Biology unit 3

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  • Created by: Megan
  • Created on: 03-04-13 18:34



1. A partially permeable membrane is just one with very small holes in it. So small, in fact tiny molecules can pass through them, and bigger molecules can't. 

2. The water molecules actually pass both ways through the membrane during osmosis. This happens because water molecules move about randomly all the time. 

3. But because there are more water molecules on one side than on the other, there's a steady net flow of water into the region with fewer water molecules.

4. This means the strong sugar solution gets more dilute. The water acts like it's trying to even up the concentration either side of the membrane. 

5. Osmosis is a type of diffusion - passive movement of water particles from an area of high water concentration to an area of low water concentration

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Water moves in and out of cells by osmosis

1. Tissue fluid surrounds the cells in the body - it's basically just water with oxygen, glucose and stuff dissolved in it. It's squeezed out of the blood capillaries to supply the cells with everything they need. 

2. The tissue fluid will usually have a different concentration to the fluid inside a cell. This means that water will either move into the cell from the tissue fluid, or out of the cell, by osmosis. 

3. If a cell is short of water, the solution inside it will be quite concentrated. This ususally means the solution outside is more dilute, so water will move into the cell by osmosis. 

4. If a cell has lots of water, the solution inside it will be more dilut, and water will be drawn out of the cell and into the fluid outside by osmosis. 

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Diffusion, osmosis and active transport

1. Life processes need gases or other dissolved substances before they can happen.

2. Waste substances also need to move out of the cells so that the organism can get rid of them. 

3. These substances move to where they need to be by diffusion, osmosis and active transport

4. In life processes, the gases and dissolved substances have to move through some sort of exchange surface. The exchange surface structures have to allow enough of the necessary substances to pass through.

5. Exchange surfaces are adapted to maximise effectiveness: 

  • They are thin, so substances only have a short distance to diffuse.
  • They have a large surface area so lots of a substance can diffuse at once.
  • Exchange surfaces in animals have lots of blood vessels, to get stuff into and out of the blood quickly. 
  • Gas exchange surfaces in animals are often ventilated too - air moves in and out.
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Diffusion in leaves

1. Carbon dioxide diffuses into the air spaces within the leaf, then it diffuses into the cells where photosynthesis happens. The leaf's structure is adapted so that this can happen easily. 

2. The underneath of the leaf is an exchange surface. It's covered in biddy little holes called stomata which the carbon dioxide diffuses through.

3. Oxygen and water vapour also diffuse out through the stomata. 

4. The size of the stomata are controlled by guard cells. These close the stomata if the plant is losing water faster than it is being replaced by the roots. Without these guard cells the plant would soon wilt

5. The flattened shpae of the leaf increases the area of this exchange surface so that it's more effective. 

6. The walls of the cells inside the leaf form another exchange surface. The air spaces inside the leaf increase the area of this surface so there's more chance for carbon dioxide to get into the cells. 

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The lungs

1. The lungs are in your thorax - top part of your body.

2. It's seperated from the lower part of the body by the diaphragm.

3. The lungs are like big pink sponges and are protected by the ribcage

4. The air that you breathe in goes through the traches. This splits into two tubes called bronchi. One going into each lung. 

5. The bronchi split into progressively smaller tubes called bronchioles.

6. The bronchioles finally end at small bags called alveoli where the gas exchange takes place.

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is breathing in...

  • Intercostal muscles and diaphragm contract.
  • Thorax volume increases.
  • This decreases the pressure, drawing air in.

... and breathing out

  • Intercostal muscles and diaphragm relax.
  • Thorax volume decreases.
  • This increases the pressure, so air is forced out.
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Gas exchange happens in the lungs

1. The job of the lungs is to transfer oxygen to the blood and to remove waste carbon dioxide from it. 

2. To do this the lungs contain millions of little air sacs called alveoli where gas exchange takes place. 

3. The alveoli are specialised to maximise the diffusion of oxygen and CO2. They have:

  • An enormous surface area.
  • A moist lining for dissolving gases. 
  • Very thin walls.
  • A good blood supply. 
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Root hairs

