AQA Biology unit 3

Hope this helps - a brief summary of everything you need for the AQA biology unit 3 exam =)

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  • Created by: Joanne
  • Created on: 20-05-12 17:45

Gas exchange in the lungs

Gas exchange

  • occurs in the lungs which are in the thorax (upper part of the body) separated from the abdomen (lower part) by the diaphragm
  • breathe in: ribs move up and out, diaphragm flattens
  • breathe out: ribs move down and in, diaphragm returns to dome shape
  • oxygen from the air diffuses into the bloodstream
  • carbon dioxide diffuses out of the bloodstream into the air


  • tiny air sacs in the lungs adapted for gas exchange
  • provide a large surface area for more diffusion of oxygen
  • moist lining allows gases to dissolve and diffuse more efficiently
  • rich blood supply maintains a concentration gradient
  • thin walls gives a short diffusion path
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Exchange in the gut


  • occurs in the small intestine where foods have been digested into: glucose, amino acids, fatty acids and glycerol
  • these nutrients need to be absorbed into the blood to be made available for the body cells for respiration
  • done by a combination of diffusion (passive movement of particles from an area of high concentration to an area of low concentration) and active transport (movement of particles from and area of low concentration to an area of high concentration using energy)
  • small intestine lined with finger-like projections called villi


  • provide a large surface area for more diffusion
  • walls are only one cell thick for a short diffusion path
  • good blood supply to give a steep concentration gradient
  • moist lining for substances to dissolve
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Exchange in other organisms


  • have gills - water flows over them constantly to maintain a concentration gradient where oxygen diffuses into the bloodstream
  • only works in water, if they are out of water the gill stacks stick together


  • undergo metamorphosis from a tadpole into a frog
  • adult frog has moist skin for gas exchange to take place through
  • has simple lungs for gas exchange on land


  • have spiracles which open when they need oxygen and close when they don't
  • spiracles lead to the tracheoles which come into close contact with cells for gas exchange
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Exchange in plants

Water absorption

  • surface area of roots increased by root hairs
  • water is absorbed by the root hairs by osmosis
  • mineral ions are absorbed by the root hairs by active transport

Gas exchange

  • large flat shape to increase surface area of the leaf for diffusion
  • need carbon dioxide for photosynthesis
  • constantly lose water by evaporation
  • stomata are small openings where the gases diffuse into and out of the leaf
  • stomata are opened and closed by the guard cells
  • when open, carbon dioxide diffuses in but water vapour is lost
  • closed at night as it's dark and no photosynthesis takes place
  • stomata mostly found on the underside of the leaf (less light and therefore less water vapour lost by transpiration)
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  • process by which plants lose water vapour from their leaves
  • water vapour lost through the stomata when opened
  • losing water through the stomata is a side effect of opening them to let carbon dioxide in for photosynthesis
  • transpiration is more rapid in hot, dry and windy conditions

Transpiration stream

  • water is absorbed by the roots by osmosis
  • water is pulled up through the stem by capillary action (pressure forces water to go upwards)
  • water vapour is lost by evaporation through the stomata
  • transpiration can be investigated by the potometer experiment
  • if a plant loses more water than it is replacing it will wilt
  • wilting is a protection mechanism against further water loss
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Circulatory system


  • pumps blood around the body
  • it is a double pump: one to the lungs, the other to the rest of the body
  • blood is pumped to the lungs by the right ventricle through the pulmonary artery
  • the blood returns to the lungs to the left atrium through the pulmonary vein
  • blood is pumped to the organs in the body from the left ventricle through the aorta
  • blood returns to the heart from the organs into the right atrium through the vena cava
  • the right side pumps blood to the lungs and it has thin muscular walls
  • the left side pumps blood to the rest of the body so the walls are more muscular and thicker
  • double circulatory system: one to the lungs, one to the rest of the body
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Blood vessels


  • carry oxygenated blood away from the heart
  • under high pressure
  • thick muscular walls and elastic to withstand pressure and create a pulse
  • rarely contain valves


  • carry deoxygenated blood towards the heart
  • under low pressure
  • thin walls with little muscle
  • contain valves to prevent the back flow of blood


