide in and out. Another

adaptation is that they have a waxy cuticle covering them, which is both gas-proof and waterproof.

Roots have been adapted for uptake of water and mineral ions. Water is vital for shaping cells and for photosynthesis. Minerals are needed to make proteins and other chemicals. The roots themselves are thin and have a large surface area. The root hair cells have also adapted to increase surface area and increase efficiency of water uptake. The cell membranes of root hair cells have microvilli which further increase surface area for diffusion and osmosis. The distance between here and the xylem (transport tissue for the water) is minimal, also.

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Transpiration

The loss of water vapour through the surface of the leaves is called transpiration. As water is lost through the opening in stomata, more water is pulled up through the xylem to take its place. This constant movement of water around the plant is known as the transpiration stream. Because it is all caused by evaporation, anything affecting evaporation on a plant will also affect transpiration. Factors which increase evaporation will also increase transpiration. Sunny and warm conditions increase rate of photosynthesis, which means more carbon dioxide is needed, which

means stomata are opened, which means water is lost – so these conditions also increase transpiration rate: hot, dry and windy.

Other than having a waxy cuticle, another adaptation to help with the problem of water loss is that a plant can wilt. This happens when water is being lost faster than it is being gained. Wilting of leaves involves them collapsing and hanging downwards to prevent much water loss by minimising the surface area.

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The Circulatory System

The blood circulation system we have is made up of three main components: blood vessels, the heart and the blood. It is made up of two different blood systems – a double circulation. The diagram shows that one transports blood from the heart to the lungs and back again, the other takes blood around the rest of the body. Having a double circulation is vital in animals like ourselves because we are constantly active and in need of a rich blood supply – and with this system, we are constantly receiving oxygenated blood from the lungs which is sent around the body in one cycle.

There are three main blood vessels in the system, which have all adapted to carry out specific functions. The diagram below shows each of them…

The arteries (left) carry blood away from the heart to the organs in the body. This is usually oxygenated blood, explaining the red tubes. When you feel your pulse, that is the arteries stretching as blood is forced through them and returning back into their original shape.

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The veins carry blood towards the heart, usually low in oxygen and hence are deep purple-red in colour. No pulse in veins, but they do contain valves usually which prevent the backflow of blood.

The capillaries are found in junctions between the arteries and veins. These are found in huge networks. The walls are a single cell thick so that substances which need to get out of the blood and into body cells can easily via diffusion.

The Human Heart

Our hearts are made of two pumps, for the double circulation. These together beat around seventy times a minute. The walls of the heart are made pretty much entirely from muscle, which gets oxygen from the coronary blood vessels.

Blood Transport

The liquid part of our blood is called plasma. It transports red blood cells, white blood cellsand platelets. White blood cells (for immune system) and platelets (blood clotting) are not involved in transporting materials around the blood – it is the red blood cells and plasma. Blood plasma is a yellow liquid which transports all blood cells and other substances around the body. Carbon dioxide produced in the organs is carried in plasma back to the lungs. It is the red blood cells which give blood its red colour.

Urea, a waste product formed in the liver is carried in the plasma to the kidneys. In the kidneys, urea is removed from the blood and transformed into urine. All the small, soluble

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products of digestion pass into the blood from the gut, which are carried around the body by plasma to the individual cells which need the certain substances.

Red blood cells are the most common cell type in the human body. There are around 5 million per each square millimetre of blood. The function of red blood cells is to pick up oxygen from the lungs and deliver it to cells and tissues where it is needed. Their adaptations to improve efficiency at their job include:

being shaped like biconcave discs (concave/pushed in on both sides), this increases surface area : volume ratio over which diffusion takes place

being packed full of haemoglobin, pigments which can carry oxygen having no nucleus, more room for haemoglobin and diffusion!

