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
Exam Tip
Make sure that you refer to metabolic rate per cell, to take out the effect of the massive differences in body sizes of various animals. The metabolic rate inside a single mouse cell will be very much higher than inside a single hippopotamus cell.
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.