The Krebs cycle primarily takes place in which part of the cell?

• A. Cytosol
• B. Intermembrane space
• C. Mitochondrial matrix
• D. Outer mitochondrial membrane

Answer: (C)

The Krebs cycle takes place in the mitochondrial matrix because that's where the soluble enzymes of the cycle are located and where acetyl-CoA (derived from pyruvate in the matrix) is available to initiate the cycle. The matrix provides the aqueous, enzyme-rich environment needed for the sequence of reactions that convert acetyl-CoA into CO2, generating NADH and FADH2 on the way, which then feed the electron transport chain in the inner mitochondrial membrane.

Other compartments don’t house the full set of Krebs-cycle enzymes or the substrates in the right context. The intermembrane space is where the proton gradient builds up to drive ATP synthase, and the outer mitochondrial membrane is permeable to many small molecules, not the site of the cycle’s enzyme reactions. The cytosol hosts glycolysis in eukaryotes, not the Krebs cycle.