A haemoglobin is a large protein molecule folded around four iron atoms. In an area of high oxygen concentration, haemoglobin can react with oxygen to form oxyhaemoglobin, which is bright red in colour, ergo blood being the colour it is. Oxygen is delivered by red blood cells, we know, to where it’s needed, but the way it is done is via the reverse reaction to the formation of oxyhaemoglobin. It happens when the oxyhaemoglobin arrives at an area of low oxygen concentration (i.e. where it is needed), and so the reaction reverses – the oxygen splits and diffuses into the cells where it is required. Lone haemoglobin after is purple/deep red, which explains the colour of veins.

Because haemoglobin are made from iron, a diet lacking iron can results in anaemia making you pale and have no energy. This is because your body cannot make enough red blood cells and you cannot carry enough oxygen around the body for your needs.

Exercise and Your Body

Muscles in our bodies need a lot of energy. They are made of protein fibres which contract when they receive energy from respiration. They contain many mitochondria to supply this energy. Muscles also contain glycogen stores – glycogen is a carbohydrate which can turn into glucose. This supplies the fuel needed for cellular respiration when muscles contract:

glucose + oxygen → carbon dioxide + water (+ energy)

When resting, your muscles are using up a certain amount of oxygen, but when exercisingyour muscles contract harder and faster, so need more glucose and oxygen to supply their energy needs. More carbon dioxide is obviously produced – which has to be removed to keep muscles working efficiently. So during exercise…

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heart rate increases and arteries dilate – these changes increase the blood flow to exercising muscles, this in turn increases oxygen and glucose supply and increases the rate of carbon dioxide removal

breathing rate increases, and breaths become more deep – these changes mean you breathe more often and draw more air into the lungs with each breath, again increasing amount of oxygen being brought into the body and picked up by red blood cells – and this oxygen is carried to the exercising muscles

Exercise is definitely very beneficial to us when done regularly. Regular exercise increases the size of both the heart and the lungs, and they develop a bigger and more efficient blood supply – meaning they will begin to function as effectively as possible, whether you are exercising or not.

Anaerobic Respiration

When you are doing extremely vigorous exercise over a long period of time, the muscles need so much oxygen that even an increase in breathing rate and heart rate does not supply sufficient amounts. So respiration which does not involve oxygen must be done – anaerobic respiration. Muscles only switch to anaerobic respiration when they have been exercising for a long time and fatigue. This is because anaerobic respiration is not as efficient as aerobic respiration, because the glucose molecules are not completely broken down and so less energy is released. The end products of anaerobic respiration are lactic acid and water:

glucose → lactic acid (+ energy)

After finishing a lot of exercise, you are out of breath for quite some time – this length of time depends on your fitness. The reason your body does this post-exercise is because your body needs to get rid of this waste lactic acid, which would otherwise cause you problems –but it cannot be breathed out unlike carbon dioxide. As a result, the lactic acid has to be broken down into carbon dioxide and water (the products of aerobic respiration) which requires oxygen. The amount of oxygen required to break down all of the lactic acid is called the oxygen debt. Even though your muscles have stopped exercising, this is why your heart rate and breathing rate remain high for a while after the exercise – to supply the oxygen needed to repay the oxygen debt. Oxygen debt repayment:

lactic acid + oxygen → carbon dioxide + water

Human Kidneys

Your kidneys are vital in maintaining homeostasis (see B2). For example, they filter out urea and remove it in urine because urea is poisonous. Another example is that water balance in the body must be maintained because too much water (turgid) or too little water (flaccid) in

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cells can destroy them – so the kidneys can remove excess water and release it from the body in urine. Similarly, the kidneys can remove excess salt from the body in the same way.

The kidneys filter the blood and then reabsorb (take back) everything your body needs. So: sugar (glucose), amino acids, mineral salts and urea all move out of the blood and into the kidneys along a concentration gradient. The blood cells are too big to pass through the tubules and so are left behind. Next, ALL of the sugar is reabsorbed back into the blood by active transport. But the amount of water and the dissolved mineral ions which are reabsorbed vary. It depends on how much of each is needed by the body – this is selective reabsorption.

Urine contains waste urea along with excess mineral ions and water not needed by the body. The quantities vary depending on how much you have taken in and given out. For example, on a hot day if you drink little and exercise a lot, you will produce very little urine, which will be concentrated, a dark yellow. Whereas if you drink lots and do no exercise on a cold day, you will produce a lot of dilute, pretty colourless urine.

