The Krebs Cycle Animation

The second-stage reactions of aerobic respiration. The second-stage reactions occur in a mitochondrion's inner compartment. In the first preparatory reaction, a carbon atom is stripped from pyruvate and released as carbon dioxide. The remaining carbons combine with coenzyme A and give up hydrogen and electrons to NAD. The product of these reactions is acetyl-CoA. The acetyl-CoA will enter the Krebs cycle. In the first steps of the cycle, acetyl-coA transfers two carbons to oxaloacetate to form citrate. Next, another carbon atom is released as carbon dioxide, and NADH forms as NAD+ picks up hydrogen and electrons. Another carbon dioxide is released and another NADH forms. At this point, three carbon atoms have been released as carbon dioxide. This balances out the three carbon atoms that entered the mitochondrion as pyruvate. One ATP forms by substrate-level phosphorylation. FADH2 forms when FAD picks up electrons and hydrogen. The final steps regenerate oxaloacetate and form one more NADH. For each molecule of pyruvate, the second-stage reactions produce one ATP and reduce four NAD+ to NADH and one FAD to FADH2.

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