Exchange surfaces

• Cells can use diffusion to take in substances they need and get rid of waste products
• Organisms exchange substances with their environment
• Oxygen and carbon dioxide are transferred between cells and the environment during gas exchange
• 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 (SA:V)
• 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 ratios
• The area of a surface is found by the equation: Length x Width
• The volume of a block is found by the equation: Length x Width x Height
• Multicellular organisms need exchange surfaces
• In single-celled organisms, gases and dissolved susbtances can diffuse directly into (or out of) the cell across the cell membrane
• Because single-celled organisms have a large surface area compared to their volume, so enough substances can diffuse from their outside surface to supply their entire volume
• As single-celled organisms exchange substances from their environment, they need some sort of exchange surface 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:

  • 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
• Exchange surfaces are adapted to maximise effectiveness:

  • Exchange surfaces in animals have lots of blood vessels, to get stuff into and out of the blood quickly
  • Gas exchange surfaces in animals (e.g. alveoli) are often ventilated too - air moves in and out