The Dialysis Machine

The human kidneys are not immune from damage. And when they are damaged and stop functioning, those toxins like urea stop being removed from the body, leading to death. There are two ways we can deal with this problem, the first being dialysis. The machineused in dialysis is called the dialysis machine, and relies on a process called dialysis to clean the blood.

A person’s blood leaves their body and flows into the machine, through partially permeable membranes. After the membranes comes the dialysis fluid, which contains a certain concentration of substances to ensure diffusion of unwanted substances from the blood into the fluid. However, glucose remains in the blood.

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The dialysis machine prevents unwanted substances from building up and restores them to normal levels, so the patient can live a normal life – but they will build up again after a couple of days, which means regular dialysis must be done, which sometimes means dialysis machines are fitted in the patients’ homes. They have to remain attached to the machine for about eight hours each time.

But it is essential the patient does not lose vital substances from the blood like glucose and important mineral ions. The way this is done is by having the dialysis fluid at exactly the right concentration so there is no net movement of glucose and mineral ions from blood plasma out into the fluid. It also contains the normal content of mineral ions, so that any excess mineral ions are lost by diffusion, but no more.

There is no urea in the dialysis fluid, so there is a very strong concentration gradient for the urea – so it simply leaves the blood in its entirety.

Disadvantages of these machines include:

repeated use at 8 hours per use must also follow a strict, healthy diet after some years, the levels can be hard to maintain

Kidney Transplants

The other solution to the problem of kidney failure is a kidney transplant. A replacement kidney is required for this, which must be healthy and donated by a donor. The patient receiving the kidney is the recipient, and the kidney is attached to the normal blood vessels of their groin. Hopefully, it would then fully function – job done.

Unfortunately, the majority of the time this is not the case. One issue is that the kidney the recipient receives was not originally theirs, so the antigens on its surface will differ from the antigens they already have. Of course, the problem with this is that the recipient’s immune

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system may reject the new kidney – which means your body will destroy it. During a transplant, everything is done to prevent such a thing, but it is always a risk.

There are certain things that can be done to minimalise the risk of rejection. The main method is to use a donor similar to the recipient in terms of body tissue, for example blood type. This means they will share some of the same antigens. Another method, which sounds more worrying, is using immunosuppressant drugs. These drugs are given to recipients which suppress (shut down) their immune system – for the rest of their lives. However, as these drugs are developed and get better, the need for matching tissue type is decreasing in importance.

Of course receiving immunosuppressant drugs means you are prone to disease and your body cannot deal well with any infection once caught. But most people think it is a price worth paying since no kidney would mean certain death.

A downside to kidney transplants is the fact that they are not permanent. A borrowed kidney will not last forever, in fact on average of about nine years, before shutting down, in which case the patient must return to hospital dialysis until another kidney is found and another transplant operation takes place.

A reason why transplant is favourable compared to dialysis machines is that once you have a kidney via transplant, you can proceed to live a normal life and not worry about it – you can eat what you want and don’t have to attend regular dialysis sessions. However, a disadvantage is rejection – you will have to take your medicine everyday for the rest of your life in case it is ever rejected by the immune system. A reason why dialysis is favoured is because it is much more readily available. Waiting lists for kidney recipients can go on for years. And, one of the main problems with kidney transplants is finding a suitable donor, which often proves hard.

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Growing Microbes

The study of microorganisms is called microbiology. These include bacteria, viruses and fungi, all too small to be seen by the naked eye. Many microorganisms can be grown in a lab, where we can research them and find out what they need to survive – and learn which are useful for us and which want to kill us.

Learning more about microorganisms requires culturing them (i.e. growing large numbers to see their behaviour as a colony). For this to happen, you must provide them with everything they need: a culture medium with carbohydrate(s) to act as an energy source, and necessary nutrients (mineral ions, and sometimes proteins and vitamins – included). The nutrients are usually contained in an agar medium – a substance which dissolves in hot water and will set to form a jelly. Hot agar containing the nutrients is poured into a Petri dish and left to cool before any microorganisms are added. Warmth and oxygen is usually needed for growth too.

