Exchanging Substances

• Surface area to volume ratio (SA:V)

  How easily stuff moves between an organism and its environment depends on its surface area to volume
• Cells can use diffusion

  1. Take in substances they need

  2. Get rid of waste products
• Substances exchanged by diffusion

  • Oxygen and carbon dioxide
  • Urea
• Surface area to volume ratio (SA:V)

  The importance of an organism's SA:V ratio is that it determines how big one value is compared to another. The larger an organism is, the smaller its surface area to volume ratio
• Multicellular organisms have a smaller surface area compared to their volume, so they need some sort of exchange surface for efficient diffusion
• Exchange surface structures


  • They have a thin membrane so substances only have a short distance to diffuse
  • They have a large surface area so lots of a substance can diffuse at once
  • They have lots of blood vessels to get stuff into and out of the blood quickly
  • They are often ventilated so air moves in and out
• Gas exchange in the lungs

  1. Transfer oxygen to the blood

  2. Remove waste carbon dioxide from the blood
• Alveoli


  • They have an enormous surface area (about 75 m² in humans)
  • They have a moist lining for dissolving gases
  • They have very thin walls
  • They have a good blood supply
• Villi


  • They increase the surface area in the small intestine so that digested food is absorbed much more quickly into the blood
  • They have a single layer of surface cells
  • They have a very good blood supply to assist absorption
• Gills


  • They have lots of thin plates called gill filaments which give a big surface area for exchange of gases
  • They have lots of tiny structures called lamellae which increase the surface area even more
  • They have lots of blood capillaries in the lamellae to speed up diffusion
  • They have a thin surface layer of cells to minimise the distance that the gases have to diffuse
  • They have blood flowing through the lamellae in one direction and water flowing over in the opposite direction to maintain a large concentration gradient
• Leaf structure


  • The underneath of the leaf is an exchange surface covered in little holes called stomata which carbon dioxide diffuses in through
  • Oxygen and water vapour also diffuse out through the stomata
  • The size of the stomata is controlled by guard cells which can close them if the plant is losing water faster than it is being replaced
  • The flattened shape of the leaf increases the area of this exchange surface
  • The air spaces inside the leaf increase the area of the exchange surface inside the leaf cells
• Big surface areas mean substances can diffuse through quickly