Advanced Placement (AP) Biology Practice Exam

In which location of the mitochondria does the Krebs cycle occur?

1. Outer mitochondrial membrane

2. Intermembrane space

3. Mitochondrial matrix

4. Cytoplasm

The correct answer is: Mitochondrial matrix

The Krebs cycle, also known as the citric acid cycle, takes place in the mitochondrial matrix. This is the innermost compartment of the mitochondria, where the enzymes required for the Krebs cycle are located. The mitochondrial matrix provides the necessary environment for the cycle to occur, including the appropriate pH and concentration of substrates. During the Krebs cycle, acetyl-CoA is oxidized, releasing carbon dioxide and transferring high-energy electrons to carriers like NAD+ and FAD. These carriers then transport the electrons to the electron transport chain, which is crucial for ATP production. The specific conditions and enzymes present in the mitochondrial matrix are optimized for these reactions, differentiating it from other compartments of the mitochondria or cellular structures. The outer mitochondrial membrane, intermembrane space, and cytoplasm do not contain the enzymes or the conditions suitable for the biochemical reactions of the Krebs cycle, which is why they do not serve as the site for this important metabolic process.