Adaptation to Facilitate Exchange (AQA A Level Biology): Revision Note

Exchange in larger organisms

• Single-celled organisms have a high SA:V ratio, meaning that simple diffusion at the cell surface is sufficient to meet the needs of the cell

  • The large surface area allows for maximum diffusion of nutrients and gases

  • The small volume means the diffusion distance to all parts of the cell is short

• As organisms increase in size their SA:V ratio decreases

  • There is less surface area for diffusion in relation to the size of the organism

  • The greater volume results in a longer diffusion distance to all parts of the organism, e.g. there may be many layers of cells

• Large, multicellular animals and plants have evolved adaptations to facilitate the exchange of substances with the environment

  • E.g. the gas exchange system and digestive system in mammals, and the leaves of plants