Biology A-Level AQA Chapter 6

6.1 Exchange between organisms and their environment
1. Exchange can occur
  1. Activley
    1. Active transport
    2. Co-Transport
  2. Passivley
    1. Osmosis
    2. Diffusion
2. 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
3. SA:Vol Ratio
  1. Simple diffusion across outer surface only works for small organisms
  2. Larger organisms evolved
    1. Flattened shape so no cell is far from surface
      1. Flatworm
      2. Leaf
    2. Specialised exchange surface with large area to increase SA:Vol ratio
      1. Lungs in mammal
      2. Gills in fish
4. Specialised exchange surface features
  1. Large SA:Vol ratio >increases rate of exchange
  2. Thin > short diff. distance > materials can cross rapidly
  3. Selectivley permeable > allows selected material to cross
  4. Movement of environmental medium > maintain diff. grad
  5. Transport system > ensure movement of internal medium
5. Diffusion = (surface area x difference in conc.) / length of diff. path
6.2 Gas exchange in single-celled organisms and insects
1. Single-Celled
  1. Large SA:Vol ratio
  2. Diffusion of oxygen across body surface
    1. Body surface covered by cell-surface membrane
  3. CO2 diffuses out in same way
2. Insects
  1. Increased SA needed for diffusion increases waterloss
  2. Internal network -> Tracheae
    1. Supported by strengthened rings
    2. Divide into smaller dead-end tubes -> tracheoles
      1. Extend through body tissue of insect
        Atmospheric air brought directly to tissues
  3. Gases move in and out in different ways
    1. Along a diffusion gradient
      1. 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
      2. Diffusion in air more rapid than in water so quicker diffusion
    2. Mass transport
      1. Muscle contration squeezes tracheae enables mass movement of air in and out
      2. Speeds up exchange of repiratory gases
    3. The ends of the tracheoles are filled with water
  4. Spiracles
    1. Open and close by valve
      1. Stops water loss
  5. Insects only small due to short diff. pathway
6.3 Gas exchange in fish
1. Gills
  1. Behind head
  2. Gill Filaments
    1. Stack up at right angle to lamelle
      1. Increase surface area
  3. Water in through mouth
    1. Forced over gills
      1. Out opening
2. Coounter-current flow
  1. Oxygen loaded blood meets water which has max conc. of oxygen
    1. Diffusion of oxygen from water to blood
  2. Blood with little oxygen meets water with little oxygen
    1. Diffusion of oxygen from water to blood
6.4 Gas exchange in the leaf of a plant
1. Gases produced in photosynthesis can be used in respiration and vice versa
  1. This reduces gas exchange
  2. Volume of gases exchanged depends on rates of photosynthesis and respiration
  3. Most Co2 used in photosynthesis comes from the external air and most oxygen produce diffuses out of the plant
  4. When photosynthesis isn't occuring oxygen diffuses in and Co2 out
2. Structure of a plant leaf and gas exchange
  1. No living cell far from external air
    1. Source of oxygen and CO2
  2. Diffusion takes place in air
    1. More rapid than water
  3. Air spaces in leaf have large surface area compared to tissue volume
  4. Adaptions for rapid diffusion
    1. Stomata
      1. Underside of leaf
      2. Guard cells up and close them to prevent water loss
      3. Short diffusion pathway as no cell is far from stoma
    2. Interconnecting air spaces through mesophyll
      1. gases regually come into contact with mesophyll cells
    3. Large SA of mesophyll cells for rapid diffusion
6.5 Limiting Water loss
1. Insects
  1. Small SA:Vol Ratio
    1. Minimise area from where water is lost
  2. Waterproof Covering
    1. rigid outer-skeleton of chitin covered in waterproof cuticle
  3. Spiracles
    1. Can be closed to reduce waterloss
2. Plants
  1. Thick, waxy cuticle
    1. thicker cuticle = less water escapes
  2. Rolling leaves
    1. protects stomata and lower epidermis
    2. traps a region of air within rolled leaf
      1. Region becomes saturated by water vapour
        no wp grad. inside and outside so no water loss
  3. Hairy leaves
    1. Traps still moist air next to leaf surface
      1. Water potential grad. between inside and outside reduced
        Less water lost by evaporation
  4. Stomata in pits or grooves
    1. trap still moist air
      1. reduce wp grad.
  5. Reduced SA:Vol ratio
    1. Slower rate of diffusion
    2. Must be balanced for photosynthesis needs
3. xerophytes
  1. Plants which have adapted to living where there is little water supply, without their adaptions they would die
6.6 Structure of the human gas exchange system
1. aerobic organisms need oxygen to release ATP and CO2 must be removed as build up is harmful
  1. There is a large volume which must be removed and absorbed because...
    1. large organisms have large volume of living cells
    2. they maintain high body temp due to high metabolic and respiratory rates
    3. Due to this animals have specialised exchange surfaces called lungs for efficent exchange
2. Mammalian lungs
  1. located inside body because...
    1. air not dense enough to support and protect delicate structure
    2. body as a whole would otherwise loose a lot of water
  2. supported and protected by ribcage
    1. ribs can be moved by muscles so lungs are ventilated by tidal stream of air ensuring air is constantly replenished
  3. structure and functions of main parts
    1. lungs
      1. lobed structure made of series of tubules > bronchioles, which end with air sacs > alveoli
    2. trachea
      1. flexible airway supported by rings of cartilage
        cartilage prevents trachea collapsing when air pressure falls
      2. walls made of muscle lined with epithelium and goblet cells
    3. bronchi
      1. two divisions of trachea leading to a lung
      2. similar to trachea and produce mucus to trap dirt and have cilia to move such to throat
      3. supported by cartilage
    4. bronchioles
      1. branching subdivisions of bronchi
      2. walls = muscle lined with epithelia cells
      3. constrict to control flow of air in and out
    5. alveoli
      1. air sacs
      2. collagen and elastic fibres between them
      3. fill with air when breathing in and spring back when breathing out to expel CO2 rich air
6.7 The mechanism of breathing
1. Inspiration
  1. External intercostal muscles contract and internal relax
    1. Ribs move up and out increasing thorax volume
      1. 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
2. Expiration
  1. Internal intercostal muscles contract, external relax
    1. ribs move down and in decreasing thorax volume
      1. 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
6.8 Exchange of gases in the lungs
6.9 Enzymes and digestion
6.10 Absorption of the products of digestion
1. Ileum
  1. Villi
    1. Microvilli
      1. Increase SA
    2. Thin walled-> reduce diff. distance
    3. Increase SA for diff.
    4. able to move > helps maintain diff. grad.
    5. Well supplied with blood to carry away absorbed molecules
2. Amino acids and monosaccharides
  1. proteins produce amino acids
  2. carbohydrates produce monosaccharides (e.g. glucose, fructose, galactose)
  3. Absorbed by diffusion and co-transport
3. Triglycerides
  1. Monoglycerides and fatty acids are formed in digestion
    1. Stay in association with bile salts which initially emulsifies the lipid droplets
      1. 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

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Biology A-Level AQA Chapter 6

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