Exchange Surfaces

• Three key features of a good exchange surface:

  • Good Blood Supply
  • Large Surface Area
  • Thin Walls
• Surface area to volume ratio determines the transport mechanisms for respiratory gases, nutrients, and excretory products, in or out of organisms, as an organism increases in size, the surface area to volume ratio decreases; this is a negative correlation
• Organisms (e.g. bacteria) are very small but can achieve efficient internal transport by just diffusion because: high SA:V, short diffusion distance, low activity, lower demand for O_2 and glucose and necessary diffusion gradients maintained
• Organisms (e.g. elephants ) have many cells but a low SA:V, diffusion is too slow and distances are too far for it, also greater metabolic activity (endothermic higher than exothermic) so they have specialised exchange surfaces with large surface area
• Gas exchange: Exchange of oxygen and carbon dioxide between an organism and its environment; this occurs across a specialised exchange surface, which may be a cell wall/membrane or a single layer of cells, depending on the organism
• Gases pass over exchange surfaces by diffusion; any factor that speeds this up will speed up the exchange of gases: good blood supply (keeps steep diffusion gradient), large SA (more area for diffusion) and thin walls (short diffusion distance)
• Single-celled organisms e.g. amoeba have a larger SA:V and are less active than multicellular animals, so gas exchange occurs across the plasma membrane of the cell and this is adequate to supply the needs of the organism
• Multi-cellular organisms, with a smaller surface area to volume ratio, and organisms with high oxygen requirements, have lungs to take in the large volumes of oxygen they need
• There should be mechanisms on either side of the exchange surface designed to maintain a steep diffusion gradient of molecules/ions, also all gas exchange surfaces are moist because they allow water to diffuse across, by Osmosis, but it is not included as a useful feature of gas exchange; slows diffusion down, not speeds it up