Where do the reactions of the citric acid cycle occur?

• A. In the inner mitochondrial membrane
• B. In the matrix of the mitochondria
• C. In the cytoplasm
• D. In the intermembrane space

Answer: (B)

The citric acid cycle runs in the mitochondrial matrix. In this compartment, the enzymes that drive the cycle are soluble and work on acetyl-CoA to produce citrate, then regenerate oxaloacetate while generating NADH and FADH2 to fuel the electron transport chain. The inner mitochondrial membrane hosts the electron transport chain and ATP synthase, which are responsible for oxidative phosphorylation, not the cycle itself. The cytoplasm is where glycolysis happens, and the intermembrane space is just the gap between mitochondrial membranes, not where the TCA cycle enzymes reside. So the matrix is the correct site because it houses the enzyme set necessary for the cycle to proceed.