Describe the urea cycle. Add a note on its regulation and disorders.

1. 1 · Open in two lines. The urea cycle (Krebs–Henseleit or ornithine cycle) turns toxic ammonia into harmless urea. It runs in the liver, starting in the mitochondria and finishing in the cytoplasm.
2. 2 · Draw the cycle. [Diagram: cycle across mitochondria and cytoplasm; five enzymes, ATP used, aspartate in, fumarate out, urea out, ornithine returning]
3. 3 · Write the 5 steps.
   • Mitochondria: CPS I forms carbamoyl phosphate; ornithine transcarbamoylase forms citrulline.
   • Cytoplasm: argininosuccinate synthetase adds aspartate; argininosuccinate lyase releases fumarate; arginase releases urea and ornithine.
4. 4 · Energy and regulation.
   • 3 ATP per urea (4 high-energy bonds).
   • N-acetylglutamate switches on CPS I; a high-protein diet increases the enzymes.
   • Fumarate links the cycle to the TCA cycle.
5. 5 · Disorders. Enzyme defects (OTC deficiency is the commonest) and liver disease cause hyperammonaemia: vomiting, confusion, coma.
6. 6 · Significance. Removes ammonia safely; urea is the main form in which the body excretes nitrogen.
7. 7 · Nutrition link. More protein means more urea; in liver disease ammonia builds up, so protein intake is planned carefully.
8. 8 · Close in one line. Sum up the urea cycle as the liver's ammonia clean-up system.

Scoring tip: Draw one big circle split by a dotted line for the mitochondrial membrane. Labelled enzymes and the 3 ATP note carry most of the marks.