Safety in the Lab

It is essential that all microbe culture is done carefully, even when growing the safe microorganisms. This is because they can be pathogenic, and the safe ones can mutate to become harmful pathogens. The other problem is cross-contamination between microorganisms, which can upset experiments – but more importantly is a health and safety issue when they get onto human skin and you bring them everywhere with you.

Yeast

Yeast is probably the most important microorganisms for us. Yeasts are single-celled organisms with a nucleus, cytoplasm and membrane surrounded by a cell wall. They reproduce by asexual budding (splitting into two to form two new yeast cells).

Provided with a lot of oxygen, yeast cells will respire aerobically. They break down sugar as an energy source,

producing the waste products carbon dioxide and water. However, sometimes there is a lack of oxygen, so they respire anaerobically, which produces ethanol and carbon dioxide.

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Ethanol is alcohol. And this process of anaerobic respiration in yeast cells is called fermentation.

Like with humans, aerobic respiration is better for the cells because it produces more energy. This energy allows them to grow and reproduce; however, when there is a large number of yeast cells, they can survive longer without oxygen and so can respire anaerobically, breaking down sugars into ethanol.

Yeast is used to produce bread and alcoholic drinks. In bread production, the yeast grows and respires – producing carbon dioxide which causes the bread to rise. The gas bubbles expand when baked due to the high temperatures, giving the bread its light, wafery texture. All yeast cells are killed by the heat in the cooking process.

We can make beers and wines using yeast. Making beer relies on the process of malting, where barley grains are soaked in water to keep them warm. Germination begins and enzymes break down the starch in the grains into a sugary solution. This solution is extracted and used as an energy source for the yeast. The yeast and sugar mixture is fermented to produce alcohol, when hops are often added to give the drink its flavour. The beer is then left to settle, clear and develop fully its flavour.

Making wines however, uses the natural sugars found in fruit, e.g. grapes, for the yeast’s energy source. The grapes would be pressed for their juice, which is mixed with yeast and water. The yeast is then left to respire anaerobically until all of the sugar is used up. The wine is later filtered to remove the yeast, and stored in bottles, where it is left for some time to mature.

Food Production Using Other Bacteria

Yoghurt always used to be made by fermenting whole milk, but nowadays semi-skimmed, skimmed and even soya milk can be used. Yoghurt is formed by the bacteria action on the lactose (milk sugar). You can make yoghurt by:

1 adding a culture of the right type of bacteria to warm milk2 keeping the mixture warm so the bacteria grow, reproduce and ferment3 as the bacteria break down the lactose, lactic acid is produces (this gives yoghurt the

sharp, tangy taste) – this process is lactic fermentation4 the lactic acid causes the milk to clot and solidify to form a yoghurt5 further bacterial action gives the yoghurt its creamy texture

The same bacteria used to make yoghurt will also keep it from going off – yoghurts tend to last for around three weeks, whereas fresh milk lasts a couple of days. Colourings,

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flavourings and other additives can be added to the yoghurt to improve its taste, appearance and texture after the above steps.

Cheese-making also depends on the bacterial action in milks. They change the texture, taste and help preserve the milk. Some cheeses can last decades without decaying. The stages in cheese production are fairly similar, although a different type of bacteria is added. This bacteria still converts the lactose into lactic acid as before, but in makes far more lactic acid – to the extent that the solid parts (the curds) have solidified almost completely, unlike with yoghurts. Enzymes are also added to help separate the milk – when it has completely curdled, you can see separate curds from the liquid whey. The curds can then be used to make cheeses, whey generally goes on to be animal feed.

Next, the curds are mixed with other bacteria and moulds and left to dry out. The bacteria and moulds added at this stage affect the end texture and flavour, depending on how much they allow it to ripen – so the types and amounts are particularly important here. The ripening stage may take months or even years depending on the type of cheese being made.

