3.1 Surface area to volume ratio

• Describe the relationship between the size and structure of an organism and its surface area to volume ratio (SA:V)

  ● As size increases, SA:V tends to decrease

  ● More thin / flat / folded / elongated structures increase SA:V
• How is SA:V calculated?

  Divide surface area (size length x side width x number of sides) by volume (length x width x depth)
• Suggest an advantage of calculating SA:mass for organisms instead of SA:V

  Easier / quicker to find / more accurate because irregular shapes
• What is metabolic rate? Suggest how it can be measured

  • ● Metabolic rate = amount of energy used up by an organism within a given period of time
  • ● Often measured by oxygen uptake → as used in aerobic respiration to make ATP for energy release
• Explain the relationship between SA:V and metabolic rate

  As SA:V increases (smaller organisms), metabolic rate increases because:

  ● Rate of heat loss per unit body mass increases

  ● So organisms need a higher rate of respiration

  ● To release enough heat to maintain a constant body temperature ie. replace lost heat
• Explain the adaptations that facilitate exchange as SA:V reduces in larger organisms

  1. Changes to body shape (eg. long / thin)

  • Increases SA:V and overcomes (reduces) long diffusion distance / pathway

  1. Development of systems, such as a specialised surface / organ for gaseous exchange e.g. lungs:

  • Increases (internal) SA:V and overcomes (reduces) long diffusion distance / pathway
  • Maintain a concentration gradient for diffusion eg. by ventilation / good blood supply