  • carry blood into organs and back out again
  • one cell thick in size with small amount of pressure and no valves
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Red blood cells

  • contain haemoglobin which combines with oxygen to form oxyhaemoglobin
  • no nucleus and biconcave to increase surface area for more haemoglobin
  • carries oxygen to cells for respiration

White blood cells

  • fight against disease - involved in the immune system


  • liquid part of the blood which transports glucose, amino acids, carbon dioxide, urea and products of digestion around the body


  • fragments of cells involved in the clotting of blood
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  • muscles contract more and so need more energy from respiration
  • glucose + oxygen = water + carbon dioxide (+energy)
  • this is aerobic respiration - it involves oxygen
  • exercise means the muscles need more oxygen and glucose for more aerobic respiration and therefore more energy


  • rate and depth of breathing increase to increase rate of gas exchange, more oxygen is diffused into the blood and more carbon dioxide diffused out
  • heart rate increases to increase the rate of blood flow to the muscles
  • arteries dilate (get wider) so more blood flows to muscles
  • glycogen stores in the muscles are converted into glucose
  • changes increase the rate and amount of glucose and oxygen to muscles and the removal of carbon dioxide
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Anaerobic respiration

Anaerobic respiration

  • muscles exercising for long periods or time and become fatigued due to: oxygen supply gets low or they stop contracting efficiently
  • anaerobic respiration is respiration without oxygen
  • glucose = lactic acid (+energy)
  • it releases less energy than aerobic respiration but means the muscles can contract and exercise for longer

Oxygen debt

  • lactic acid is poisonous and so needs to be removed
  • lactic acid + oxygen = carbon dioxide + water
  • after exercise, we continue to breathe faster to get in the oxygen needed to oxidise the lactic acid in our muscles to pay the oxygen debt
  • the fitter you are, the less oxygen debt you have or the quicker your breathing rate returns back to normal after exercise
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  • involved in homeostasis - maintaining internal conditions inside the body
  • control water content and ion content in the blood
  • remove urea from the blood (a toxin produced by the liver from the breakdown of amino acids)


  • where substances are removed from the blood and released in the urine
  • the blood is filtered - everything is removed apart from proteins and the red blood cells which are too big for the filters
  • all glucose is reabsorbed by active transport
  • some ions are reabsorbed by active transport
  • some water is reabsorbed depending on the levels in the body (ADH controls how permeable the kidneys are to water loss)
  • urine is released from the kidney containing: all urea, some ions and water which is stored in the bladder
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Kidney failure

  • kidney failure is fatal as urea levels build up in the body and it is poisonous
  • dialysis acts as an artificial kidney


  • blood vessels connected to machine
  • blood flows between dialysis fluid by a partially permeable membrane
  • dialysis fluid contains same concentration of useful substances in the blood
  • this ensures things like glucose are not removed from the blood by diffusion
  • dialysis fluid contains no urea so there is a steep concentration gradient and urea diffuses into the dialysis fluid from the blood
  • dialysis fluid is constantly circulated to remain the steep concentration gradient of the urea diffusing out of the blood into it
  • dialysis must be done at regular intervals and a restricted diet is needed
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Kidney transplant

Kidney transplant

  • relies on a healthy person donating their kidney or the kidney of a deceased person carrying a donor card
  • person with kidney failure is given a new kidney and no longer has to do dialysis
  • donor kidney may be rejected by the immune system so to prevent this, a close tissue match is needed
  • antigens on cells of donated kidney may be different to person with kidney failure so the white blood cells may destroy the new kidney
  • to prevent this, the close tissue match is needed and the patient will need to take immunosuppressant drugs to suppress the immune system from destroying the new kidney
  • kidney transplants mean people can live more normal lives
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Dialysis V kidney transplant


  • no major surgery needed
  • no waiting lists - prevents people from dying
  • restricted diet
  • restricts normal life - takes about 8 hours several times a week

