The Song of the Cell: An Exploration of Medicine and the New Human

by Siddhartha Mukherjee

How does a cell generate energy? There are two pathways: one fast and one slow. The fast route occurs mainly in the protoplasm of the cell. Enzymes serially break down glucose into smaller and smaller molecules, and the reaction produces energy. Because the process doesn’t use oxygen, it is called anaerobic. In terms of energy, the end product of the fast pathway is two molecules of a chemical called adenosine triphosphate, or ATP. ATP is the central currency of energy in virtually all living cells. Any chemical or physical activity that requires energy—for instance, the contraction of a muscle or the synthesis of a protein—utilizes, or “burns,” ATP. The deeper slow burn of sugars to produce energy occurs in mitochondria. (Bacterial cells, lacking mitochondria, can use only the first chain of reactions.) Here the end products of glycolysis (literally, the chemical breakdown of sugar) are fed into a cycle of reactions that ultimately produce water and carbon dioxide. This cycle of reactions involves the use of oxygen (and is therefore called aerobic) and is a small miracle of energy production: it generates a much larger harvest of energy, again, in the form of ATP molecules.