AQA GCSE Biology Unit 3 Summary Notes by huangyuarong


									   AQA GCSE Biology – Unit 3 summary notes

GCSE Biology
   Unit 3
Summary Notes

                    Page 1                   1/13/2013
                           AQA GCSE Biology – Unit 3 summary notes

       13.1 How do dissolved materials get into and out of animals and plants?

Absorption of substances by cells

   Dissolved substances move by diffusion, osmosis and active transport.

Diffusion (covered in more detail in unit 2):
 The movement of particles in a liquid or gas from an area where they are in higher
    concentration to an area where they are in lower concentration.
 The greater the difference in concentration, the faster the rate of diffusion.
 2 examples of diffusion through the cell membrane:
       o Oxygen required for respiration diffuses into cells.
       o Carbon dioxide produced by respiration diffuses out of cells.


Osmosis (covered in more detail in unit 2):
 The diffusion of water from a dilute to a more concentrated solution through a partially
  permeable membrane that allows the passage of water molecules.
 2 examples:
     o Absorption of water by the root hair cells in roots in plants.
     o Absorption of water by epithelial cells in the small intestine.

Active Transport:
 The process by which cells absorb substances against a concentration gradient.
 The substance being transported attaches to large protein molecules in the cell
 Energy released through respiration is used to change the shape of the protein.
 This releases the substance on the other side of the membrane.
 This process enables cells to absorb substances from very dilute solutions.
 2 examples:
      o Absorption of mineral ions by the root hair cells in roots in plants.
      o Absorption of glucose by epithelial cells in the small intestine.

                                        Membrane          Energy


                                            Page 2                                     1/13/2013
                            AQA GCSE Biology – Unit 3 summary notes

Gas Exchange in the lungs

   The lungs are in the upper part of the body (thorax).
   They are protected by the ribcage.
   They are separated from the lower part of the body (abdomen) by the diaphragm.

   The breathing system takes air into and out of the body.
   Oxygen from the air diffuses into the bloodstream.
   Carbon dioxide can diffuse out of the bloodstream into the air.
   The alveoli provide a very large, moist surface, richly supplied with blood capillaries so
    that gases can readily diffuse into and out of the blood.

                                             Page 3                                      1/13/2013
                             AQA GCSE Biology – Unit 3 summary notes

       Many small spherical alveoli provide a large surface area.
       The moist surface helps oxygen to dissolve so that it can diffuse into the cells.
       The capillaries are very close to the alveoli so that the gases have a short distance
        to diffuse.
       Concentration gradients are maintained by:
           o The blood removing oxygen from and bringing carbon dioxide to the lungs.
           o Breathing, which replenishes oxygen and removes carbon dioxide.
                   Muscles in the diaphragm and between the ribs are used to increase
                      and decrease the volume in the thorax.

Absorption of nutrients in the small intestine

   Food in the small intestine has been digested into:
        o Glucose
        o Amino acids
        o Fatty acids
        o Glycerol
   These nutrients need to be absorbed into the blood across the wall of the small
   They are absorbed by diffusion and active transport.
   The wall of the small intestine is covered with finger-like extensions called villi.

   Many villi provide a large surface area.
   The wall is only one cell thick so the diffusion pathway is short.
   Villi have an extensive network of capillaries to absorb the products of digestion.
   Concentration gradients are maintained because the blood removes the absorbed

                                               Page 4                                      1/13/2013
                            AQA GCSE Biology – Unit 3 summary notes

Absorption of water by roots

   The surface area of the roots is increased by root hairs.
   Most of the water and mineral ions are absorbed by root hair cells.
   Water is absorbed by osmosis.
   Most of the mineral ions are absorbed by active transport.

Gas Exchange in the leaf

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

   Most photosynthesis takes place in the palisade cells.
   Carbon dioxide needs to reach the palisade cells.
   Plants have stomata to obtain carbon dioxide from the atmosphere.
   This carbon dioxide is used in photosynthesis.

