(also called internal respiration), the aggregate of enzymatic processes occurring during the absorption of atmospheric oxygen in the cells of organs and tissues. The products of the breakdown of carbohydrates, fats, and proteins subsequently become oxidized into carbon dioxide and water, and a large amount of the released energy is stored in the form of high-energy compounds. Tissue respiration differs from external respiration—the aggregate of physiological processes that take oxygen into the body and eliminate carbon dioxide. Many enzymes that catalyze these processes are located in cytoplasmic organelles called the mitochondria.

All manifestations of life—growth, movement, excitation, and reproduction—consume energy. The form of energy used by cells is the energy of the chemical bonds in high-energy compounds, mainly such phosphates as adenosinetriphosphate (ATP). An intake of energy from external sources is necessary for the synthesis of ATP. The principal difference between an autotrophic and heterotrophic organism is the organism’s method of obtaining energy. The cells of green plants—the most typical autotrophes—use solar energy for the synthesis of ATP and glucose during the process of photosynthesis. Within plant cells, more complex molecules are formed from glucose during tissue respiration. In the cells of heterotrophes—animals and man—energy is derived solely from the molecular chemical bonds of food substances. The molecules of various compounds function as biological fuel and include glucose, fatty acids, and some amino acids. These molecules are formed in the cells or enter the blood from the digestive tract; they subsequently undergo a number of chemical changes.

There are three basic stages in the process of tissue respiration. The first consists of the oxidation formation of acetyl coenzyme A (an active form of acetic acid) from pyruvic acid (an intermediate product of glucose breakdown), fatty acids, and amino acids. The second stage involves breakdown of acetyl residues during the tricarboxylic acid cycle and the release of two molecules of carbon dioxide and four pairs of hydrogen atoms. The latter are partly accepted by the coenzymes nicotinamide adenine dinu-cleotide and flavin adenine dinucleotide, and partly become dissolved into protons. The third stage consists of the transfer of electrons and protons to molecular oxygen (the formation of H₂O), a process that is catalyzed by a set of respiratory enzymes and is conjugated with the formation of ATP. This catalysis is known as oxidizing phosphorylation. The first two stages are a preparation for the third, during which most of the energy produced in the cell is released owing to oxidation-reduction reactions. Approximately 50 percent of the energy resulting from oxidizing phosphorylation is stored in the form of high-energy ATP bonds; the remainder is released as heat.

Tissue respiration ensures the formation and constant replenishment of ATP in the cells. When the supply of oxygen to the cells of animals and man is insufficient, the reserves of ATP are not immediately exhausted. They may be replenished through such auxiliary mechanisms as glycolysis and glycogenolysis, that is, the anaerobic breakdown of carbohydrates. However, in terms of energy, this method is far less efficient and cannot ensure the functioning and structural integrity of the organs and tissues. The biological role of tissue respiration extends beyond its already significant contribution to the energy metabolism of the organism. During different stages of tissue respiration, molecules of organic compounds are formed that are used by cells as intermediate products for various biosyntheses.