Body Size & Metabolic Rate (College Board AP® Biology)

Body Size & Metabolic Rate

Surface Area to Volume Ratio

• 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

• The size of an organism is best thought of as its surface area-to-volume ratio when considering the relationship between size and metabolic rate

Body Mass and Metabolic Rate Graph

Metabolic rate per organism 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 hippo consumes more oxygen within a given period of time compared to a single mouse

Surface Area-to-Volume 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

• The resting heart rates of animals can be a proxy for their base metabolic rates

  • Elephant: 25-35 beats per minute (bpm)

  • Hamster: 300-600 bpm

• Different apparatus can be used for investigating metabolic rates in organisms:

  • Respirometers

  • Oxygen/carbon dioxide probes

  • Calorimeters