Aerobic means that an organism needs oxygen to live. Some microorganisms can live without oxygen and they are called anaerobic. Bacteria are not dependent on oxygen to burn food for energy, but most other living organisms do need oxygen. Fats, proteins, and sugars in the diet of organisms are chemically broken down in the process of digestion to release energy to drive life activities. If oxygen is present, maximum energy is released from the food, and the process is referred to as aerobic respiration. The analogy of a bonfire with the energy metabolism of living organisms is appropriate up to the point that both processes require fuel and oxygen to produce energy and yield simpler compounds as a result of the oxidation process. There are, however, a number of important differences between the energy produced by the fire and the energy that comes from organism metabolism. The fire burns all at once and gives off large quantities of heat and light. Aerobic oxidation in an organism, on the other hand, proceeds in a series of small and controlled steps. Much of the energy released in each step is recaptured in the high-energy bonds of a chemical called adenosine triphosphate (ATP), a compound found in all cells and serving as an energy storage site. Part of the energy released is given off as heat.
Energy metabolism begins with an anaerobic sequence known as glycolysis. Since the reactions of glycolysis do not require the presence of oxygen, it is termed the anaerobic pathway. This pathway does not produce very much energy for the body, but it establishes a base for further aerobic steps that do have a much higher yield of energy. It is believed that cancer cells do not have the necessary enzymes to utilize the aerobic pathway. Since these cells rely on glycolysis for their energy metabolism, they place a heavy burden on the rest of the body.
The aerobic pathway is also known as the Krebs citric acid cycle and the cytochrome chain. In these two steps the by-products of the initial anaerobic glycolysis step are oxidized to produce carbon dioxide, water, and many energy-rich ATP molecules. All together, all these steps are referred to as cell respiration. Forty percent of the glucose "burned" in cell respiration provides the organism with energy to drive its activities, while 60% of the oxidized glucose is dissipated as heat. This ratio of heat and energy is about the same as a power plant that produces electricity from coal.