Mitochondria carry out ATP synthesis that occurs as a result of oxidation of organic substrates and the phosphorylation of ADP. In the cells of the oxidation processes and the allocation of energy released by this process, taking place in several interrelated steps. At the same time as the initial substrates, various carbohydrates, fatty acids and amino acids. The first stages of oxidation of lead besides the formation of ATP to the emergence of intermediate products, final oxidation in the function of mitochondria, which allows the cell to use this process for the synthesis of the basic amount of ATP.

The initial stages of oxidation of carbohydrates occur in Hyaloplasm and do not require the participation of oxygen. Therefore they are called anaerobic oxidation, or glycolysis. Main substrate oxidation during anaerobic energy generation is the hexoses, primarily glucose and some bacteria have the property to extract energy, oxidizes pentose, fatty acids or amino acids. In the glucose amount of potential energy contained in the bonds between atoms C, N and O, is about 680 calories per 1 mol (i.e. 180 g of glucose), this energy is released when the complete oxidation of glucose according to the following reaction:

S6N12O6 6O2Þ + 6H2O + 6SO2 + 680 kcal

In a living cell is a tremendous amount of energy is not released at the same time as the combustion in the flame. Energy release comes in the form of a stepped process controlled by a number of oxidative enzymes, and is not associated with the transition of the chemical bond energy to heat, as in combustion, and the transition it macro energetic bond in the molecule ATP, which is synthesized by using the energy liberated from ADP and phosphate .

In the process of glycolysis is an incomplete oxidation of the substrate. As a result of glycolysis splits glucose to triose, while spent two ATP molecules are synthesized and 4 molecules of ATP. So the end result is a cell "earns" only 2 ATP molecules. The energy for this process is inefficient, and therefore out of 680 kcal, involves the connection of 1 mol of glucose released less than 10% of energy. Despite the low power output, anaerobic oxidation, glycolysis, is widely used in wildlife. It is a major process of supplying energy for many microorganisms, some of the anaerobic intestinal parasitic protozoa, for cells of higher organisms in the early stages of embryonic development, many tumor cells to tissue culture cells and other mammalian erythrocytes, for example, get all they need energy from the glycolysis, since they have no mitochondria.

Formed by glycolysis triose, and primarily pyruvic acid, are involved in further oxidation, which occurs already in the function of mitochondria. This results in energy splitting of chemical bonds, which leads to release of CO2 to oxygen consumption and synthesis of a large number of ATP. These processes are associated with oxidative citric acid cycle and respiratory electron transport chain, where the phosphorylation of ADP and synthesis of cellular "fuel" molecules of ATP.