The citric acid cycle completes the oxidation of glucose to CO2

Acetyl CoA is the starting point for the citric acid cycle. This pathway of eight reactions completely oxidizes the two-carbon acetyl group to two molecules of CO2. The free energy released from these reactions is captured by GDP (guanosine diphosphate) and the electron carriers NAD+ and FAD (Figure 9.6). (Remember from Key Concept 7.2 that GDP is a nucleoside diphosphate like ADP.) This is a cycle because the starting material, oxaloacetate, is regenerated in the last step and is ready to accept another acetate group from acetyl CoA. The citric acid cycle operates twice for each glucose molecule that enters glycolysis (once for each pyruvate that enters the mitochondrion).

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Figure 9.6 The Citric Acid Cycle The citric acid cycle has eight steps; in the last step, the starting material acceptor, oxaloacetate, is regenerated. Energy is released and captured by reducing NAD+ or FAD, or by producing GTP. “6C,” “5C,” and so on indicate the number of carbon atoms in each intermediate in the cycle.

Activity 9.3 The Citric Acid Cycle

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Let’s focus on the final reaction of the cycle (Step 8 in Figure 9.6), as an example of the kind of reaction that occurs:

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This oxidation reaction (see the carbon atom highlighted in blue) is exergonic, and the released energy is trapped by NAD+, forming NADH. With four such reactions (the FADH2 produced in Step 6 is a reduced coenzyme similar to NADH), the citric acid cycle harvests a great deal of chemical energy from the oxidation of acetyl CoA.

To summarize:

GTP + ADP → ATP + GDP

Thus the citric acid cycle releases two carbons as CO2 and produces four reduced electron carrier molecules.

Overall, for each molecule of glucose that is oxidized, two molecules of pyruvate are produced during glycolysis, and after oxidation these feed two turns of the citric acid cycle. So the oxidation of one glucose molecule yields: