Biology A-Level AQA Chapter 6

Biology A-Level AQA Chapter 6

1. 6.1 Exchange between organisms and their environment

   • Exchange can occur
     • Activley
       • Active transport
       • Co-Transport
     • Passivley
       • Osmosis
       • Diffusion
   • Most cells too far from exchange surface for diffusion alone to supply / remove their tissue fluid with the various materials needed to keep its compisition relativley constant
   • SA:Vol Ratio
     • Simple diffusion across outer surface only works for small organisms
     • Larger organisms evolved
       • Flattened shape so no cell is far from surface
         • Flatworm
         • Leaf
       • Specialised exchange surface with large area to increase SA:Vol ratio
         • Lungs in mammal
         • Gills in fish
   • Specialised exchange surface features
     • Large SA:Vol ratio >increases rate of exchange
     • Thin > short diff. distance > materials can cross rapidly
     • Selectivley permeable > allows selected material to cross
     • Movement of environmental medium > maintain diff. grad
     • Transport system > ensure movement of internal medium
   • Diffusion = (surface area x difference in conc.) / length of diff. path

2. 6.2 Gas exchange in single-celled organisms and insects

   • Single-Celled
     • Large SA:Vol ratio
     • Diffusion of oxygen across body surface
       • Body surface covered by cell-surface membrane
     • CO2 diffuses out in same way
   • Insects
     • Increased SA needed for diffusion increases waterloss
     • Internal network -> Tracheae
       • Supported by strengthened rings
       • Divide into smaller dead-end tubes -> tracheoles
         • Extend through body tissue of insect
           • Atmospheric air brought directly to tissues
     • Gases move in and out in different ways
       • Along a diffusion gradient
         • Conc. at end of tracheoles fall
           • Creates diff. grad. causing oxygen to diffuse from atmosphere to cells (through tracheae and tracheoles)
             • CO2 produced creates conc. grad. in opposite direction
               • CO2 diffuses from cells into atmosphere along tracheoles and tracheae
         • Diffusion in air more rapid than in water so quicker diffusion
       • Mass transport
         • Muscle contration squeezes tracheae enables mass movement of air in and out
         • Speeds up exchange of repiratory gases
       • The ends of the tracheoles are filled with water
     • Spiracles
       • Open and close by valve
         • Stops water loss
     • Insects only small due to short diff. pathway

3. 6.3 Gas exchange in fish

   • Gills
     • Behind head
     • Gill Filaments
       • Stack up at right angle to lamelle
         • Increase surface area
     • Water in through mouth
       • Forced over gills
         • Out opening
   • Coounter-current flow
     • Oxygen loaded blood meets water which has max conc. of oxygen
       • Diffusion of oxygen from water to blood
     • Blood with little oxygen meets water with little oxygen
       • Diffusion of oxygen from water to blood

4. 6.4 Gas exchange in the leaf of a plant

   • Gases produced in photosynthesis can be used in respiration and vice versa
     • This reduces gas exchange
     • Volume of gases exchanged depends on rates of photosynthesis and respiration
     • Most Co2 used in photosynthesis comes from the external air and most oxygen produce diffuses out of the plant
     • When photosynthesis isn't occuring oxygen diffuses in and Co2 out
   • Structure of a plant leaf and gas exchange
     • No living cell far from external air
       • Source of oxygen and CO2
     • Diffusion takes place in air
       • More rapid than water
     • Air spaces in leaf have large surface area compared to tissue volume
     • Adaptions for rapid diffusion
       • Stomata
         • Underside of leaf
         • Guard cells up and close them to prevent water loss
         • Short diffusion pathway as no cell is far from stoma
       • Interconnecting air spaces through mesophyll
         • gases regually come into contact with mesophyll cells
       • Large SA of mesophyll cells for rapid diffusion

