AS LEVEL- Biology- Exchange surfaces + Breathing (1)

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  • Exchange surfaces + Breathing (1)
    • Specialised exchange surfaces
      • Why do organisms need specialised exchange surfaces?
        • Multicellular organisms have a higher metabolic rate than single-celled organisms, so they use up oxygen and glucose faster
        • Large animals have a small SA:V ratio- it's difficult to exchange enough substances to supply a large volume of animal through a relatively small outer surface
        • The distance between the cells that need the oxygen and the environment is a lot longer + is too far for diffusion to take place
      • Ratio = SA/V
      • Features of an efficient exchange surface
        • Ventilation to maintain diffusion gradient- eg. gills + alveoli *Maintains a steep concentration gradient
          • Breathing in = air into alveoli. Refreshes oxygen concentration in alveoli
          • *Breathing out = remaining a carbon dioxide gradient from alveoli. You need the concentration in alveoli to be higher in blood
        • Increased SA- eg. root hair cells *Increases rate of diffusion *Overcomes the limitation of a small SA:V ratio
        • Thin layers-   eg. alveoli *Quicker rate of diffusion *Gases have a short distance to travel- gives you a short diffusion pathway
        • Good blood supply- eg. gills + alveoli *Maintains a steep concentration gradient *Steeper concentration gradient = faster rate of diffusion
    • Mammalian gaseous exchange system
      • Bronchioles- *Bronchi divides to form many of these *No cartilage in the walls *Walls contain smooth muscle + flattened epithelium
      • Alveoli- *Tiny air sacs that are the main site of gas exchange *200-300um in diameter *300-500 million per lung increasing the SA:VA ratio in lungs *No cartilage, mucus membrane or muscle *Wall made out of flattened squamous epithelial cells, collagen + elastic fibres *Good blood supply- millions of capillaries in close contact so there's a short diffusion pathway *Constant flow of blood maintains a steep concentration gradient


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