Mass Production of Microbes

We need microbes in large quantities for production of drugs, like antibiotics, and food. To grow microbes on an industrial scale, large vessels called fermenters are used. These have been developed to prevent occurrences which stop bacterial growth. They react to changes, to try and maintain a stable environment (i.e. if external temperature increases, the fermenter will decrease internal temperature to restore balance). Industrial fermenters usually have:

1 An oxygen supply so the microorganisms can respire2 A stirrer to keep the microbes in suspension – this maintains a constant temperature

and makes sure that the oxygen and food are evenly spread out throughout the culture

3 A water-cooled jacket which removes excess heat produced from the respiration4 Measuring devices for pH and temperature so changes can be made if necessary (i.e.

when a dependent change is caused)

The graph below shows bacterial growth for real-life conditions, not the suitable, convenient conditions we give microbes in an industrial fermenter.

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Mycoprotein

A new substance was discovered not long ago, a food based on fungi, and it is called mycoprotein. It is produced using the fungus Fusarium, which grows and reproduces very rapidly based on a cheap energy supply (an inexpensive sugar syrup made from waste carbohydrates) in a large fermenter. It does require aerobic conditions to grow. Its mass doubles every 5 hours or so, and this biomass is harvested, purified and dried to leave mycoprotein. On its own, it is pale yellow in colour and tastes faintly of mushrooms – but a range of colours and flavours can be added to it to enhance it. Mycoprotein serves as a high-protein, low-fat meat substitute. This means it is good for dieters and vegetarians.

Antibiotic Production

In 1928, Alexander Fleming left some bacteria culture he had been growing on some plates near an open window. When he returned to look at them, microbes had grown, but there were patches of mould surrounding the agar and bacteria had stopped growing there. Whatever blew in from the wind had killed the bacteria. He analysed the mould and found it to be the fungus and Penicillium notatum extracted penicillin from it which he used to treat a wound. However, it was hard to extract much penicillin without the available technology, and even when extracted was very unstable, so he gave up on penicillin and returned to other areas of research.

It was Howard Florey and Ernst Chain who returned to penicillin during the Second World War and extracted enough to fully understand it. Firstly, they used it on animal tests, which were successful, so they tried it on a policeman dying from a blood infection. He recovered, but died later when the penicillin ran out. However, several months later they were prepared and saved the life of a child. Unfortunately, Fleming’s original mould was hard to

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harvest on a mass scale, but a mould growing on a melon was discovered which yielded more than 200 times the penicillin from before. It was grown in deep tanks industrially, so that by 1945 enough was being produced to treat 7 million people a year.

Modern Penicillin Production

Nowadays we use modern strains of the Penicillium mould which give even higher yields. We grow the mould in a sterilised medium which contains sugar, amino acids, mineral salts and other nutrients. The mould grows rapidly in the first forty hours of production, where most of the nutrients are used up. Only when most of these nutrients

have gone can penicillin begin to produce, which explains why there is a 40 hour lagbetween the start of fermentation and the start of penicillin production. Over the next 140 hours, broth is removed and nutrients are repeatedly added. This allows us to get the maximum yield of the drug.

Biogas

A flammable mixture of gases, known as biogas, forms when bacteria break down the waste material of dead animals or plants in anaerobic conditions. The main component is methane, although the contents varies. The methane content tends to be around 50 to 80 per cent of the gas, the rest is made of carbon dioxide, water, hydrogen and hydrogen sulphide.

Animal waste, dead animal and plant material and garden waste all contain carbohydrates which make them good energy sources for biogas generators. They tend to work best at around 30°C so are usually in hot countries, although the reactions which take place are exothermic, so if kept insulated, the generator can still be in a cold country.

On average, every 10kg of dry dung can produce 3 cubic metres of biogas. That 3m³ can be three hours of cooking, three hours of lighting or 24 hours of running a refrigerator. Another advantage of these generators is that the other product, the waste, can be used as a fertiliser.

In China’s biodigesters, they will put anything in there – vegetables, human and animal faeces and urine, etc – which produces low quality gas but high amounts of fertiliser.

