Describe glycolysis, the citric acid cycle, and oxidative phosphorylation, including where each occurs and the net ATP yield per glucose in eukaryotes.

• A. Glycolysis in cytosol with a net of 2 ATP; pyruvate to acetyl-CoA in mitochondria; CAC in mitochondrial matrix yields 2 GTP/ATP and NADH/FADH2; oxidative phosphorylation on the inner mitochondrial membrane yields ~26-28 ATP; total around 30-32 ATP per glucose.
• B. All steps occur in the cytosol; total ATP 4.
• C. Glycolysis occurs in the nucleus; CAC occurs in chloroplast.
• D. Glycolysis yields 40 ATP; CAC yields 40 NADH.

Answer: (A)

Glycolysis, the citric acid cycle, and oxidative phosphorylation work together to extract energy from glucose, with each step having a distinct location and role. Glycolysis happens in the cytosol and converts one glucose molecule into two pyruvate molecules, yielding a net two ATP through substrate-level phosphorylation and producing two NADH molecules. The pyruvate then enters the mitochondria and is converted to acetyl-CoA by the pyruvate dehydrogenase complex, generating NADH in the process and linking glycolysis to the next stage.

The citric acid cycle occurs in the mitochondrial matrix. Each acetyl-CoA that enters the cycle is oxidized to CO2, and per turn yields 3 NADH, 1 FADH2, and 1 GTP (which is equivalent to ATP). Since two acetyl-CoA molecules come from one glucose, the cycle produces 6 NADH, 2 FADH2, and 2 GTP per glucose.

Oxidative phosphorylation takes place on the inner mitochondrial membrane. The NADH and FADH2 deliver electrons to the electron transport chain, creating a proton gradient that powers ATP synthase to produce ATP. The typical yield is about 26–28 ATP per glucose from this stage in most eukaryotes, with NADH contributing roughly 2.5 ATP per molecule and FADH2 about 1.5 ATP. When you add the ATP from glycolysis and the GTP from the citric acid cycle, you arrive at a total of roughly 30–32 ATP per glucose. The exact number can vary a bit depending on the shuttle systems that move electrons from cytosolic NADH into the mitochondrion.