Getting the energy out

There is energy in food. But to get it out, the food has to be broken down inside the cell. That process is cellular respiration.

It cannot be watched directly

Important as it is as a characteristic of the living state, respiration cannot be observed directly. Cellular respiration is a chain of biochemical reactions inside a cell — not something the eye can see. So it has to be confirmed by experimental means.

Then how is it confirmed?

The process is invisible, but its inputs and outputs can be measured. If respiration needs oxygen, the oxygen going missing can be measured; if it releases carbon dioxide, that can be detected. The two activities on the next two pages do exactly this.