1. Are specialised for absorbing water and minerals.

2. The cells on the surface of plant roots grow into long hairs which stick out into the soil.

3. This gives the plant a big surface area for absorbing water and mineral ions from the soil.

4. Most of the water and mineral ions that get into a plant are absorbed by the root hair cells. 

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Active transport in root hair cells

1. The cocentration of minerals is usually higher in the root hair cell than in the soil around it. 

2. So normal diffusion doesn't explain how minerals are taken up into the root hair cell. 

3. They should go the other way if they followed the rules of diffusion. 

4. The answer is that a process called active transport is responsible. 

5. Active transport allows the plant to absorb minerals from a very dilute solution, against a concentration gradient. This is essential for its growth. But active transport needs energy from respiration to make it work.

6. Active transport also happens in humans, for example in taking glucose from the gut and from the kidney tubules. 

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Phloem tubes transport food

1. Made of columns of living cells with small holes in the ends to allow stuff to flow through. 

2. They transport food substances (mainly dissolved sugars) made in the leaves to growing regions and storage organs of the plant. 

3. The transport goes in both directions

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Xylem tubes take water up

1. Made of dead cells joined end to end with no end walls between them and a hole down the middle. 

2. They carry water and minerals from the roots to the stem and leaves in the transpiration system

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1. Is the loss of water from the plant.

2. Transpiration is caused by the evaporation and diffusion of water inside the leaves.

3. This creates a slight shortage of water in the leaf, and so more water is drawn up from the rest of the plant through the xylem vessels to replace it. 

4. This in turn means more water is drawn up from the roots, and so there's a constant transpiration stream of water through the plant. 

5. Transpiration is just a side-effect of the way leaves are adapted for photosynthesis. They have to have stomata in them so that gases can be exchanged easily. Because there's more water inside the plant than in the air outside, the water escaped from the leaves through the stomata. 

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Double circulatory system

Humans have a double circulatory system - two circuits joined together:

1. The first one pumps deoxygenated blood (blood without oxygen) to the lungs to take in oxygen. The blood then returns to the heart. 

2. The second one pumps oxygenated blood around all the other organs of the body. The blood gives up its oxygen at the body cells and the deoxygenated blood returns to the heart to be pumped out to the lungs again. 

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The heart

1. The heart is a pumping organ that keps the blood flowing around the body. The walls of the heart are mostly made of muscle tissue.

2. The heart has valves to make sure that blood goes in the right direction (not backwards).

3. This is how the heart uses its four chambres to pump blood around: 

  • Blood flows into the two atria from the vena cava and the pulmonary vein.
  • The atria contract, pushing the blood into the ventricles.
  • The ventricles contract, forcing the blood into the pulmonary artery and the aorta, and out of the heart
  • The blood then flows to the organs through arteries, and returns through veins. 
  • The atria fill again and the whole cycle starts over
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Blood vessels

There are three different types of blood vessel

1. ARTERIES - these carry the blood away from the heart.

2. CAPILLARIES - these are involved in the exchange of materials at the tissues. 

3. VEINS - these carry the blood to the heart. 

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1. Carry blood under pressure.

2. The heart pumps the blood out at high pressure so the artery walls are strong and elastic.

3. The walls are thick compared to the size of the hole down the middle.

4. They contain thick layers of muscle to make them strong, and elastic fibres to allow them to stretch and spring back

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1. Arteries branch into capillaries

2. Capillaries are really tiny - too small to see. 

3. They carry the blood really close to every cell in the body to exchange substances with them. 

4. They have permeable walls, so substances can diffuse in and out.

5. They supply food and oxygen, and take away waste like CO2

6. Their walls are usually only one cell thick. This increases the rate of diffusion by decreasing the distance over which it occurs. 

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1. Capillaries eventually join up to form veins. 

