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

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Phil

Written by: Phil

Reviewed by: Lára Marie McIvor

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-vs-body-size-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

Examiner Tips and Tricks

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.

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Phil

Author: Phil

Expertise: Biology Content Creator

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.

Lára Marie McIvor

Author: Lára Marie McIvor

Expertise: Biology Lead

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.