Human Anatomy and Physiology - I. V. Gayvoronsky 2011

Metabolism and Energy Balance
Breakdown and Oxidation of Organic Substances in Cells

The body requires a continuous supply of energy to sustain its vital Functions. This energy is generated through The breakdown of Organic compounds—primarily CARBOHYDRATES and fats, and to a lesser extent, Proteins, which are essential for driving anabolic processes. Energy is released when the chemical bonds between the atoms of these molecules are broken. Part of this energy is dissipated as heat, while the remainder is stored as ATP (adenosine triphosphate). The ratio of dissipated energy to stored energy is approximately 1:1.

The ATP molecule contains high-energy bonds between its phosphoric acid residues; the Cleavage of these bonds releases a substantial amount of energy. During ATP Hydrolysis, these bonds are broken sequentially, yielding ADP (adenosine diphosphate) and AMP (adenosine monophosphate). The energy stored in ATP can be utilized by cellular mechanisms as needed, making ATP the universal energy currency of The Cell.

The core Mechanism of ATP generation is phosphorylation—the attachment of a phosphoric acid residue to ADP. However, this process requires energy derived from the breakdown of complex organic molecules and tissue Respiration. A classic example is The formation of ATP via the breakdown of a single glucose molecule (С₆Н₁₂О₆). The complete cellular degradation of glucose into carbon dioxide and Water requires both anaerobic (oxygen-free) and aerobic (oxygen-dependent) oxidation pathways (Fig. 10.1).

Glycolysis (anaerobic oxidation). This process occurs in the Cell Cytoplasm without the involvement of oxygen. Recent findings indicate that glycolysis can also proceed actively and at high rates under aerobic conditions. Glycolysis involves a sequence of 10 biochemical reactions, each catalyzed by a specific enzyme. When oxygen is abundant in the cell, the end product of anaerobic oxidation is pyruvic acid (PA). When oxygen is deficient, an eleventh glycolytic reaction takes place, converting pyruvic acid into lactic acid. Through these 10 primary reactions, two molecules of pyruvic acid and two molecules of ATP are produced.

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Fig. 10.1. Breakdown and oxidation of glucose in the cell

Oxygen deficiency commonly occurs in Cells during periods of intense physical exertion. Under such conditions, glycolytic pathways in the cytoplasm are upregulated, leading to the copious production of lactic acid (lactate) from glucose. This substance cannot be further utilized by the cell and must be removed. Significant accumulation of lactate leads to discomfort associated with the acidification of the body's internal environment.

Aerobic oxidation. Pyruvic acid is transported from the cell cytoplasm into the Mitochondria, where it undergoes decarboxylation to form acetic acid. This acid is then completely oxidized in the Krebs cycle into carbon dioxide, releasing hydrogen protons. In the Electron Transport Chain, these hydrogen protons combine with oxygen to form water, driving the synthesis of 36 ATP molecules. The overall reaction for glucose breakdown can be represented as follows:

С₆Н₁₂О₆ + 6О₂→ 6СО₂ + 6Н₂О + Q (energy)

Tissue respiration. This term refers to the gas exchange occurring within cells during the Biological Oxidation of nutrients. Through these oxidative processes, cells produce carbon dioxide as a metabolic end product while simultaneously taking up oxygen from Blood capillaries. Concurrently, the hydrogen atoms generated during glucose oxidation are transferred to Enzymes located on The inner mitochondrial membrane—a pathway known as the Respiratory Electron Transport chain. Hydrogen reacts with oxygen to form water. The flow of hydrogen protons releases a significant amount of energy, which is harnessed to synthesize ATP from ADP and inorganic phosphate. As a result of these reactions, The oxidation of a single glucose molecule yields 38 ATP molecules. Notably, oxygen deficiency limits oxidative reactions far more severely than the inadequate removal of carbon dioxide. The energy stored in ATP is utilized by the Organism to support all its functions and life processes:

1) the synthesis of novel, organism-specific organic compounds (proteins, Lipids, carbohydrates, DNA) and the formation of new Cellular Structures and Organelles;

2) the execution of fundamental cellular processes (such as mitosis and transmembrane substance transport);

3) the Maintenance of the body's Temperature Homeostasis.