Students know plant and animal cells break down sugar to obtain energy, a process resulting in carbon dioxide (CO₂) and water (respiration).

Respiration is actually the burning of sugar to release the chemical energy stored in the sugar molecule. As in other burning reactions, oxygen combines with the fuel, and the products are carbon dioxide and water.

Our bodies cannot handle the high temperature that would result from actually burning sugar. Also our bodies could not capture all the energy that is released when a bunch of sugar rapidly burns. Instead, our cells combine the sugar with the oxygen (i.e., burn it) in a series of small chemical steps. As a result the temperature never gets too high, and the energy is released in small amounts that can be packaged in other convenient chemical forms. Cellular respiration is this controlled burning of sugar in many small chemical steps that efficiently release the stored energy and completely convert the sugar to carbon dioxide and water.