                                             Page 5                                     1/13/2013
                            AQA GCSE Biology – Unit 3 summary notes

   The size of stomata is controlled by guard cells, which surround them.
   Stomata open during daylight hours, to enable carbon dioxide to diffuse in.


   The process by which plants lose water vapour from the surface of their leaves.
   It evaporates into the air spaces in the leaf, and then diffuses out through the stomata.

                                              Page 6                                      1/13/2013
                           AQA GCSE Biology – Unit 3 summary notes

   Transpiration is more rapid in hot, dry and windy conditions:
        o Heat causes the water to evaporate quicker.
        o Dry conditions increases the water vapour concentration gradient.
        o Wind moves the water vapour away from the leaf, maintaining the concentration
   Most of the water lost by transpiration leaves through the stomata.
   Stomata close when it is dark, when carbon dioxide is not required.
   This reduces the amount of water lost by the plant at a time when it is not needed for
   If plants lose water faster than it is replaced by the roots, the stomata can close to
    prevent wilting.

                                            Page 7                                     1/13/2013
                            AQA GCSE Biology – Unit 3 summary notes

             13.2 How are dissolved materials transported around the body?

   Substances are transported around the body by the circulation system:
        o The heart
        o The blood vessels
        o The blood
   They are transported from where they are taken into the body to the cells, or from the
    cells to where they are removed from the body.
   Substances are taken in:
        o In the digestive system
        o In the lungs
   Substances are removed:
        o From the lungs
        o By the kidneys

The Heart

   The heart pumps blood around the body.
   Its walls are mainly made of muscle.
   This muscle contracts to pump blood through the circulation systems.
   It is a double pump, because there are two separate circulation systems:
         o One to the lungs
         o One to all the other organs of the body.
   The blood passes through the heart twice in order to pass round both of these
    circulation systems.

       Blood is pumped to the lungs from the right ventricle through the pulmonary artery.
       The blood returns from the lungs to the left atrium through the pulmonary vein.
                                             Page 8                                      1/13/2013
                             AQA GCSE Biology – Unit 3 summary notes

       Blood is pumped to the organs of the body from the left ventricle through the aorta.
       Blood returns to the heart from the organs into the right atrium through the vena

Blood vessels

   Blood flows from the heart to the organs through arteries and returns through veins.

   In the organs, blood flows through capillaries.

                                              Page 9                                       1/13/2013
                            AQA GCSE Biology – Unit 3 summary notes

   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.

          Arteries                     Capillaries                      Veins
Carry blood away from the    Carry blood away from           Carry blood away from the
heart to the organs.         arteries into organs then       organs back to the heart.
                             back into veins.
High blood pressure.         Decreasing blood pressure.      Low blood pressure.

Elastic to withstand high    No elastic tissue – walls one Not elastic.
blood pressure.              cell thick.

Muscular walls, to create a No muscle – walls one cell       Very little muscle.
pulse, to maintain blood    thick.

Rarely contain valves.       No valves.                      Contain valves to control
                                                             the direction of blood flow.


   Blood has two main components:
       o Plasma
       o Cells


   The liquid part of the blood.
   Blood plasma transports:
       o Carbon dioxide from the organs to the lungs
       o Soluble products of digestion from the small intestine to other organs:
                Glucose
                Amino acids
       o Urea from the liver to the kidneys.

Blood cells:

                                             Page 10                                        1/13/2013
                             AQA GCSE Biology – Unit 3 summary notes

   White blood cells – involved in the immune system (covered in Unit 1).

   Platelets – fragments of cells; involved in blood clotting.

   Red blood cells transport oxygen from the lungs to the organs.

   Red blood cells have no nucleus.
   They are packed with a red pigment called haemoglobin.
   In the lungs:
        o Oxygen diffuses into the red blood cells.
        o Haemoglobin combines with oxygen to form oxyhaemoglobin.
   In the other organs:
        o Oxyhaemoglobin splits up into haemoglobin and oxygen.
        o Oxygen diffuses out of the red blood cell.
   They have a biconcave shape.
   This increases their surface area to increase rate of diffusion across the cell membrane.

