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.