2. The blood is at lower pressure in the veins so the walls don't need to be as thick as artery walls. 

3. They have a bigger lumen than arteries to help the blood flow despite the lower pressure. 

4. They also have valves to help keep the blood flowing in the right direction, back to the heart

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White blood cells

Defend against disease: 

  • They can change shape to gobble up unwelcome microorganisms.
  • They produce antibodies to fight microorganisms, as well as antitoxins to neutralise any toxins produced by the microorganisms.
  • Unlike red blood cells, they do have a nucleus
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1. These are small fragments of cells. They have no nucleus

2. They help the blood to clot at a wound - to stop all your blood pouring out and to stop microorganisms getting in. 

3. Lack of platelets can cause excessive bleeding and bruising. 

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This is a pale straw-coloured liquid which carries just about everything in the blood:

1. Red and white blood cells and platelets

2. Nutrients like glucose and amino acids. These are the soluble products of digestion which are absorbed from the gut and taken to the cells of the body. 

3. Carbon dioxide from the organs to the lungs. 

4. Urea from the liver to the kidneys. 

5. Hormones

6. Antibodies and antitoxins produced by the white blood cells. 

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Artificial blood

1. When someone loses a lot of blood, their heart can still pump the remaining red blood cells around, as long as the volume of their blood can be topped up.

2. Artificial blood is a blood substitute, which is used to replace the lost volume of the blood. It's safe and can keep people alive even if they lose 2/3 of their red blood cells This may give the patient enough time to produce new blood cells. If not, the patient will need a blood transfusion

3. Ideally, an artificial blood product would replace the function of the red blood cells, so that there's no need for a blood transfusion. These are being developed but currently have problems with side - effects

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The blood - artificial parts

1. Artificial hearts are mechanical devices that are put into a person to pump blood if their own heart fails. They're usually used as a temporary fix, to keep a person alive until a doner heart can be found. In some cases they're used as a permanent fix, which reduces the need for a doner heart. 

2. The main advantage of artificial hearts is that they're not rejected by the body's immune system. This is because they're made from metals or plastics, so the body doesn't recognise them as foreign and attack in the same way as it does with living tissue

3. But surgery to fit an artificial heart can lead to bleeding and infection. Also, artificial hearts don't work as well as healthy natural ones - parts of the heart could wear out or the electrical motor could fail. Blood doesn't flow through artificial hearts as smoothly, which can cause blood clots and lead to strokes. The patient has to take drugs to thin their blood and make sure this doesn't happen, which can cause problems with bleeding if they're hurt in an accident. 

4. If it's just the heart valvedthat are defective, they can be replaced with mechanical valves. Replacing valves is still major surgery and can still cause blood clots.

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1. Coronary heart disease is when the arteries that supply the blood to the muscle of the heart get blocked by fatty deposits. This causes the arteries to become narrow and blood flow is restricted - this can result in a heart attack.

2. Stents are tubes that are inserted inside arteries. They keep them open, making sure blood can pass through to the heart muscles. This keeps the person's heart breathing.

3. Stents are a way of lowering the risk of a heart attack in people with coronary heart disease.

4. But over time, the artery can narrow again as stents can irritate the artery and make scar tissue grow. The patient also has to take drugs to stop blood clotting on the stent. 

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There are 6 main things that need to be controlled: 

1. The body temperature can't get too hot or too cold. 

2. Water content musn't get too high ot too low, or too much water could move into or out of cells and damage them. 

3. If the ion content of the body is wrong, the same thing could happen. 

4. The blood sugar level needs to stay within certain limits. 

5. Carbon dioxide is a product of respiration. It's too toxic in high quantities so it's got to be removed. It leaves the body by the lungs when you breathe out. 

6. Urea is a waste product made from excess amino acids. 

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Body temperature

All enzymes work best at a certain temperature. The enzymes within the human body work best at about 37C. If the body gets too hot or too cold, the enzymes won't work properly and some really important reactions could be disrupted.

1. There is a thermoregulatory centre in the brain which acts as your own thermostat.

2. Contains receptors that are sensitive to the temp of the blood flowing through the briain.

3. The thermoregulatory centre also receives impulses from the skin, giving temp info.

4. Your body responds to your temp: 

Too hot...  1. Hairs lie flat 2. Sweat is produced by glands and evaporates from the skin, which removes heat 3. The blood vessels dilate so more blood flows close to the surface, so its easier for heat to transfer to environment. 

