Explain the integration of carbohydrate, protein and fat metabolism.

1. 1 · Open in two lines. Carbohydrate, protein and fat are broken down by separate pathways, but these meet at shared intermediates. The body links them to keep a steady energy supply in fed and fasting states.
2. 2 · Draw the hub chart. [Diagram: glucose → pyruvate → acetyl-CoA → TCA cycle; fatty acids → acetyl-CoA; glycerol → DHAP; amino acids → pyruvate, acetyl-CoA and TCA intermediates; one-way arrow at pyruvate → acetyl-CoA]
3. 3 · Junction points.
   • Glucose-6-phosphate: glycogen, glycolysis, HMP shunt.
   • Pyruvate: from glucose, alanine, lactate.
   • Acetyl-CoA: from glucose, fatty acids, ketogenic amino acids; makes fat, cholesterol, ketone bodies.
   • TCA cycle: common final pathway; its intermediates link to amino acids.
4. 4 · Interconversions. Glucose → fat (yes). Protein → glucose (glucogenic amino acids) and fat. Fat → glucose (no, except glycerol), because pyruvate dehydrogenase is irreversible.
5. 5 · Fed, fasting, starvation and organ roles.
   • Fed: insulin; glycogen, fat and protein are built.
   • Fasting: glucagon; glycogenolysis, gluconeogenesis, lipolysis.
   • Starvation: ketone bodies rise; brain adapts; protein is spared.
   • Liver supplies glucose and ketones; muscle, adipose tissue and brain each play a part.
6. 6 · Significance. Keeps blood glucose steady for brain and red blood cells, stores extra fuel as fat, and protects body protein in long fasts.
7. 7 · Nutrition link. Enough carbohydrate spares protein. B1, B2, B3 and B5 are needed for pyruvate → acetyl-CoA.
8. 8 · Close in one line. Sum up acetyl-CoA and the TCA cycle as the hub that ties all three fuels together.

Scoring tip: One labelled hub diagram earns most of the marks. Mark the one-way arrow at pyruvate → acetyl-CoA clearly.