Biochemical Foundations of Human Vital Activity - Volkov, N. I., & Nesen, E. N. 2000

Biochemical Foundations of Human Vital Activity
Metabolism in the Body
Stages of Nutrient Breakdown and Energy Extraction in Cells

METABOLISM is characterized by the multi-stage nature of its underlying processes. The breakdown of nutrients within body Cells and the extraction of energy from them occur gradually, encompassing three main stages: the preparatory stage, the stage of universalization, and the stage of oxidative breakdown (Fig. 8).

During the preparatory stage, complex molecules of CARBOHYDRATES, fats, and Proteins are broken down into simple structural monomers: proteins into 20 different Amino Acids, complex carbohydrates into Monosaccharides (primarily glucose), and fats into glycerol and Fatty acids. This stage releases a negligible amount of energy: 0.6% of the potential energy from the Breakdown of Proteins and carbohydrates, and about 1% from the breakdown of fats. This breakdown of nutrients takes place in the Digestive System under The Influence of digestive Enzymes and in various Tissues through the action of intracellular enzymes.

At the universalization stage, the resulting diverse substances are converted into a single common compound—acetyl-CoA, which is the active form of acetic acid:

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Fig. 8. Stages of nutrient breakdown and energy extraction in cells

Acetyl-CoA plays a central role in the metabolism of carbohydrates, fats, and proteins, as it integrates the pathways of transformation for various Organic compounds. This breakdown stage releases 1/3 of the potential energy contained in the oxidized substances.

The nutrient oxidation stage is the final metabolic pathway for the breakdown of all nutrients. It includes The Citric Acid Cycle, the terminal oxidation system (Respiratory Chain), and Oxidative Phosphorylation, all of which take place on the mitochondrial membranes. Through complex oxidative transformations, acetyl-CoA is broken down into the end products СО₂ and Н₂О. This process releases about 2/3 of the energy stored in the nutrients. Part of this energy is released as heat, while another part is stored in the chemical bonds of ATP molecules, which are formed during oxidative phosphorylation—The addition of phosphoric acid to ADP:

АДФ + Н₃РО₄ + ∆О → АТФ