AQA Biology unit b3 (Higher revision)

Biology higher, unit 3, gcse revision notes of EVERY topic: 2011


Exchange of gases in the lungs

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. 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
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Exchange of gases in 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. Carbon dioxide constantly being removed from the blood and oxygen constantly entering the lungs means that gas exchange happens at the highest concentration gradients to make it rapid and effective.

an alveolus adapted to make gaseous exchange as efficient as possible:
-  large surface area
- moist surface makes diffusion easy as gases can dissolve
- thin walls make diffusion easy
- good bloody supply and capillary network


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Exchange in the gut

The food we eat is broken down in the gut. It forms simple sugars, such as glucose, amino acids, fatty acids and glycerol.

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.

The lining of the small intestine is folded into thousands of tiny villi. These greatly increase the uptake of digested food by diffusion because they 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, a steep concentration gradient is constantly maintained

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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. The gills are thin so there is only a short distance for the gas to diffuse across.

tadpoles begin with a set of gills to help them survive in water – but when they become adult frogs, they can live on land and yet breathe in water at the same time.  As the creature grows into an adult frog, the gills are reabsorbed back into the body, a process called metamorphosis. An adult frog has moist skin with a rich blood supply.  Gas exchange can be done through the mouth using the frog’s lungs system. In water, all respiration is done through the skin.

Insects’ muscles require a lot of oxygen because they are so active. They 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 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

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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 and decreases the distance between the air and the photosynthesising cells.

They have openings known as stomata which can open and close at specific times to allow carbon dioxide in and out and reduce the amount of water loss. They have a waxy cuticle covering them, which is both gas-proof and waterproof.

The roots are thin and have a large surface area. The root hair cells increase surface area and increase efficiency of water uptake. The cell membrances 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 also minimal

 Water is vital for shaping cells and for photosynthesis. Minerals are needed to make proteins and other chemicals.

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The loss of water vapiour 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.

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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Transport in the blood

The liquid part of our blood is called plasma. It transports red blood cells, white blood cells and platelets. White blood cells (for immune system) and platelets (blood clotting) are not involved in transportation – 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.  Urea, a waste product formed in the liver is carried in the plasma to the kidneys nd transformed into urine.

Red blood cells are the most common cell type in the human body. There are around 5 million per each square milimetre 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 protein molecule folded around four iron atoms. It can react with oxygen to form oxyhaemoglobin and is delivered by red blood cells to the cells:  the reaction reverses and the oxygen splits and diffuses into the cells. Lone haemoglobin after is purple/deep red: the colour of veins.

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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. 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.

  • arteries (left diagram)carry blood away from the heart to the organs in the body. This is usually oxygenated blood.
  • veins carry blood towards the heart, usually low in oxygen and hence are deep blue-purple-red in colour. They contain valves usually which prevent the backflow of blood.
  • 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.

    Artery ( ( (

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The effects of exercise

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 exercising your 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…

  • heart rate increases and arteries dilate
  • breathing rate and depth increases  

Regular exercise increases the size of both the heart and the lungs, and they develop a bigger and more efficient blood supply

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Anaerobic respiration

When you are doing extremely vigorous exercise over a long period of time, insufficient amounts of oxygen are supplied to the muscles. 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. 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)

your body needs to get rid of this waste lactic acid, which would otherwise cause you problems. It 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.  Oxygen debt repayment:

lactic acid + oxygen → carbon dioxide + water

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Active transport

Diffusion is the movement of a substance along a concentration gradient and osmosis is the movement of water depands on the gradient of water and a partially-permeable membrane.

Active transport is the movement of a substance against the concentration gradient, using energy from respiration and mitochondria.


Active transport is widely used in cells e.g. the reabsorption of glucose from the kidneys into the blood.

However, the energy is only used if the substance involved is really needed.

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The Kidney

Your kidneys are vital in maintaining homeostasis. They filter out urea and remove it in urine because urea is poisonous. The water balance in the body must be maintained because too much water (turgid) or too little water (flaccid) in cells can destroy them – so the kidneys can remove excess water and release it from the body in urine.  The kidneys can remove excess salt from the body in the same way.

The kidneys filter the blood and then reabsorb 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 drank, exercised and the temperature, for example.

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 when kidneys 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.

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 the normal content of mineral ions, so that any excess mineral ions are lost by diffusion and is at exactly the right concentration so there is no net movement of glucose and mineral ions from blood plasma out into the fluid) to ensure diffusion of unwanted substances from the blood into the fluid. However, glucose remains in the blood.

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, 

Disadvantages 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
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Kidney Transplant

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 kidney is attached to the normal blood vessels of the recipients 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 recipient’s immune 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.

Matching blood type will minimise the risk of rejection as they will share some of the same antigens. Another method is using immunosuppressant drugs. These drugs are given to recipients which suppress  their immune system – for the rest of their lives meaning you are prone to disease and infection. However, as these drugs are developed and get better, the need for matching tissue type is decreasing in importance.

A reason why transplant is favourable compared to dialysis machines is that  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 they are not permanent. On average, A borrowed kidney will last about nine years, before shutting down and the cycle starts again.

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Growing microbes

The study of microorganisms is called microbiology.These include bacteria, viruses and fungi.

Learning more about microorganisms requires culturing them (i.e. growing large numbers to see their behaviour as a colony). The nutrients needed to grow them 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. The inoculating loop is sterilised in a red-hot flame before transferring a microorganism to the agar to prevent cross-contamination.

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 or cross-contamination between microorganisms can occur. We culture bacteria at 25C in schools to reduce the likelihood of pathogens growing which might be harmful to people.

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Large-scale microbe production

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. Industrial fermenters usually have:

  • An oxygen supply so the microorganisms can respire
  • 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
  • A water-cooled jacket which removes excess heat produced from the respiration
  • pH sensor 

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

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.  Mycoprotein serves as a high-protein, low-fat meat substitute. This means it is good for dieters and vegetarians.

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Antibiotic production

In 1928, Alexander Fleming left some bacteria culture he had been growing on some plates near an open window.  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 Penicillium notatum.

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 several months later they used it to save the life of a child. 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.

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Yeast in food production

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

Aerobic respiration is better for the cells because it produces more energy. 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.

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,  hops are often added to give the drink its flavour. The beer is then left to settle, clear and develop fully its flavour.

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Bacteria in food production

Yoghurt is made by fermenting whole milk:

  • adding a culture of the right type of bacteria to warm milk
  • keeping the mixture warm so the bacteria grow, reproduce and ferment
  • as the bacteria break down the lactose, lactic acid is produces (this gives yoghurt the sharp, tangy taste) – this process is lactic fermentation
  • the lactic acid causes the milk to clot and solidify to form a yoghurt
  • further bacterial action gives the yoghurt its creamy texture
  • flavourings and other additives can be added to the yoghurt to improve its taste, appearance and texture

Cheese-making also depends on the bacterial action in milks. The stages in cheese production are 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; so that the curds have solidified almost completely, unlike with yoghurts. 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, bacteria and moulds are added and the cheese is left to ripen. The ripening stage may take months or even years depending on the type of cheese being made.

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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.


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 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. 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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where did you get this information from? :)

Molly Howarth-Maddison


Just my books; what I've learnt from school :)

Tanya Edwards


these are really good:)

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