The Krebs Cycle occurs in which part of the mitochondrion?

• A. Matrix
• B. Outer membrane
• C. Intermembrane space
• D. Nucleolus

Answer: (A)

The Krebs cycle takes place in the mitochondrial matrix because that is the fluid-filled interior where the necessary enzymes are located and where acetyl-CoA is produced and used. In this matrix, acetyl-CoA combines with oxaloacetate to form citrate, and the cycle proceeds through a sequence of enzyme-catalyzed steps that release carbon dioxide and generate NADH and FADH2. Those reduced carriers then feed the electron transport chain, which is embedded in the inner mitochondrial membrane rather than in the matrix. The intermembrane space, outer membrane, and other structures do not house the full set of cycle enzymes. The nucleolus is a nuclear structure, not in the mitochondrion. So the matrix is the correct site.