Through a series of reactions involving the elimination of two carbons and four oxygens (as two CO2 molecules) and the removal of hydrogens, citric acid is eventually converted to ox-aloacetic acid, which completes the cyclic metabolic pathway (fig. 5.8). In this process, these events occur:

1. One guanosine triphosphate (GTP) is produced (step 5 of fig. 5.8), which donates a phosphate group to ADP to produce one ATP.

2. Three molecules of NAD are reduced to NADH (steps 4, 5, and 8 of fig. 5.8).

3. One molecule of FAD is reduced to FADH2 (step 6).

The production of NADH and FADH2 by each "turn" of the Krebs cycle is far more significant, in terms of energy production, than the single GTP (converted to ATP) produced directly by the cycle. This is because NADH and FADH2 eventually donate their electrons to an energy-transferring process that results in the formation of a large number of ATP.

Chapter Five

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Essentials of Human Physiology

Essentials of Human Physiology

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