Biology Unit 3

Biology unit 3 notes, suitable for higher seperate AQA science.

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Exchange of Materials

Water & dissolved substances automatically move along a concentration gradient, from high concentrations to low concentrations by osmosis & diffusion

When substances are absorbed against a concentration gradient, it is called active transport. This requires energy from respiration

Villi in the Small Intestine

  • They have a massive surface area.
  • They have an extensive network of capillaries
  • Absorbs the products of digestion by diffusion and active transport.

Alveoli in the Lungs

  • The trachea divides into two tubes called the bronchi.
  • Which then divide into several times to form the bronchioles
  • These then divide again until they finally form alveoli
  • Alveloi have a large, moist surface area
  • They have an excellent blood supply.
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Exchange of Materials 2

Exchange in Plants - The leaf.

  • Leaves are broad, thin & flat with lots of internal air spaces to provide a large surface area to make them efficient at photosynthesis. 
  • They have stomata on their undersurface to allow carbon dioxide in & oxygen out by diffusion.
  • This leads to a loss of water vapour in a process called transpiration. 
  • Transpiration is quicker in hot, dry, windy conditions. 
  • Water vapour from the internal leaf evaporates through the stomata
  • The size of the stomata is controlled by a pair of guard cells
  • If plants loose water faster than it is taken up by the root hair cells in the roots, the stomata closes to prevent wilting & dehydration
  • In periods of intense drought, photosynthesis may be impossible to occur as the stomata will close up to prevent water loss. 
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The Circulation System

The circulation system carries blood from the heart to all the cells of the body to provide them with food and oxygen (oxygenated blood), & carries waste products including carbon dioxide away from the cells (deoxygenated blood). 

Blood is pumped to the lungs so that carbon dioxide can be exchanged for oxygen.
The system consists of the heart, the blood vesses and the blood.

Deoxygenated blood is on the right, and oxygenated blood on the left.

The heart & major blood vessels.

  • The heart acts as a pump, in a double circulation system
  • Blood travels away from the heart by arteries, and returns to the heart through veins
  • In the organs, blood flows through capillaries
  • Substances needed by cells pass out of the blood, and substances produced by cells pass into the blood through capillary walls
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The Circulation System 2

The blood

  • If the blood is allowed to stand without clotting it seperates into plasma, red blood cells, white blood cells & platelets. 
  • The plasma & red blood cells play an important role.
  • Plasma is a straw coloured liquid which transports carbon dioxide from organs to lungs, glucose from small intestine to organ & other wastes from the liver to the kidneys. 
  • Red blood cells transport oxygen from the lungs to the organs
  • They have no nucleus so they can contain lots of haemoglobin.
  • In the lungs, haemoglobin combines with oxygen to form oxyhaemoglobin. In other organs, oxyhaemoglobin splits up into haemoglobin & oxygen
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The effects of exercise within the body.

Aerobic respiration.

  • When glucose is combined with oxygen inside living cells it breaks down and releases energy. This is called aerobic respiration. 
  • The energy that is released during respiration is used to enable muscles to contract.
  • One molecule of glucose produced by aerobic respiration can provide twenty times as much energy as anaerobic respiration. 

Glucose + Oxygen -----> Water + Energy.

Anaerobic Respiration.

  • The waste product is lactic acid, which accumulates in the tissues. 
  • When this happens the muscles become fatigued
  • After excercise, the body needs oxygen to break down this, this is called acid debt.

Glucose -----> Energy + Lactic Acid.

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The effects of exercise within the body 2

During exersise a number of changes take place. 

  • The heart rate increases.
  • Arteries supplying the muscles dilate.
  • The rate & depth of breathing increases
  • The blood flow to muscles increase.
  • Supply of oxygen and sugar is increased, which speeds up removal of carbon dioxide
  • Glycogen stored in muscles is broken down to glucose to be used in respiration.
  • If muscles are subjected to long periods of vigorous excercise they become fatigued.  
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The kidney

The function of the kidney.

  • Maintain the concentrations of dissolved substances in the blood. 
  • Remove urea.
  • If the kidneys fail then there is no way of removing excess substances.
  • This will ultimately result in death.

Control of ion content & excretion of urea. 

  • Made up of two important tissues; blood vessels & tubules
  • Blood vessels take the blood through the kidney where unwanted substances end up in millions of tiny tubules which join to form the ureter.
  • The substances flow through the tubules into the ureter, which leaves the kidney & ends up at the bladder
  • The kidney regulates the amount of water and ions in the blood and removes all urea.


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The kidneys 2

Using a dialysis machine.

