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Exchange Surfaces

  • All living cells need to be able to take up and excrete substances from and to their environment.
  • Cells need:
    • Water for many different things
    • Minerals for many different things
    • Oxygen for respiration
    • Glucose for energy
    • Fats for membranes
    • Proteins for growth and repair

  • Specialised cells in multicellular organisms may also need to take up other molecules.
  • Many of these substances can be produced inside the cytoplasm as part of metabolism, but the basic building blocks must still be taken up from the environment.
  • Cells may need to excrete:
    • Carbon Dioxide
    • Oxygen
    • Ammonia
    • Again, specialised cells in multicellular organisms may also need to excrete special molecules.
    • Single-celled organisms can exchange all the substances they need to via their outer surfaces; however, most multicellular organisms require special Exchange Surfaces.
    • This is because single-celled creatures have a very high surface-area-to-volume ratio, so have lots of available surface to exchange substances. Multi-celled creatures on the other hand tend to have a low surface-area-to-volume ratio, meaning their outer surfaces cannot exchange substances fast enough for all their cells.
    http://alevelnotes.com/content_images/i93_surface_area_to_volume_ratio.png
    • Exchange surfaces have a number of adaptations to make them efficient:
      • Thin barriers to minimise the diffusion distance
      • A high concentration gradient to the substance
      • A large surface area
    • Exchange surfaces are found in abundance in nature. For example:
      • The Nephron in the Kidney
      • The root hairs of plants
      • The Hyphae of fungi
    http://alevelnotes.com/content_images/i94_Fungal_hyphae_620.JPG