                                               Page 11                                   1/13/2013
                            AQA GCSE Biology – Unit 3 summary notes

      13.3 How does exercise affect the exchanges taking place within the body?

The role of respiration during exercise

   Muscles contract to move the bones in our bodies.
   Respiration releases energy, which is used to contract the muscles:
    (Refer to Unit 2 for more details on aerobic respiration.)

glucose + oxygen                          carbon dioxide + water + energy released

   When we exercise, our muscles contract more quickly and with more force.
   This requires more energy.
   This requires more glucose and oxygen.
   Also, more carbon dioxide is created which needs to be removed.
   The human body needs to react to the increased demand for energy during exercise.

Changes during exercise

   During exercise a number of changes take place:
        o The rate and depth of breathing increases.
                This increases the rate of gaseous exchange.
                More oxygen is taken into the blood.
                More carbon dioxide is removed from the blood.
        o The heart rate increases.
                This increases rate of blood flow to the muscles
        o The arteries supplying the muscles get wider (they dilate).
                This allows more blood to enter the capillaries in the muscles.
   All of these changes increase the supply of sugar and oxygen to the muscles and
    increase the rate of removal of carbon dioxide from the muscles.


   Glucose is stored as glycogen is in the muscles (refer to unit 2).
   During exercise, glycogen is broken down into glucose in the muscles.
   This increases the amount of glucose that can be respired.

Anaerobic respiration

   If muscles are subjected to long periods of vigorous activity they become fatigued,
         o Oxygen supply gets too low.
         o They stop contracting efficiently.
   If insufficient oxygen is reaching the muscles they use anaerobic respiration to obtain
   Anaerobic respiration is the incomplete breakdown of glucose and produces lactic acid.
   As the breakdown of glucose is incomplete, much less energy is released than during
    aerobic respiration.

    glucose                        lactic acid + small amount of energy released

   However, lactic acid is poisonous. We can only tolerate small amounts in our body.
   During and after exercise, we breathe heavily to take in extra oxygen to oxidise the lactic
                                             Page 12                                      1/13/2013
                            AQA GCSE Biology – Unit 3 summary notes

   The extra oxygen is called the oxygen debt

    lactic acid + oxygen                         carbon dioxide + water

   The heart continues to pump faster.
   The breathing rate remains high.
   This delivers the extra oxygen to the muscles.
   This pays back the oxygen debt.

                                             Page 13                      1/13/2013
                           AQA GCSE Biology – Unit 3 summary notes

13.4 How do exchanges in the kidney help us to maintain the internal environment in
        mammals and how has biology helped us to treat kidney disease?

   Most humans have two kidneys.
   Kidneys have an important role in homeostasis.
    (More detail on homeostasis in Unit 1 and 2.)
   They control the water content of the blood.
   They control the ion (salt) content of the blood.
   They remove the urea from the blood.
       o Urea is a toxin produced when the liver breaks down excess amino acids.

   Each kidney receives blood from the aorta (via a renal artery).
   The artery branches into millions of capillaries inside each kidney.
   Each kidney also contains about one million microscopic tubules, which are responsible
    for forming urine.

                                            Page 14                                   1/13/2013
                              AQA GCSE Biology – Unit 3 summary notes

   A healthy kidney produces urine by:

    1.   Filtering the blood
        Blood enters the capillaries under high pressure.
        Most of the liquid leaves the blood and enters a tubule.
        The cells and large molecule remain in the blood.
        The blood becomes very concentrated.

    2. Reabsorbing all the sugar
     All of the sugar is reabsorbed from the tubule, back into the blood by active transport.
     It moves against a concentration gradient.

    3. Reabsorbing the dissolved ions needed by the body
     Some of the ions are also reabsorbed back into the blood by active transport.
     Some salt is left behind to balance what the body needs.

    4. Reabsorbing as much water as the body needs
     Water is reabsorbed into the blood by osmosis.
     This occurs because there is a much higher solute concentration (lower water
       concentration) in the blood than in the tubule.
     The body balances how much water it needs by changing how much water is lost in
       the urine.

    5.   Releasing urine
        Urine is released from the kidney.
        It contains urea, excess ions and water.
        Urine is stored in the bladder, before being expelled from the body.

Treating Kidney Failure

   People whose kidneys do not function properly can die because toxic substances
    accumulate in their blood.
   Their lives can be saved by using dialysis machines or by having a healthy kidney

 The blood vessels in the arm are connected to a dialysis machine.
 In the dialysis machine a person’s blood flows between partially permeable membranes.
 The dialysis fluid contains the same concentration of useful substances as the blood.
 This ensures that glucose and useful mineral ions are not lost.
 There is a high concentration of urea in the person’s blood, and no urea in the dialysis
 There is a concentration gradient of urea between the blood and the dialysis fluid.
 Urea passes out from the blood into the dialysis fluid by diffusion.
 Treatment by dialysis restores the concentrations of dissolved substances in the blood
   to normal levels.
 It has to be carried out at regular intervals.

                                               Page 15                                   1/13/2013
                            AQA GCSE Biology – Unit 3 summary notes

 Kidney transplants
  A kidney transplant enables a diseased kidney to be replaced with a healthy one from a
  However, the donor kidney may be rejected by the immune system:
       o The antigens on the cells of the donated kidney may be different to the recipient’s
       o His white blood cells will be stimulated and the immune system will attempt to
           destroy the new kidney.
  To prevent rejection of the transplanted kidney:
       o A donor kidney with a tissue-type similar to that of the recipient is used.
       o The recipient is treated with drugs that suppress the immune system.

                     Advantages                         Disadvantages
Dialysis             No major surgery needed.           Diet needs to be controlled carefully.

                     No waiting lists – it prevents     Restricts normal life – it takes about 8
                     people dying.                      hours, several times a week.
Kidney transplant    No need for dialysis.              Major surgery has many risks.

                     The recipient can lead a           Possibility of rejection.
                     relatively normal life.
                                                        Immunosuppresant drugs need to be
                     Diet does not need to be           taken for life.
                                                        The donated kidney only lasts for
                                                        about 9 years.

                                                        Some religious groups do not agree
                                                        with organ donation.

                                              Page 16                                        1/13/2013
                            AQA GCSE Biology – Unit 3 summary notes

             13.5 How are microorganisms used to make food and drink?

 The accepted scientific principle is that life can only be passed on by living things.
     o Biogenesis is the process of life forms producing other life forms.
 Until the 19th Century, people believed in spontaneous generation (abiogenesis):
     o That life can arise from non-living things.
     o Eg the appearance of maggots or mould in food.

   Spallanzi (1765):
      o Boiled flasks of meat broth to kill microorganisms.
      o The flasks were sealed.
      o No microorganisms grew in the broth.
      o Scientists criticised his work – they asserted that spontaneous generation
          required fresh air in the flask.

   Scwhann (1836):
      o Used sulphuric acid and heated glass tubes to sterilise air.
      o He showed that the amount of oxygen remained the same.
      o This was passed through boiled broth in a flask.
      o No microorganisms grew in the broth.
      o Then, when exposed to normal air, microorganisms grew.
      o Scientists criticised this work too – they asserted that something had been
         destroyed in the air that enabled life to form.

   Pasteur (1864):
      o Developed a swan-neck flask.
      o Air could enter but microbes were trapped in curved neck of the flask.
      o He boiled the broth inside the flask.
      o The broth remained clear.

       o When the neck was broken off, the broth went cloudy.

       o Scientists accepted this as proof that spontaneous generation does not occur.

                                             Page 17                                       1/13/2013
                           AQA GCSE Biology – Unit 3 summary notes

Use of microorganisms to make food and drink

   People from many different cultures have known for thousands of years how to use
    microorganisms to make various types of food and drink.

