The ability to harness energy from the environment is a key attribute of life. We have seen that energy is needed for all kinds of tasks—among them cell movement and division, muscle contraction, growth and development, and the synthesis of macromolecules. Organic molecules such as carbohydrates, lipids, and proteins are good sources of chemical energy. Some organisms, like humans and other heterotrophs, obtain organic molecules by consuming them in their diet. Others, like plants and other autotrophs, synthesize these molecules from inorganic molecules like carbon dioxide, as we saw in Chapter 6. Regardless of how organic molecules are obtained, nearly all organisms—animals, plants, fungi, and microbes—break them down in the process of cellular respiration, releasing energy that can be used to do the work of the cell. Cellular respiration is a series of chemical reactions that convert the chemical energy in fuel molecules into the chemical energy of adenosine triphosphate (ATP), which can be readily used by cells.

It is tempting to think that organic molecules are converted into energy in this process, but this is not the case. Recall from Chapter 6 that the first law of thermodynamics (the law of conservation of energy) states that energy cannot be created or destroyed. Biological processes, like all processes, are subject to the laws of thermodynamics. As a result, the process of cellular respiration converts the chemical potential energy stored in organic molecules to chemical potential energy that is useful to cells: the chemical potential energy in ATP. ATP is the universal energy currency for all cells.

It is also easy to forget that organisms other than animals, such as plants, use cellular respiration. If plants use sunlight as a source of energy, why would they need cellular respiration? As we will see in the next chapter, plants use the energy of sunlight to make carbohydrates. Plants then break down these carbohydrates in the process of cellular respiration to produce ATP.

In this chapter, we discuss the metabolic pathways that supply the energy needs of a cell: the breakdown, storage, and mobilization of sugars such as glucose, the synthesis of ATP, and the coordination and regulation of these metabolic pathways.