A third key feature of cells is the ability to harness energy from the environment. Let’s go back to our introductory example of eating an apple. The apple contains sugars, which store energy in their chemical bonds. By breaking down sugar, our cells harness this energy and convert it into a form that can be used to do the work of the cell. Energy from the food we eat allows us to grow, move, communicate, and do all the other things that we do.

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Organisms acquire energy from just two sources—the sun and chemical compounds. The term metabolism describes chemical reactions by which cells convert energy from one form to another and build and break down molecules. These reactions are required to sustain life. Regardless of their source of energy, all organisms use chemical reactions to break down molecules, releasing energy in the process that is stored in a chemical form called adenosine triphosphate, or ATP. This molecule enables cells to carry out all sorts of work, including growth, division, and moving substances into and out of the cell.

Many metabolic reactions are highly conserved between organisms, meaning the same reactions are found in many different organisms. This observation suggests that the reactions evolved early in the history of life and have been maintained for billions of years because of their fundamental importance to cellular biochemistry.