Which statement about the citric acid cycle is correct? 1. The citric acid cycle oxidizes glucose to carbon dioxide. 2 The citric acid cycle produces most of the ATP that is subsequently used by the electron transport chain. 3 The citric acid cycle depends on the availability of NAD+, which is a product of glycolysis. 4 The last reaction in the citric acid cycle produces a product that is a substrate for the first reaction of the citric acid cycle. 5 The oxidation of compounds by the citric acid cycle requires molecular oxygen.

The correct statement is:

4. The last reaction in the citric acid cycle produces a product that is a substrate for the first reaction of the citric acid cycle.

Explanation:

• Statement 1 (Incorrect): The citric acid cycle does not directly oxidize glucose to carbon dioxide. It oxidizes acetyl-CoA, which is derived from glucose (via glycolysis and pyruvate oxidation), to carbon dioxide.
• Statement 2 (Incorrect): The citric acid cycle produces some ATP (or GTP), but the majority of ATP production occurs in the electron transport chain via oxidative phosphorylation.
• Statement 3 (Incorrect): The citric acid cycle depends on NAD+^++, but NAD+^++ is regenerated during the electron transport chain, not directly from glycolysis.
• Statement 4 (Correct): The citric acid cycle ends with the regeneration of oxaloacetate, which is then used as a substrate for the first reaction of the cycle by combining with acetyl-CoA to form citrate.
• Statement 5 (Incorrect): The citric acid cycle does not require molecular oxygen directly. However, the regeneration of NAD+^++ and FAD+^++ via the electron transport chain does depend on oxygen as the final electron acceptor.