Extraction of Energy from Nutrients — Functionof the Mitochondria The principal
substances from which cells extract energy are foodstuffs that react chemically with oxygen—
carbohydrates, fats, and proteins. In the human body, essentially allcarbohydrates are converted into
glucose by the digestive tract and liver before they reach the other cells of the body. Similarly, proteins
are converted into amino acids and fats into fatty acids. Figure 2-14 shows oxygen and the
foodstuffs— glucose, fatty acids, and amino acids—all enteringthe cell. Inside the cell, the foodstuffs
react chemically with oxygen, under the influence of enzymes that control the reactions and channel
the energy released in the proper direction. The details of all these digestive and metabolic functions
are given in Chapters 62 through 72. To reconstitute the cellular ATP as it is used up, energy derived
from the cellular nutrients causes ADP and phosphoric acid to recombine to form new ATP, and the
entire process repeats over and over again. For these reasons, ATP has been called the energy currency of the cell because it can be spent and remade continually, having a turnover time of only a few
minutes.
