Fundamentals of Molecular Biology. Part 1: Molecular Cell Biology - A. N. Ogurtsov 2011

Glucose and Fatty Acid Oxidation
Anaerobic Metabolism

Many eukaryotes are obligate aerobes—they grow only in the presence of oxygen and metabolize glucose (or similar sugars) completely to СО₂, while synthesizing a large amount of ATP.

Most eukaryotes, however, can synthesize a certain amount of ATP via anaerobic METABOLISM (Figure 152).

Some eukaryotes are facultative anaerobes, meaning they grow both in the presence and absence of oxygen. For example, Annelids (segmented worms), Mollusks, and certain Yeasts can live and reproduce without oxygen for many days.

In the absence of oxygen, facultative anaerobes convert a glucose molecule into one or more two- and three-carbon compounds, which are typically excreted into the surrounding environment.

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Figure 152 - Anaerobic glucose metabolism: a - in Yeast; b - in Muscles

For instance, yeast metabolize glucose into two pyruvates while synthesizing additional ATP. In this process, two molecules of NADH are produced from НАД⊕ for every glucose molecule (Figure 150).

In the absence of oxygen, yeast Cells convert each Pyruvate molecule into ethanol and СО₂. Simultaneously, for every two pyruvate molecules converted to СО₂ and ethanol, two NADH molecules are oxidized back to НАД⊕, thereby restoring the initial НАД⊕ concentration (Figure 152(a)). This anaerobic breakdown of glucose, known as Fermentation, serves as the technological basis for brewing and winemaking.

During prolonged Muscle activity, when oxygen reserves are depleted, muscle cells convert a glucose molecule into two molecules of lactic acid, once again synthesizing an additional two molecules of ATP per glucose molecule (Figure 152(6)).

This lactic acid is precisely what causes muscle soreness. It is primarily secreted into the bloodstream, while a portion is retained by Liver cells where it is re-oxidized to pyruvate and either further metabolized aerobically to СО₂ or converted back into glucose.

The majority of lactate is metabolized to СО₂ by The Heart, which receives a continuous, robust Blood flow and can maintain aerobic metabolism while working muscles secrete lactate.

Lactic acid Bacteria (lactobacilli, the organisms responsible for milk souring) and other prokaryotes utilize glucose metabolism into lactate to synthesize ATP.

When sufficient oxygen is available within The Cell, the pyruvate produced via Glycolysis is transported into the mitochondrion, where it is oxidized by oxygen to СО₂ through a cycle of reactions known as cellular Respiration (Figure 153).

These reactions yield an additional 28 ATP molecules per glucose molecule, which significantly exceeds the ATP yield of anaerobic glucose metabolism.