Foods are the obvious source of energy for animals. Most of our packaged foods are now labelled with Calories to help consumers adjust their energy consumption to their energy needs. (Remember that food Calories are actually kilocalories.) Excess of Calories results in energy being stored as fat molecules for later use in exercise. Calories from exercise can be related to specific foods by using the information on the packaging. The exercise numbers are averages and depend on specific metabolism rates on individuals, but it gives students an approximation.

Energy is used up by muscles contracting, an endogonic process driven by exergonic process resulting from the reactions of foods in our bodies. The linkage of these processes in muscle contraction is illustrated here.

A more complete discussion of energy in biological systems is available as a complete additional course. See the course on the Chemistry of life processes-energy and metabolism.