Which organelle is the site where oxaloacetate combines with acetyl-CoA to initiate the citric acid cycle?

• A. Mitochondria
• B. Nucleus
• C. Cytosol
• D. Chloroplast

Answer: (A)

The citric acid cycle starts in the mitochondria. In the mitochondrial matrix, acetyl-CoA (from pyruvate or fatty acid breakdown) combines with oxaloacetate to form citrate, a reaction catalyzed by citrate synthase. This condensation is the first step of the cycle and sets up the sequence that extracts energy-rich electrons through successive oxidation steps. The cycle remains in the mitochondria because the enzymes needed are there and because acetyl-CoA is produced in that same compartment. Oxaloacetate is regenerated at the end of each turn, allowing the cycle to continue as long as substrates are present. Other locations listed don’t host this process in eukaryotic cells: the nucleus houses genetic material, the cytosol is where glycolysis occurs, and chloroplasts carry out photosynthesis.