AQA AS Biology

Revision Notes

3.1.4 SA:V Ratio & Metabolic Rate

The Relationship between Surface Area: Volume Ratio & Metabolic Rate

  • The metabolic rate of an organism is the amount of energy expended by that organism within a given period of time
  • The basal metabolic rate (BMR) is the metabolic rate of an organism when at rest. The BMR is significantly lower than when an organism is actively moving
  • During periods of rest, the body of an organism only requires energy for the functioning of vital organs such as the lungs, heart and brain
  • The metabolic rate of an organism can be measured/estimate using different methods:
    • Oxygen consumption
    • Carbon dioxide production
    • Heat production

Body Mass and Metabolic Rate (1), downloadable AS & A Level Biology revision notes

Metabolic rate increases with body mass

Body Mass

  • Experiments conducted by scientists have shown that the greater the mass of an organism, the higher the metabolic rate
    • Therefore, a single rhino consumes more oxygen within a given period of time compared to a single mouse

SA:V Ratio

  • Although metabolic rate increases with body mass the BMR per unit of body mass is higher in smaller animals than in larger animals
    • Smaller animals have a greater SA:V ratio so they lose more heat, meaning they have to use up more energy to maintain their body temperature
  • Different apparatus can be used for investigating metabolic rates in organisms:
    • Respirometers
    • Oxygen/carbon dioxide probes
    • Calorimeters

Exam Tip

Plants have much lower metabolic rates than animals as they do not move around their habitat and don’t have to maintain a high body temperature.

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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