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On the other hand, in India (where there are social taboos against using human waste), only animal waste is fed in – which produces high quality gas, but very little amounts of fertiliser.

Ethanol

A crop like sugar cane can grow fast – roughly 4 or 5 metres a year, and has a juice rich in carbohydrates (like sucrose). Maize (or sweet corn) can have its starches broken down into glucose by carbohydrase enzymes. If the sugar-rich products from sugar cane or maize are fermented with yeast anaerobically, the sugars break down to give ethanol and water. Ethanol can be extracted by distillation and used as a car fuel. Many cars run on a mixture of petrol and ethanol, which can prove cheaper than fully petrol.

Ethanol is an ideal fuel in the sense than it does not produce toxic gases when burned; it does not pollute as much as other fuels which produce carbon monoxide and sulphur dioxide; and it can be mixed with petrol to form gasohol (although this means there is still half the problem with the petrol side of the mixture). Using a fuel like ethanol is called carbon neutral because you are not contributing to carbon dioxide levels in the atmosphere by using it. The original plants to make ethanol took carbon dioxide from the air, and you are returning the same amount.

Gasohol is in common use in the USA, and is around 90% petrol, 10% ethanol, and most of the ethanol comes from America’s own maize crops. But they don’t have that many, like us, so making enough ethanol will always be a struggle for MEDCs (who don’t have the resources) and LEDCs (who don’t have the money). This is the drawback to ethanol as a biofuel.

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B3-1 : Exchange of Materials

1 Explain the process of active transport2 How does the rate of respiration affect the rate of active transport?3 Where does the energy needed for active transport come from?4 Explain why plant roots use active transport5 What gaseous exchange takes place in the lungs?6 When you breathe in/out, what happens to your ribs and diaphragm?7 Explain the function of alveoli8 Name the projections that line the small intestine wall9 Explain the function of fish gills and how they have adapted to better perform at

their job10 Name and explain the process undergone by tadpoles when becoming adult frogs11 How does the respiratory system of an insect work?12 Explain two adaptations of a plant to reduce water loss13 What is the function of the guard cells in a leaf?14 Explain the process of water and mineral ion uptake through the roots of a plant15 What is meant by the term transpiration?16 What is the constant movement of water called?17 Name two environmental factors which affect transpiration

B3-2 : Transporting Substances Around the Body

1 What three features are there to our circulatory system?2 Why do we have double circulation?3 Explain the features of arteries, veins and capillaries4 What colour is normal blood plasma?5 Name and explain three adaptations of the red blood cell which help it better

perform at its job6 What is oxyhaemoglobin?7 When you exercise, how does your body respond?8 What are the benefits of regular exercise?9 Write the word equation for anaerobic respiration10 Why is aerobic respiration favourable?11 How is lactic acid removed from the body?12 Explain the term “oxygen debt”

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13 What is the function of the kidney?14 Explain the process of selective reabsorption15 Describe what is contained in human urine16 What is kidney dialysis?17 Why must air bubbles be removed in the dialysis machine?18 Why might some people prefer a kidney transplant?19 Explain the drawback(s) to a transplant20 Why might it be hard to find donors for a kidney transplant?

B3-3 : Microbiology

1 How can we grow simple microbes in a lab?2 Why must safety precautions be taken when doing this?3 What does fermentation mean?4 Explain the function of yeast in bread production5 Describe the process of beer production6 Describe the process of wine production7 How is yoghurt made?8 How is cheese made?9 Name the equipment used to industrially produce microbes10 What features do these have to help grow microbes well?11 What is mycoprotein?12 Explain why it is suitable for vegetarians and dieters13 How was penicillin discovered?14 Which two scientists recovered the penicillin work after it was abandoned?15 Explain the process of modern penicillin production16 What is biogas, and what is it composed of?17 Explain what a biogas generator is?18 What are the two outputs of a biogas generator and what are they used for?19 What can sugar cane and maize be used for?20 Why is ethanol a good fuel to use?21 What are the drawbacks to ethanol as a fuel?22 Explain the term “carbon neutral”23 Describe the advantages to using gasohol

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