Too cold... 1. Hairs stand up 2. no sweat 3. blood vessels supplying capillaries constrict to close off the skins blood supply 4. When your cold you shiver  this needs respiration which releases energy to warm the body. 

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The kidneys perform three main roles

1. REMOVAL OF UREA  - This process occurs in the liver. Urea is produced as a waste product from the reactions. The kidneys then filter it out of the blood.

2. ADJUSTMENT OF ION CONTENT - If the ion content of the body is wrong this will upset the balance between ions and water, meaning too much or too little water is drawn into cells through osmosis. Having the wrong amount of water in cells can damage them. The balance is maintained by the kidneys. 

3. ADJUSTMENT OF WATER CONTENT  - Water is lost from the body in three main ways: in urine, in sweat and in the air we breathe out. The body has to constantly balance the water coming in against the water going out. 

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Kidney function

Nephrons are the filtration units in the kidneys


  • A high pressure is built up which squeezes water, urea, ions and sugar out of the blood and into the Bowman's capsule.
  • The membranes between the blood vessels and the Bowman's capsule act like filters, so big molecules like proteins and blood cells are not squeezed out. They stay in the blood. 


  • All the sugar is reabsorbed. This involves the process of active transport against the concentration gradient. 
  • Sufficient ions are reabsorbed. Excess ions are not. Active transport is needed. 
  • Sufficient water is reabsorbed. 


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Dialysis machines

1. Dialysis has to be done regularly to keep the concentrations of dissolved substances in the blood at normal levels, and to remove waste substances. 

2. In a dialysis machine the person's blood flows alongside a selectively permeable barrier, surrounded by dialysis fluids. It's permeable to things like ions and waste substances, but not big molecules like proteins. 

3. The dialysis fluis has the same concentration of ions and glucose as healthy blood

4. This means that useful ions and glucose wont be lost from the blood during dialysis. 

5. Only waste substances and excess ions and water diffuse across the barrier. 

6. Many patients with kidney failure have to have dialysis sessions three times a week. Each session takes 3-4 hours

7. Plus, dialysis may cause blood clots or infections.

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Kidney transplant

At the moment, the only cure for kidney disease is to have a kidney transplant. Healthy kidneys are usually transplanted from people who have died suddenly, say in a car accident, and who are on the organ donor register or carry a donor card. But kidneys can also be transplanted from people who are still alive - as we all have two of them. 

The donor kidney can be rejected by the patient's immune system - the foreign antigens on the donor kidney are attacked by the patient's antibodies. To help prevent this happening, precautions are taken: 

1. A donor with a tissue type that closely matches the patient is chosen. Tissue type is based on antigens which are proteins on the surface of most cells. 

2. The patient is treated with drugs that suppress the immune system, so that their immune system won't attack the transplanted kidney. 

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Insulin and glucagon

1. Eating foods containing carbohydrates puts glucose into the blood from the gut. 

2. The normal metabolism of cells removes glucose from the blood. 

3. Vigorous exercise removes much more glucose from the blood. 

4. Levels of glucose in the blood must be kept steady. Changes in blood glucose are monitored and controlled by the pancreas, using the hormones insulin andglucagon

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World population

1. The population of the world is currently rising very quickly, and it's not slowing down. 

2. This is mostly due to modern medicine and farming methods, which have reduced the number of people dying from disease and hunger. 

3. This is great for all of us humans, but it means we're having a bigger effect on the environment we live in. 

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Increasing demands on the environment

When the earth's population was much smaller, the effects of human activity were usually small and local. Nowadays though, our actions can have a far more widespread effect. 

1. Our increasing population, puts pressure on the environment, as we take the resources we need to survive

2. But people around the world are also demanding a higher standard of living. So we use more raw materials, but we also use more energy for the manufacturing processes. This all means we're taking more and more resources from the environment more and more quickly

3. Unfortunately, many raw materials are being used up quicker than they're being replaced. So if we carry on like we are, one day we're going to run out

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Increase waste

As we make more and more things we produce more and more waste. And unless this waste is properly handled, more harmful polution will be caused. This affects water, land and air. 

