AQA Biology Unit 3

overview of biology unit 3

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13.1 - The body

Dissolved substance move by active transport, diffusion and osmosis

Many human organs are specialised for gas exchanges such as the villi in the small intestine and the alveoli in the lungs both increase the surface area.

They both have extensive networks of capillaries so that oxygen and carbon dioxide can pass out of the blood quicker

The villi absorb the soluble products of digestion through active transport and diffusion

The breathing system takes air in and out the body so that oxygen can diffuse in and carbon dioxide can diffuse out

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13.1 - Plants

Carbon dioxide enters the leaf through diffusion

Root hair cells absorb water and nutrients neeeded in the plants. They also increase the surface area (the hairs increase the s.a)

The surface area of the leaves is maximised by the flattened shape and internal air spaces.

Plants have stomata to obtain co2 from the atmosphere. 

Plants lose water vapour from their leaf surfaces through transpiration. Hot, dry conditions increase transpiration

The stomata size is controlled by guard cells 

If plants lose water faster than it is replaced by the roots, the stomata can close to prevent wilting.

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  • The heart pumps blood around the body
  • It passes through capillaries, veins and arteries
  • Substances needed by cells in the body tissues pass out of the blood, and substances produced by the cells pass into the blood through the walls of the capillaries
  • There is one circulation heart->lungs and one heart->rest of body
  • Blood contains red and white blood cells, plateles and plasma
  • Plasma transports substance around the body to and away from the organs
  • It transports co2, urea and soluble products of digestion


They transport oxygen from the lungs around the body. They have no nucleus and a biconvex shape so they are adapted to transport the maximum amount of oxygen around the body. They are packed with a red pigment called haemoglobin. This pigment mixtures with oxygen in the lungs becoming oxyhaemoglobin. It then is broken down to haemoglobin and oxygen in different organs that need oxygen to function.

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13.3-Exercise and the body

The body respires to release energy that muscles can use to contract

During exercise: the heart rate increases, rate and depth of breating increases and the arteries supplying the muscles dilate.

These changes increase the blood flow to muscles to: increase the removal of co2, increase the supply of sugar and oxygen and thus to increase the rate of anaerobic respiration.

Glycogen stores in the muscle are used during exercise

After a lot of exercise, muscles become fatigued and stop contracting efficently.

If there is not enough oxygen reaching muscles, they respire anaerobically

Anaerobic respiration is the incomplete breakdown of glucose that produces lactic acid and much less energy is released to contract muscles.

Anaerobic respiration results in an oxygen debt that has to be repaid in order to oxidise lactic acid to carbon dioxide and water

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A healthy kidney produces urine by first filtering the blood in the Bowman's capsule, reabsorbing all the sugar, water and dissolved ions needed by the body and releasing urea, excess ions and water as urine

Sugar and dissolved ions are often absorbed by active transport

People who suffer from kidney failure can have dialysis or a transport

In a dialysis operation, a person's blood flows between paritally permeable membranes. The dialysis fluid contains the same concentration of useful substances as the blood should contain so that glucose and useful mineral ions are not lost.

Urea passes out from the blood into dialysis fluid. 

Dialysis treatment restores the concentrations of dissolved substances to normal levels

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13.4-Kidney Transplants

A kidney transplant enables a diseased kidney to be replaced with 

a healthy one from a donor. However, the donor kidney may be 

rejected by the immune system unless precautions are taken. 

• To prevent rejection of the transplanted kidney: 

− a donor kidney with a ‘tissue-type’ similar to that of the 

recipient is used

− the recipient is treated with drugs that suppress the immune system

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13.5-Microbes in food and drink

Micro-organisms are used in the production of yogurt, cheese, bread and alcohol

They are made from yeast, a single celled organism with a nucleus, cytoplasm and a cell membrane

Fermentation: when yeast respires anaerobically to form carbon dioxide and ethanol

When yeast respires aerobically it produces carbon dioxide and water. 

This provides more energy for the yeast to respire

It is then necessary for the yeast to grow and reproduce

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13.5-Making food and drink from microbes

  • In brewing beer and wine-making, carbohydrates are used as an energy source for yeast to respire.  For making beer: 

              − the starch in barley grains is broken down into a sugary solution by                     enzymes in the germinating grains, in a process called malting 

              − the sugary solution is extracted then fermented 

               − hops are then added to give the beer flavour. 

  •  In wine-making the yeast uses the natural sugars in the grapes as its energy source. 
  •  In the production of yoghurt: 

            − a starter of bacteria is added to warm milk 

            − the bacteria ferment the milk sugar (lactose) producing lactic acid 

            − the lactic acid causes the milk to clot and solidify into yoghurt. 

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Mysterious Stranger..


Thanks for this, the notes are very good, Letts and Lonsdale is it? You need to put up the last bit about biofuels soon~ :)

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