MORE ON METABOLIC RATE1. . . . . .
In both endothermic and ectothermic animals, the normal metabolic rate is inversely
related to body size; the smaller the organism, the higher the relative metabolic rate---that is, the
higher the metabolic rate per gram of body tissue. Why is this so? Recall that temperature is an
important factor in enzyme activity. As the temperature increases up to about 40oC, the velocity
of an enzyme-catalyzed reaction increases because the substrates collide with active sites more
frequently as the molecules move more rapidly due to thermal agitation. Because the metabolic
reactions are enzyme catalyzed, metabolic rate is also proportional temperature, up to the point
where the enzyme. The relationship between metabolic rate and temperature is often expressed
as Q10, which measures the rate increase for each 10o rise in temperature. For example, if the rate
triples for each 10 o rise, then the value of Q10 is 3. Metabolic rates often have a Q10 of around 2.
If the metabolic rate of an animal at 0 o is X, then at 10 o the rate would be 2X, at 20 o, 4X, etc.
Notice that the rate increases more and more rapidly as the temperature increases.
Changes in metabolic rate with changes in
temperature: The hypothetical organism has a
Q10 of 2, that is, its metabolic rate doubles with
every 10°C rise in temperature. This rise is the
result of the greater thermal energy of the
reactants in the cell and the increasing
effectiveness of the cellular enzymes. The
abrupt decline above 40° represents the point at
which the weak bonds that hold enzymes in
their specific active conformations begin to
break. As a result the enzymes become
denatured and metabolic activity is severely
disrupted.
Why then is there and inverse relationship then between metabolic rate and body size?
The answer is easily understood in the case of endotherms; smaller animals have a greater
surface-to-volume ratio, and consequently a larger relative heat loss to the environment per unit
time. To maintain a constant high body temperature despite rapid heat loss across a body
surface, a small animal must oxidize food at a high rate. Because the relative amount of food
consumed and the pace of digestion, respiration, and so on must rise with decreasing body size,
there is a lower limit on the size of endotherms. The smallest mammals are shrews, which weigh
only about 4 grams. They must eat nearly their own body weight of food every day, and can
starve to death is just a few hours if deprived of food.
1
Adapted from Gould, J. and W. Keeton, Biological Science, 1996. W. W. Norton & Co., Inc. New York
There is also an inverse relationship between size and relative metabolic rate in
ectothermic animals, which is not as easily understood. Ectotherms lose their metabolic heat to
the environment and do not normally respond to heat loss by increased metabolism, so larger size
and its concomitant smaller surface-to-volume ratio should actually retard heat loss somewhat,
and the conserved heat ought then to speed up metabolism. In fact, why larger heat size in
ectotherms tends to be correlated with lower metabolic rate is still not completely understood.
One factor is that increasing size involves a disproportionate increase in the mass of skeletal and
other connective tissue in animals—an alligator, for instance, requires more inactive support
structure than a salamander. Since these tissues are relatively inactive metabolically, the average
metabolic rate per unit weight for the organism as a whole may fall as the proportion of these
less active but necessary structural tissues rises.
Inverse relationship between metabolic rate and body size in mammals.