1. WATER - Sewage and toxic chemicals from industry can pollute lakes, rivers and oceans, affecting the plants and animals that rely on them for survival. And the chemicals used on land can be washed into water. 

2. LAND - We use toxic chemicals for farming. We also bury nuclear waste underground, and we dump a lot of household waste in landfill sites. 

3. AIR - Smoke and gases released into the atmosphere can pollute the air. 

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Land use

Humans also reduce the amount of land and resources available to other animals and plants. The four main human activities that do this are: 





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Carbon dioxide removed from atmosphere

1. Carbon is presented in the atmosphere as carbon dioxide

2. Many processes lead to CO2 being released.

3. Too much CO2 in the atmosphere causes global warming

4. Luckily, the CO2 can be sequestered in natural stores, including: 

  • Oceans, lakes and ponds. 
  • Green plants, where it's stored as carbon compounds. Green plants remove CO2 from the atmosphere during photosynthesis. 
  • Peat bogs. 

5. Storing CO2 in these ways is really important because it means CO2 is removed from the atmosphere

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Effects of CO2 and methane

1. The temperature of the Earth is a balance between the heat it gets from the sun and the heat it radiates back out into space. 

2. Gases in the atmosphere naturally act like an insulating layer. They absorb most of the heat that would normally be radiated out into space, and re-radiate it in all directions. 

3. If this didn't happen, then at night there'd be nothing to keep and heat in, and we'd quickly get very cold indeed. But recently we've started to worry that this effect is getting a bit out of hand... 

4. There are several different gases in the atmosphere which help keep the heat in. They're called greenhouse gases and the main ones whose levels we worry about are carbon dioxide and methane - because the levels of these two gases are rising quite sharply. 

5. The Earth is gradually heating up because of the increasing levels of greenhouse gases - this is global warming. Global warming is a type of climate change and causes other types of climate change. 

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Deforestation is the cutting down of forests. This causes big problems when it's done on a large - scale, such as cutting down rainforests. It's done for various reasons: 

1. To provide timber to use as building material. 

2. To clear more land for farming, which is important to: 

  • provide more food 
  • or, grow crops from which biofuels based on ethanol can be produced. 

3. To produce paper from wood. 

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Deforestation problems


  • Rice is grown in warm, waterlogged conditions - ideal for decomposers. These organisms produce methane, so more is released in the atmosphere. 
  • Cattle produce methane and rearing cattle means that more methane is released. 


  • Carbon dioxide is released when trees are burnt to clear land. 
  • Microorganisms feeding on bits of dead wood release CO2 during respiration. 


  • Cutting down loads of trees means less CO2 is removed from the atmosphere.


  • Biodiversity is the variety of different species in a habitat.
  • When habitats are destroyed, it causes species to become extinct.
  • This causes a number of lost opportunities.
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Destroying peat bogs

1. Bogs are areas of land that are acidic and waterlogged. Plants that live in bogs don't fully decay when they die, because there's not enough oxygen. The partly-rotted plants gradually build up to form peat.

2. So the carbon in the plants is stored in peat instead of being released into the atmosphere. 

3. However, peat bogs are oftern drained so that the area can be used as farmland, or the peat is cut up and dried to use as fuel. Peat is also sold to gardeners as compost.

4. Peat starts to decompose when the bogs are drained, so carbon dioxide is released. If we continue to destroy peat bogs, more carbon dioxide will be released, adding to the greehouse effect.

5. So one way people can do their bit is by buying peat-free compost for their gardens to reduce the demand for peat. 

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Consequences of global warming

1. As the sea gets warmer, it expands, causing sea level to rise. Sea level has risen as little bit over the last 100 years. If it keeps rising it'll be bad news for people living in low-lying places.

2. Higher temperatures make ice melt. Water that's currently trapped on land (as ice) runs into the sea, causing sea levels to rise even more.

3. Global warming has changes weather patterns in many parts of the world. It's thought that many regions will suffer more extreme weather, because of this. 

4. The distribution of many wild animal and plant species may change. Some species may become more widely distributed. Other species may become less widely distributed.

5. Biodiversity could be reduced if some species are unable to survive a change in the climate, so become extinct

6. There could be changes in migration patterns

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Fuels can be made by fermentation

1. Fuels can be made by fermentation of natural products - luckily enough, waste products can often be used. 

2. Fermentation is when bacteria or yeast break sugars down by anaerobic respiration. 

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1. Is made by anaerobic fermentation of sugar. 

2. Yeast make ethanol when they break down glucose by anaerobic respiration


3. Sugar cane juices can be used, or glucose can be derived from maize starch by the action of carbohydrase (an enzymes). 

4. The ethanol is distilled to seperate it from the yeast and remaining glucose before it's used. 

5. In some countries, e.g. Brazil, cars are adapted to run on a mixture of ethanol and petrol - this is known as gasohol

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1. Is made by anaerobic fermentation of waste material

2. Biogas is usually about 70% methane and 30% carbon dioxide

3. Lots of different microorganisms are used to produce biogas. They ferment plant and animal waste, which contains carbohydrates. Sludge waste from, e.g. sewage works or sugar factories, is used to make biogas on a large scale.

4. It's made in a simple fermenter called digester or generator

5. Biogas generators need to be kept at a constant temperature to keep the microorganisms respiring away. 

6. There are two types of biogas generators - batch generators and continuous generators

7. Biogas can't be stored as a liquid, so it has to be used straight away - for heating, cooking, lighting, or to power a turbine to generate electricity

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Fuel production

1. Can happen on a large or small scale

2. Large-scale biogas generators are now being set up in a number of countries. Also, in some countries, small biogas generators are used to make enough gas for a village or a family to use in their cooking stoves and for heating and lighting

3. Human waste, waste from keeping pigs, and food waste can be digested by bacteria to produce biogas. 

4. By-products are used to fertilise crops and gardens. 

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Biogas generators

There are two main types of biogas generators: 

1. Batch generators make biogas in small batches. They're manually loaded up with waste, which is left to digest, and the by-products are cleared away at the end of each session. 

2. Continuous generators make biogas all the time. Waste is continuously fed in, and biogas is produced at a steady rate. Continuous generators are more suited to large-scale biogas projects. 

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Designing a generator

When generators are being designed four factors need to be considered: 

COST - Continuous generators are more expensive that batch ones, because waste has to be mechanically pumped in and digested material mechanically removed all the time.

CONVENIENCE - Batch generators are less convenient because they have to be continually loaded, emptied and cleaned

EFFICIENCY - Gas is produced most quickly at about 35C. If the temperature falls below this the gas production will be slower. Generators in some areas will need to be insulated or kept warm. The generator shouldn't have any leaks or gas will be lost. 

POSITION - The waste will smell during delivery, so generators should be sited away from homes. The generator is also best located fairly close to the waste source

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Economic and environmental effects of using biofue

1. Biofuels are a greener alternative to fossil fuels. The carbon dioxide released into the atmosphere was taken in by plants which lived recently, so they're carbon neutral

2. The use of biofuels doesn't produce significant amounts of sulfur dioxide or nitrogen oxides, which cause acid rain

3. Methane is a greenhouse gas and is one of those responsible for global warming. It's given off from untreated waste, which may be kept in farmyards or spread on agricultural land as fertiliser. Burning it as biogas means it's not released into the atmosphere. 

4. The raw material is cheap and readily available

5. The digested material is a better fertiliser than undigested dung - so people can grow more crops

6. In some developing rural communities women have to spend hours each day collecting wood for fuel. Biogas saves them this drudgery. 

7. Biogas generators act as a waste disposal system

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Efficiency of food production


  • There's less energy and less biomass every time you move up a stage in a food chain


  • In the UK, animals such as pigs and chickens are often intensively farmed. They're kept close together indoors in small pens, so that they're warm and can't move about
  • This saves them wasting energy on movement, and stops them giving out so much energy as heat. This makes the transfer of energy from the animal feed to the animal more efficient - so basically, the animals will grow faster on less food
  • This makes things cheaper for the farmer, and for us when they get to the supermarket.


  • Mycoprotein means proteins from fungi. 
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Very good resource which covers all the topics for b3. Saved my time - thanks

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