Adaptations for Gas Exchange

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  • Adaptations for Gas Exchange
    • All living organisms need to exchange gases with their environment
      • Gas exchange occurs by diffusion
        • Gas exchange is 'The diffusion of gases into and out of cells so respiration can take place'
    • Although living organisms have become adapted for gas exchange in different ways, all gas exchange surfaces share the following properties
      • Permeable to gases
      • Moist, because gases must dissolve before they can diffuse across membranes
      • Large surface area
      • Thin, to provide a short diffusion pathway
      • Large, active animals can also have ventilating mechanisms to maintain steep concentration gradients across the gas exchange surface
    • Surface area: Volume Ratio
      • Volume is proportional to the demand for oxygen
        • Demand will increase with an increase in metabolic rate (as an organism increases in size
          • As the cell increases in size, eventually simple diffusion will no longer satisfy demand for oxygen
      • The smaller an animal, the larger the SA:V ratio
        • The larger an animal, the smaller it's SA:V ratio
    • Unicellular Organisms
      • Amoeba
        • Small, unicellular organism
        • Has a large SA:V ratio
        • It it unicellular (thin) so the diffusion pathway is short
        • It also lives in water, so the entire surface area of the amoeba is moist
        • Diffusion of gases occurs across the whole surface membrane
        • The amoeba is unicellular, so the rate of uptake of oxygen is fast enough to satisfy the demands of oxygen by the organism
    • Multicellular Organisms
      • As an organism gets larger, the demand for oxygen increases, but SA:V ratio decreases
      • Diffusion pathways also become longer
      • Their demand for oxygen can no longer be met by simple diffusion through their surface skin


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