  • In a dialysis machine, a person's blood flows between partially permeable membranes
  • These membranes are made from a material similar to the Visking tubing.
  • As blood flows through the machine it is separated from the dialysis fluid only by the partially permeable membranes. 
  • These membranes allow all the urea, and any excess substances to pass from the blood to the dialysis fluid
  • This restores the concentrations of dissolved substances in the blood to their normal levels. 
  • Dialysis fluid contains the same concentration of useful substances of blood. 
  • This ensures glucose and essential mineral ions are not lost through diffusion.
  • Dialysis must be carried out at regular intervals in order to maintain the patients health.
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The kidneys 3

Kidney Transplants.

  • Kidney transplants are preformed if both kidneys fail - one kidney can work.
  • A main problem with kidney transplants is the possibility of rejection by the body's immune system.
  • To do this, precautions are taken. 
  • A donor kidney with a tissue type as close as possible to that of the recipient is used. This is best achieved if the donor is a close relative
  • The bone marrow of the recipient is irradiated to stop the production of white cells. This reduced the likelihood of rejection just after transplantation. 
  • The recipient is treated with drugs which suppress the immune response. This also lessens the chances of rejection in the early stages. 
  • The recipient is kept in sterile conditions for some time after the operation the lesson the risk of infection due to his/her suppressed immune system.
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  • Used to make yoghurt and cheese.
  • Vary in shape
  • They have a cell wall.
  • No distinct nucleus
  • They reproduced rapidly


  • Used to make bread and alcoholic drinks
  • It is a single-celled organism.
  • Each cell has a nucleus, cytoplasm & a membrane surrounded by a cell wall.

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Microorganisms 2

How yeast works.

  • It can reproduce without oxygen(anaerobic respiration) to produce ethanol & carbon dioxide from glucose. 
  • This is called fermentation & it has many industrial applications. 

Glucose -----> Ethanol + Carbon Dioxide + Energy

  • Yeast can also respire using oxygen (aerobic respiration) to produce water and carbon dioxide

Glucose + Oxygen -----> Water + Carbon Dioxide + Energy

  • Aerobic respiration produces more energy and is necessary for the yeast to grow & reproduce. 
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Microorganisms 3

Yeast in baking.

  • A mixture of yeast & sugar is added to flour
  • The mixture is left in a warm place
  • The carbon dioxide from the respiring yeast makes the dough rise.
  • The bubbles of gas in the dough expand when the bread is baked, making the bread 'light'.
  • As the bread is baked, any alcohol produced during respiration evaporates off. 

Yeast in brewing. 

  • In a process called malting, the starch in barley is broken down into a sugary solution by enzymes
  • Yeast is added to the solution and fermentation takes place. In beer making, hops are added to give the beer flavour. In wine making, the yeast uses the natural sugars in the grapes as its natural energy source. 
  • Carbon dioxide is bubbled off to leave just the alcohol.
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Microorganisms 4

Using bacteria to make yoghurt. 

  • A starter culture of bacteria is added to warm milk in a fermenting vessel.
  • The bacteria ferments the milk sugar (lactose) producing lactic acid which provides a sour taste. The lactic acid causes the milk to clot and solidify into yoghurt.

Growing microorganisms.

  • Microorganisms are grown in fermenters (large vessels) and used to produce products such as antibiotics.


  • Penicillin is made by growing penicillium, a mould, in a fermenter. The medium contains sugar and other nutrients which tend to be used for growth before the mould starts to make penicillin
  • Mycoprotien is a protein rich food suitable for vegetarians & is made using Fusarium. A fungus. It is grown on starch in aerobic conditions & the biomass is harvested and purified
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Culture Mediums

Preparing a culture medium. 

  • A culture medium contains various nutrients that the microorganism may need.
  • Such as carbohydrates
  • Mineral ions
  • Vitamins.
  • Proteins
  • Agar is most commonly used as a growth medium.

Preparing uncontaminated cultures. 

  • If the cultures we want to investigate become contaminated by unwanted microorganisms, these 'rouge' microorganisms may produce undesirable substances which can prove harmful
  • It is only safe to use microorganisms if we have a pure culture of one particular species of microorganism. 
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Culture Mediums 2

Sterilisation of Petri Dishes.

  • Both petri dishes & culture mediums are sterilised using an autoclave
  • This is a pressure cooker which exposes the dishes & the agar to high temperature and high pressure to kill off any unwanted microorganisms.

Sterilisation of Inoculating Loops.

  • Inoculating loops tend to be made of nichrome wire, inserted into a wooden handle. 
  • They should be picked up like a pen and the loop and half the wire should be heated to red heat in a bunsen flame, before being left to cool for five seconds. 
  • They are then sterile and can be used safely to transfer microorganisms to the culture medium.

Sealing the Petri Dish. 

  • After the agar has been poured in and allowed to cool the petri dish should be taped to prevent microorganisms entering & labelled on the base.
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Bethany Sawyer


Top Notch, cheers mate, you really helped me out :)

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