Use of bacteria:
 Yoghurt production
  1.     Take milk and standardise its fat content – mix skimmed and full fat milk.
  2.     Pasteurise the milk - kills most contaminating microorganisms.
  3.     Add the bacterial starter culture.
  4.     Incubate at warm temperature.
  5.     The sugar lactose is converted into lactic acid.
  6.     This causes the milk to clot and solidify into yoghurt.

   Cheese production
    1.   Standardise the fat content of the milk.
    2.   Pasteurise the milk.
    3.   Add the bacterial starter culture.
    4.   This creates lactic acid.
    5.   Add an enzyme that separates the curds (solid) from the whey (liquid).
    6.   Remove solid curds, squash them and allow the cheese to ripen.

Use of yeast:
 Yeast is a single-celled organism.
 The cells have a nucleus, cytoplasm and a membrane surrounded by a cell wall.

   Anaerobic respiration in yeast:
     Yeast can respire without oxygen.
     This produces carbon dioxide and ethanol (alcohol).
     This is called fermentation.

   Aerobic respiration in yeast:
     In the presence of oxygen yeast carries out aerobic respiration and produces carbon
       dioxide and water.
     Aerobic respiration provides more energy and is necessary for the yeast to grow and
                                            Page 18                                    1/13/2013
                            AQA GCSE Biology – Unit 3 summary notes

   Alcoholic drinks
     In brewing beer and wine-making, carbohydrates are used as an energy source for
       yeast to respire.
     Brewing beer:
          o Malting:
                   The barley grains are soaked in warm water.
                   They start to germinate.
                   Enzymes in barley grains break down starch into a sugary solution
                   This sugary solution is extracted.
          o Fermention:
                   Yeast and the sugar are mixed in a fermenter.
                   It is kept warm.
                   An air lock prevents oxygen getting in, ensuring anaerobic respiration.
                   Alcohol is produced.
          o Flavour development:
                   Hops are then added to give the beer flavour.
                   The beer is left for a while for the flavour to develop.
                   The yeast settles to the bottom.
                   The beer is bottled or put into barrels.

       Making wine:
          o Pressing:
                 The grapes are pressed to release their juice.
          o Fermentation;
                 The juice is mixed with yeast and water in a fermenter.
                 The yeast uses the natural sugars in the grapes as its energy source.
                 An air lock is used to ensure anaerobic respiration.
                 Alcohol is produced.
                 The yeast settles to the bottom.
          o Maturing.
                 The wine is put into bottles.
                 It is stored for the wine to mature.

                                             Page 19                                    1/13/2013
                            AQA GCSE Biology – Unit 3 summary notes

       13.6 What other useful substances can we make using microorganisms?


 Microorganisms can be grown in large vessels called fermenters to produce useful
  products such as antibiotics.
 Industrial fermenters usually have:
       o An air supply - to provide oxygen for respiration of the microorganisms
       o A stirrer - to keep the microorganisms in suspension and maintain an even
       o A water-cooled jacket - to remove heat produced by the respiring
       o Instruments to monitor factors such as pH and temperature.

Antibiotic production

 The antibiotic, penicillin, is made by growing the mould Penicillium in a fermenter.
 The medium contains:
    o Sugar
    o Other nutrients eg a source of nitrogen.
 The Penicillium only starts to make penicillin after using up most of the nutrients for

                                              Page 20                                       1/13/2013
                            AQA GCSE Biology – Unit 3 summary notes

Mycoprotein production

   The fungus Fusarium is used to make mycoprotein (Quorn).
   This is a protein-rich food suitable for vegetarians.
   The fungus is grown on starch in aerobic conditions.
   The biomass is harvested and purified.

Fuel production

 Fuels can be made from natural products by fermentation.

