Exchange Surfaces

• Cells can use diffusion to take in substances they need and get rid of waste products. For example: Oxygen and Carbon Dioxide are transferred between cells and the environment during gas exchange and in humans urea (a waste product produced from the breakdown of proteins) diffuses from cells into the blood plasma for removal from the body by the kidneys.
• How easy it is for an organism to exchange substances with its environment depends on the organism‘s surface area to volume ratio.
• A ratio shows how big one value is compared to another. The larger an organism is, the smaller its surface area is compared to its volume. You can show this by calculating surface area to volume ratio.
• An example of a surface area to volume question would be: A hippo can be represented by a 2cm x 4cm x 4cm block. The area of a surface is found by the equation: Length x Width. So the hippos total surface area is: 64cm^2. The volume of a block is found by the equation: Length x Width x Height. So the hippos volume is 4 x 4 x 2 = 32cm^3. The surface area to volume ratio of the hippo can be written as 64:32. This can be simplified to 2:1.
• In single celled organisms, gases and dissolved substances can diffuse directly into (or out of) the cell across the cell membrane. It’s because they have a larger surface area compared to their volume, so enough substances can be exchanged across the membrane to supply the volume of the cell.
• Multicellular organisms have a smaller surface area compared to their volume - not enough substances can diffuse from their outside surface to supply their entire volume. This means they need some sort of exchange surfaces for efficient diffusion. The exchange surface structures have to allow enough of the necessary substances to pass through.
• Exchange surfaces are adapted to maximise effectiveness because:

  1. They have a thin membrane, so substances only have a short distance to diffuse.
  2. They have a large surface area so lots of substances can diffuse at once.
  3. Exchange surfaces in animals have lots of blood vessels, to get stuff into and out of the blood quickly.
  4. Gas exchange surfaces in animals (e.g. alveoli) are often ventilated too - air moves in and out.