Describe the TCA cycle with its energetics and significance.

1. 1 · Open in two lines. The TCA (citric acid, Krebs) cycle oxidises acetyl-CoA to CO2 in the mitochondrial matrix. It is the final common pathway for carbohydrate, fat and protein.
2. 2 · Write the link reaction. Pyruvate → acetyl-CoA by the PDH complex, releasing CO2 and NADH.
3. 3 · Draw the cycle. [Diagram: a circle from oxaloacetate through the 8 intermediates, enzymes labelled, NADH, FADH2, GTP and CO2 marked]
4. 4 · Write the 8 steps.
   • Citrate synthase, aconitase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase.
   • Succinyl-CoA synthetase, succinate dehydrogenase, fumarase, malate dehydrogenase.
5. 5 · Energy count. 3 NADH + 1 FADH2 + 1 GTP per turn = 12 ATP (older values) or 10 ATP (newer values); two turns per glucose.
6. 6 · Significance. Final common pathway of all fuels. Amphibolic: supplies intermediates for glucose, amino acids, haem and fatty acids.
7. 7 · Nutrition link. Needs thiamine, riboflavin, niacin and pantothenic acid; thiamine deficiency (beriberi) slows it.
8. 8 · Close in one line. Sum up the TCA cycle as the hub of metabolism, where all fuels are finally burnt.

Scoring tip: Draw the cycle as a neat circle. Write enzymes outside and NADH, FADH2 and CO2 arrows inside.