 Biogas:
   o Produced by anaerobic fermentation of a wide range of plant products or waste
      material containing carbohydrates.
   o It is mainly methane.
   o On a large scale industrial waste can be used:
           From sugar factories.
           From sewage works.
   o On a small scale, biogas generators can be used to supply the energy needs of
      individual families or farms.
   o Organic waste can be used:
           Plant waste from crops.
           Animal faeces and urine.
   o Many different microorganisms are involved in the breakdown of materials in biogas
   o There are many environmental and economic advantages:
           It is a useful way to recycle waste material.
           Initial set up costs are expensive, but the raw materials for biogas production
             are very cheap (or free).
           The fuel is readily available.
           This is particularly useful in developing countries.
           Waste products from the process can be used as fertilisers.
           It reduces the use of fossil fuels.
   o There are various different designs:

                                             Page 21                                    1/13/2013
                          AQA GCSE Biology – Unit 3 summary notes

   Floating dome design

               Advantages                                   Disadvantages
Floating dome stabilises the gas pressure     Metal construction:
                                                     Expensive to set up
Straightforward to use.                              Rusts easily

   Fixed dome design

               Advantages                                    Disadvantages
Fewer metal parts:                            Gas pressure fluctuates.
      Less expensive to set up                Less straightforward to control.
      Does not rust                           Gas leakage is more likely

 Ethanol-based fuels:
   o Produced by the anaerobic fermentation of:

                                            Page 22                              1/13/2013
                            AQA GCSE Biology – Unit 3 summary notes

             Sugar cane juices, or
             Glucose derived from maize starch.
                   The starch is broken down using carbohydrase enzymes.
   o   The sugars are fermented using yeast.
   o   The ethanol is distilled from the products of the fermentation.
   o   This can be used in motor vehicle fuels.
   o   It can also be mixed with petrol to make gasohol.

                 Advantages                                    Disadvantages
Less fossil fuels used.                         Large quantities of plant material is required.

It is a renewable supply of energy.             This uses a lot of land to grow the crops.

No toxic gases produced with combustion.        Only certain countries have the climate and
                                                the land available to grow them eg Brazil.
Carbon neutral - The carbon dioxide
released is balanced by the carbon dioxide      Not all of the plant material can be used.
removed from the atmosphere by the plant.
                                                There are large amounts of plant waste

                                             Page 23                                         1/13/2013
                             AQA GCSE Biology – Unit 3 summary notes

            13.7 How can we be sure we are using microorganisms safely?

 Microorganisms are organisms that can only be seen with a microscope.
 They are mainly single-celled:
     o Bacteria
     o Fungi
     o Viruses
 Microorganisms need to be handled safely in laboratories.
 If they are contaminated, the other microorganisms that are present may be pathogens:
     o They may produce harmful substances (toxins).
     o They may cause disease.
 So it is only safe to use microorganisms if we have a pure culture containing only one
  particular species of microorganism.

Culturing microoganisms

 To study microoganisms, they need to be cultured.
 This provides them with the conditions they need to reproduce quickly.
 A typical culture medium contains:
     o Carbohydrates as an energy source.
     o Mineral ions.
     o In some cases supplementary protein and vitamins.
 These nutrients are often contained in an agar medium – a type of jelly made from
 When agar is heated up it is liquid.
 It can be poured into a Petri dish.
     o A circular plastic or glass dish with a lid:

 The agar solidifies when left to cool.

Growing pure cultures

 Petri dishes and culture media must be sterilised before use to kill unwanted
    o An autoclave is used.
    o This uses steam at high pressure to kill all microoganisms.
 Inoculating loops are used to transfer microorganisms to the media.
    o These must be sterilised by passing them through a flame:

                                              Page 24                                 1/13/2013
                           AQA GCSE Biology – Unit 3 summary notes

 The lid of the Petri dish should be sealed with adhesive tape to prevent microorganisms
  from the air contaminating the culture.

 It should not be sealed all the way round the edge, as this would prevent oxygen getting
 In school and college laboratories, cultures should be incubated at a maximum
  temperature of 25 °C.
     o This greatly reduces the likelihood of pathogens growing that might be harmful to
     o In industrial conditions higher temperatures can produce more rapid growth.

                                            Page 25                                     1/13/2013

To top