5. 6.5 Limiting Water loss

   • Insects
     • Small SA:Vol Ratio
       • Minimise area from where water is lost
     • Waterproof Covering
       • rigid outer-skeleton of chitin covered in waterproof cuticle
     • Spiracles
       • Can be closed to reduce waterloss
   • Plants
     • Thick, waxy cuticle
       • thicker cuticle = less water escapes
     • Rolling leaves
       • protects stomata and lower epidermis
       • traps a region of air within rolled leaf
         • Region becomes saturated by water vapour
           • no wp grad. inside and outside so no water loss
     • Hairy leaves
       • Traps still moist air next to leaf surface
         • Water potential grad. between inside and outside reduced
           • Less water lost by evaporation
     • Stomata in pits or grooves
       • trap still moist air
         • reduce wp grad.
     • Reduced SA:Vol ratio
       • Slower rate of diffusion
       • Must be balanced for photosynthesis needs
   • xerophytes
     • Plants which have adapted to living where there is little water supply, without their adaptions they would die

6. 6.6 Structure of the human gas exchange system

   • aerobic organisms need oxygen to release ATP and CO2 must be removed as build up is harmful
     • There is a large volume which must be removed and absorbed because...
       • large organisms have large volume of living cells
       • they maintain high body temp due to high metabolic and respiratory rates
       • Due to this animals have specialised exchange surfaces called lungs for efficent exchange
   • Mammalian lungs
     • located inside body because...
       • air not dense enough to support and protect delicate structure
       • body as a whole would otherwise loose a lot of water
     • supported and protected by ribcage
       • ribs can be moved by muscles so lungs are ventilated by tidal stream of air ensuring air is constantly replenished
     • structure and functions of main parts
       • lungs
         • lobed structure made of series of tubules > bronchioles, which end with air sacs > alveoli
       • trachea
         • flexible airway supported by rings of cartilage
           • cartilage prevents trachea collapsing when air pressure falls
         • walls made of muscle lined with epithelium and goblet cells
       • bronchi
         • two divisions of trachea leading to a lung
         • similar to trachea and produce mucus to trap dirt and have cilia to move such to throat
         • supported by cartilage
       • bronchioles
         • branching subdivisions of bronchi
         • walls = muscle lined with epithelia cells
         • constrict to control flow of air in and out
       • alveoli
         • air sacs
         • collagen and elastic fibres between them
         • fill with air when breathing in and spring back when breathing out to expel CO2 rich air

7. 6.7 The mechanism of breathing

   • Inspiration
     • External intercostal muscles contract and internal relax
       • Ribs move up and out increasing thorax volume
         • Diaphragm muscles contract causing it to flattern increasing volume of thorax
           • Increased volume = reduction in pressure
             • Atmospheric pressure greater than pulmonary pressure
               • Air forced into lungs
   • Expiration
     • Internal intercostal muscles contract, external relax
       • ribs move down and in decreasing thorax volume
         • diaphragm muscles relax so pushed up by abdomen contents, thorax volume further decreased
           • decreased volume = increased pressure
             • pulmonary pressure greater than atmospheric pressure
               • air forced out of lungs

8. 6.8 Exchange of gases in the lungs

9. 6.9 Enzymes and digestion

10. 6.10 Absorption of the products of digestion

    • Ileum
      • Villi
        • Microvilli
          • Increase SA
        • Thin walled-> reduce diff. distance
        • Increase SA for diff.
        • able to move > helps maintain diff. grad.
        • Well supplied with blood to carry away absorbed molecules
    • Amino acids and monosaccharides
      • proteins produce amino acids
      • carbohydrates produce monosaccharides (e.g. glucose, fructose, galactose)
      • Absorbed by diffusion and co-transport
    • Triglycerides
      • Monoglycerides and fatty acids are formed in digestion
        • Stay in association with bile salts which initially emulsifies the lipid droplets
          • Forms micelles
            • Via movement of material within lumen of ileum micelle contact epithelial cells lining the villi
              • Micelles break down releasing monoglycerides and fatty acids
                • Easilly diffused across cell-surface membrane into epithelial cells
                  • Once inside are transported to endoplasmic reticulum
                    • recombine to form triglyceride
                      • continue on to golgi apparatus, associate with cholesterol and lipoproteins to form chylomicrons
                        • Chylomicrons move out by excytosis and enter lacteals where they pass into the blood stream
                          • Triglycerides hydrolysed by enzyme in endothelial cells of blood capillaries where they diffuse into cells