Exchange surfaces

• Multicellular organisms have adaptations to allow effective exchange of materials between the cell and the environment
• Single celled organisms have a high surface area : volume ratios this means that:

  • there is a relatively large surface area of cell membrane for diffusion to occur
  • the distances from the cell membrane to the interior of the cell are small
• Molecules move in and out of a single celled organism well enough to meet its needs.

  However multicellular organisms need exchange surfaces to provide large surface area to volume ratios and transport systems to maintain high concentration gradients.
• The effectiveness of an exchange surface increases with:

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  increase in surface area a shorter distance for diffusion

  ↓ ↓

  (example) alveoli in lungs (example) surfaces are only one cell thick

  or villi in small intestine

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  maintenance of a high concentration gradient

  ↓

  animals have an efficient blood supply so lungs are ventilated well
• Single-celled organisms have relative large surface area to volume ratios. Larger multicellular organisms have smaller surface area to volume ratios. So, they have evolved exchange surfaces to exchange molecules with their surroundings.
• An explained example of exchange surfaces is - the lining of the small intestine is folded into many tiny finger like structures called villi these provide -

  • a large surface area for diffusion
  • a single layer of cells for a short diffusion path
  • a network of capillaries to carry food molecules away
• How are fish gills adapted for gas exchange (2 marks)

  The gills have tiny finger like gill filaments these greatly increase the surface are for gas exchange
• Describe 2 ways in which a high concentration gradient can be maintained

  having an efficient blood supply

  keeping the exchange surface ventilated
• Explain why multicellular organisms need exchange surfaces but single celled organisms don't

  Single celled organisms have a large enough surface area to volume ratio so molecules move in and out easy enough to meet their needs . Whereas in multicellular organisms have smaller surface are to volume ratios so exchange surfaces increases the surface area available for exchange of material