SECTION 5. ENERGY METABOLISM

Main subjects:

5.1. Tissue respiration and oxidative phosphorylation

of ADP

5.2. Structural organization of electron transport chain

5.3. Specific and common pathways of catabolism

5.4. Oxidative decarboxylation of pyruvate

5.5. Citric acid cycle

5.1. TISSUE RESPIRATION AND OXIDATIVE PHOSPHORYLATION

OF ADP

The energy source for humans is the degradation of organic compounds of foodstuff. Predominantly macromolecules (polysaccharides, proteins, fats) enter with food and are digested into smaller units (glucose, amino acids, fatty acids, and glycerol). Within cells these compounds undergo conversions being involved into the metabolism (substances turnover). They may be used for more complex molecules synthesis (anabolism), or degraded to final products CO₂ and H₂O in the process of catabolism (Fig. 5.1).

Fig. 5.1. Metabolism in the body

Degradation of substances in the tissues is accompanied by the consumption of oxygen and release of carbon dioxide. Energy required for cell functioning is released during this process. Inhaled oxygen (more

than 500 L/24 h) is used for synthesis of metabolic water with participation of hydrogen of oxidized substrates (SH₂) in the process called tissue respiration:

For example, oxidation of 1 mole of glucose is accompanied by release of energy equal to 2780 kJ:

C₆H₁₂0₆+60₂ → 6C0₂ + 6H₂0 +2780 kJ/mol

The energy of oxidized compounds is used for synthesis of ATP from ADP. ADP phosphorylation in cells occurs by addition of inorganic phosphate H₃PO₄. The reaction occurs with energy expenditure (Fig. 5.2).

Fig. 5.2. ADP-ATP cycle

ATP nucleotide that is formed includes two high-energy bonds (~) and is used in tissues as universal energy source for synthesis of compounds and for other types of activity (active transport of compounds through membranes, muscular contraction etc.).