Carbohydrate Metabolism: Glycolysis, TCA Cycle, Gluconeogenesis and HMP Shunt
What this domain tests
Carbohydrate metabolism questions test glycolysis, the TCA cycle, gluconeogenesis, and the HMP shunt, including locations, net yields, irreversible steps, and regulation.
Core rules
- Glycolysis converts glucose to two pyruvate in the cytosol. Investment is 2 ATP; payoff is 4 ATP and 2 NADH, so net ATP is 2. Key regulated enzymes: hexokinase/glucokinase, phosphofructokinase-1, pyruvate kinase.
- Anaerobic glycolysis reduces pyruvate to lactate, reoxidizing NADH. In aerobic cells pyruvate enters mitochondria as acetyl-CoA.
- Per acetyl-CoA, the TCA cycle yields 3 NADH, 1 FADH2, and 1 GTP. Per glucose, two turns give 6 NADH, 2 FADH2, and 2 GTP. The cycle is mitochondrial and needs oxygen indirectly through the ETC.
- Gluconeogenesis is not a simple reversal. It uses pyruvate carboxylase, PEP carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase to bypass irreversible glycolytic steps.
- HMP shunt uses glucose-6-phosphate dehydrogenase to make NADPH and ribose-5-phosphate; it produces no ATP.
Worked example
One glucose gives 2 pyruvate and net 2 ATP + 2 NADH. If both pyruvate are converted to lactate, the 2 NADH are reoxidized, so the net NADH from anaerobic glycolysis is zero. If pyruvate is oxidized in the TCA cycle, the two acetyl-CoA give 6 NADH, 2 FADH2, and 2 GTP, and the ETC converts these to ATP.
Traps
Glycolysis produces 4 ATP gross, not 2. The TCA cycle does not use O2 directly. Gluconeogenesis does not simply run glycolysis backward. HMP shunt is not a major ATP source; its NADPH is for reductive synthesis and glutathione regeneration.