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: 'The diffusion of gases into and out of cells so respiration can take place'
    • All gas exchange surfaces will share the following properties;
      • Permeable to Gases
      • Moist so gases can dissolve and diffuse across the membrane
      • 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
      • Surface area
        • Proportional to O2 supply by diffusion
          • Rate of diffusion can be increased by;
            • Increasing surface area
            • Decreasing the diffusion pathway
            • Increasing the steepness of the diffusion pathway
      • Volume
        • Proportional to demand of O2
          • Demand will increase with an increase in metabolic rate (as the organism increases in size)
            • As the cell increases in size, eventually simple diffusion will no longer satisfy demand for O2
      • The smaller the animal, the larger the surface area:volume ratio
      • The larger the animal the smaller the surface area:volume ratio
    • Unicellular Organisms e.g. Amoeba
      • Small, unicellular organisms
      • Has a large surface area:volume ratio
      • It is unicellular so has a short diffusion pathway
      • Lives in water so the entire surface of the amoeba is moist
      • Diffusion of gases occurs over the entire cell membrane
      • The amoeba is unicellular, so the rate of uptake of O2 is fast enough to satisfy the demand for O2 by the organism
    • Multicellular Organisms
      • As organisms get larger, the demand for oxygen increases but surface area to volume ration decreases
      • Diffusion pathways also become larger
      • The demand for O2 can no longer be met by simple diffusion through their surface skin


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