Kidney transplant

  • no need for dialysis
  • can lead relatively normal life
  • diet doesn't need to be carefully controlled
  • major surgery needed - risk of infection
  • immunosuppressant drugs need to be taken for life
  • donated kidney may only last for 9 years
  • shortage of kidney donors
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  • process of life forms producing other life forms
  • before this, people believed spontaneous generation where living things can arise from non-living things


  • Spallanzani boiled broths, sealed them, no microbes grew, criticised as scientists said air is needed for spontaneous generation
  • Schwann boiled air and broth, no microbes grew in the broth, criticised as scientists said something had been destroyed in the air that is needed for spontaneous generation
  • Pasteur developed the swan neck flask, air could enter flask but microbes got stuck in the curved neck, boiled the broth and the broth remained clear as no microbes grew because they couldn't get in, when the neck was broken microbes grew in - evidence that biogensis occurs
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Bacteria products


  • milk is heat treated to destroy microbes then cooled
  • started culture added of bacteria, ferments lactose sugar in milk to make lactic acid
  • acid causes milk to clot and solidify into yoghurt
  • sterilised flavour added e.g. fruit


  • culture of bacteria added to milk
  • bacteria produces solid curds in the milk
  • curds are separated from the whey by enzyme
  • curds heated and dried and cheese allowed to ripen
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  • yeast is a fungus and is used in bread
  • respires aerobically to produce carbon dioxide which makes the bread rise

Alcoholic drinks

  • barely grains soaked in water, germinate and starch is broken down into sugar by enzyme (malting)
  • yeast added to sugary solution and ferments it (respires anaerobically)
  • hops may be added to improve the flavour, solution left to develop in flavour and yeast settles to the bottom
  • when making wine, grapes are used as the carbohydrate source and the wine is left to mature at the end
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  • large scale microbe production for food or antibiotics
  • large vessel including: nutrient, air supply, temperature and pH probe, water cool jacket (keeps temperature down as microbes respire and release energy) and a stirrer to maintain the temperature and keep the microbes evenly distributed


  • mycoprotein: made from the fungus Fusarium, uses aerobic respiration, grown on starch, mycoprotein is a protein source for vegetarians
  • penicillin: antibiotic made from Penicillium (fungus), makes penicillin after nutrients used up
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  • anaerobic fermentation of plant or waste materials
  • mainly methane
  • can be produced on a large scale or small scale
  • microorganisms break down waste and produce biogas
  • needs high temperature for efficient production
  • environmentally friendly, cheap to use, initial costs expensive, useful in developing countries, waste products can be used as fertiliser, reduces the use of fossil fuels
  • biogas generator includes: inlet for waste material, outlet for digested material (fertiliser) and an outlet for the biogas
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  • ethanol is made by anaerobic fermentation of sugar
  • glucose = ethanol + carbon dioxide (+energy)
  • can be made from maize or sugar cane
  • sugar cane juices can be fermented straight to ethanol
  • maize needs to be broken down by the action of carbohydrase enzymes into sugar then it can be fermented into ethanol
  • the advantage of using maize is that it can be grown nearly anywhere
  • ethanol is then distilled
  • gasohol is a mixture of ethanol and petrol
  • advantage is that it is more environmentally friendly than petrol as it is carbon neutral and doesn't produce harmful acid rain
  • disadvantage is that the rainforests get cut down to make room for plants to be grown to be harvested to make ethanol
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Growing microbes

Growing microbes

  • grown in culture medium - carbohydrate is the energy source along with other nutrients
  • these nutrients are added to the agar jelly which is sterilised and poured into a Petri dish and is left to set
  • all equipment needs to be sterilised to prevent contamination to the culture
  • inoculating loops are used to transfer the microbes to the Petri dish and they are sterilised by being passed through a flame
  • the lid of the Petri dish should be sealed but not completely to allow the microbes to respire and grow
  • in schools, cultures of microbes should be incubated at a maximum of 25 degrees celsius as this prevents the likelihood of pathogens growing that may be harmful to humans
  • industrial conditions may use higher temperatures to increase the growth rate of microbes
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Josh Silversides


great resource, would only ask for diagrams to improve

Abby Kay


really good, include everything I need to know for the exam tomorrow!



so clever and easy to use 

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