Metabolism and Energy Transformation in Body Cells - Renata Armenakovna Petrosova 2004

Energy Metabolism
Stage Two: Anaerobic Oxidation

The Second Stage takes place at THE CELLULAR LEVEL in the absence of oxygen. It occurs in the Cell Cytoplasm. Let us examine The breakdown of glucose as one of the key metabolic substances in The Cell. All other organic substances (Fatty acids, glycerol, Amino Acids) are drawn into its transformation processes at various stages.

The oxygen-free breakdown of glucose is called Glycolysis. Glucose undergoes a series of sequential transformations (Fig. 16).

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Fig. 16. Scheme of the glycolysis process

Initially, it is converted into fructose, phosphorylated—activated by two ATP molecules—and transformed into fructose diphosphate. Next, the six-carbon molecule breaks down into two three-carbon compounds: two molecules of glycerophosphate (trioses). Following a series of reactions, these are oxidized, losing two hydrogen atoms each, and transformed into two molecules of pyruvic acid (PA). As a result of these reactions, four ATP molecules are synthesized. Since two ATP molecules were initially expended on the activation of glucose, the net yield is 2ATP. Thus, The energy released during glucose breakdown is partially stored in two ATP molecules and partially dissipated as heat. Four hydrogen atoms, which were removed during The oxidation of glycerophosphate, combine with the hydrogen carrier NAD⁺ (nicotinamide adenine dinucleotide). This is the same type of hydrogen carrier as NADP⁺, but it participates in Energy Metabolism reactions.

GENERALIZED SCHEME OF glycolysis reactions:

С₆Н₁₂O₆ + 2НАД⁺

2С₃Н₄O₃ + 2НАД · 2Н

2АДФ

2АТФ

The reduced NAD · 2H molecules enter the Mitochondria, where they are oxidized, releasing hydrogen.

Depending on the type of Cells, tissue, or organisms, pyruvic acid in an oxygen-free environment can undergo further conversion into lactic acid, ethanol, butyric acid, or other organic substances. In anaerobic organisms, these processes are called Fermentation.

Lactic acid fermentation:

С₆Н₁₂O_{6 (глюкоза)} + 2НАД⁺

2С₃Н₄O_{3 (ПВК)} + 2НАД · 2Н

2С₃Н₆O_{3 (молочная кислота)} + 2НАД⁺

Alcoholic Fermentation:

C₆H₁₂O_{6 (глюкоза)} + 2НАД⁺

2С₃Н₄O_{3 (ПВК)} + 2НАД · 2Н

2С₂Н₅OH _{(ethyl alcohol)} + 2СO₂ + 2NAD⁺