Introduction to Cellular MetabolismAll living organisms, as we have already discussed, require energy in order to survive. Animals obtain this energy by ingesting compounds that contain potential chemical energy (carbohydrates, fats, and proteins), and metabolizing them so that energy is released and stored in the form of ATP (see Chapter 3). Energy is almost always used directly in the form of ATP. What processes occur at the cellular level to accomplish these energy transformations and create ATP? The net sum of all reactions that take place in a cell or organism is called metabolism, but often the term metabolism is used to refer to the catabolic (breaking down) reactions that make energy available to the cell. These are the reactions on which we will focus in this chapter.