Human Biochemistry, Volume 1 - Murray R. 1993

Bioenergetics and Carbohydrate and Lipid Metabolism
Glycolysis and Pyruvate Oxidation
The Glycolytic Pathway

Anaerobic Glycolysis

Even in the Cytology/cytology/16.html">Early stages of Carbohydrate METABOLISM research, it was established that Fermentation in Yeast bears a striking resemblance to Glycogen breakdown in Muscle. Investigations of the glycolytic pathway were conducted precisely using these two systems.

Studies of biochemical changes During Muscle contraction revealed that when a muscle Functions in an anaerobic (oxygen-free) environment, glycogen disappears, while Pyruvate and lactate emerge as the main end products. If oxygen is subsequently supplied, “aerobic recovery” is observed: glycogen is synthesized, and pyruvate and lactate disappear. When a muscle works under aerobic conditions, lactate does not accumulate, and pyruvate is further oxidized to CO₂ and H₂O. These observations led to the traditional division of carbohydrate metabolism into anaerobic and aerobic phases. However, this division is somewhat arbitrary, since the glycolytic reactions themselves are identical in both the presence and absence of oxygen—the differences lie solely in their rates and end products. Under oxygen-deficient conditions, the reoxidation of NADH (formed from NAD during glycolysis) is coupled with the reduction of pyruvate to lactate; the resulting NAD ensures the continuation of the glycolytic reactions (Fig. 18.1). Thus, glycolysis can proceed anaerobically, but this comes at the cost of yielding less energy per mole of utilized glucose. Consequently, to generate a given amount of energy via glycolysis under anaerobic conditions, larger quantities of glucose are required than under aerobic conditions.

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Fig. 18.1. General scheme of glycolysis.