The chemical transformations that contribute to metabolism depend ultimately on the uptake of oxygen by the animal. In very simplified terms, oxygen is used in the mitochondria to complete the final stages of the breakdown of small energy-rich molecules – products of the chemical fragmentation of macromolecules mentioned earlier. In this process, usable energy is released, along with some heat.
Measuring how much oxygen is consumed by an animal over a period of time, such as a minute or an hour, is a pretty good measure of the intensity of metabolism (i.e. the metabolic rate). But when measuring and comparing oxygen consumption in two animals that are very different in size – say a mouse and an elephant – all you learn is that overall, an elephant consumes much more oxygen over a minute than a mouse does. This is simply because a massive elephant has so many more cells in which metabolism is whirring away. If your interest is in metabolic rate, you need to take size differences into account by calculating the volume of oxygen consumed for a particular amount (or mass) of animal.
Volumes of a gas such as oxygen would be measured in millilitres (ml), or the numerically equivalent unit, cubic centimetres (cm3). The mass of the animal would be measured in grams (g), or perhaps in thousandths of a gram, i.e. milligrams (mg). In practical terms, oxygen consumption by a mouse of known mass would be measured over a period of time, say 15 minutes, and the values recalculated as so many ‘cubic centimetres of oxygen per gram of mouse per hour’. (A more scientifically correct way of expressing the same thing is cm3 O2 g−1 h−1.) In principle, the calculation for the elephant would be done in the same way, though one suspects with a few practical problems along the way.
As mentioned earlier in the course, scientific data are often presented in the form of a table. Table 2 is a slightly more complex example. As with scientific diagrams, make sure to read any titles and labels carefully. Look at the headings of the rows and columns. The units in which the values are measured are usually given in the headings, rather than being written beside each value.
Take a look at Table 2, then answer Question 5 underneath.
|Metabolic rate / cm3 O2 g−1 h−1|
|Species||at rest||at peak activity|
Look at Table 2, which shows the metabolic rates of a number of animals. What is the most striking difference between the values for mammals and those for fish, reptiles and a bird?
The metabolic rates for mammals (and the hummingbird) are much higher than those for the fish and reptiles. This is true of both the ‘at rest’ levels and the ‘peak activity’ values; the mouse, dog and human (and the hummingbird) values are higher than the others, often considerably so.