Describe the beta-oxidation of palmitic acid with its energetics.

1. 1 · Open in two lines. Beta-oxidation breaks fatty acids down in the mitochondrial matrix, removing two carbons at a time as acetyl-CoA. It is named so because the β-carbon (carbon 3) is oxidised.
2. 2 · Draw the pathway. [Diagram: fatty acid → acyl-CoA → carnitine shuttle → spiral of 4 steps, each turn releasing one acetyl-CoA, one FADH2 and one NADH]
3. 3 · Activation and transport.
   • Activation: acyl-CoA synthetase forms acyl-CoA; ATP → AMP + PPi (2 ATP-equivalents).
   • Carnitine shuttle: CPT-I forms acyl-carnitine, translocase moves it in, CPT-II gives back acyl-CoA. CPT-I is rate-limiting.
4. 4 · The four repeating steps.
   • Oxidation by acyl-CoA dehydrogenase (FADH2).
   • Hydration by enoyl-CoA hydratase.
   • Oxidation by β-hydroxyacyl-CoA dehydrogenase (NADH).
   • Thiolysis by thiolase: acetyl-CoA released, chain 2 carbons shorter.
5. 5 · Energy count for palmitate. 7 cycles give 8 acetyl-CoA, 7 FADH2 and 7 NADH. Older values: 96 + 14 + 21 = 131 − 2 = 129 ATP. Newer values: 80 + 10.5 + 17.5 = 108 − 2 = 106 ATP.
6. 6 · Significance. Main fuel for heart and resting muscle, and in fasting. The liver turns surplus acetyl-CoA into ketone bodies. Malonyl-CoA blocks CPT-I, so synthesis and breakdown do not run together.
7. 7 · Nutrition link. Carnitine from meat, milk and the body's own synthesis; FAD from riboflavin, NAD+ from niacin, CoA from pantothenic acid.
8. 8 · Close in one line. Sum up beta-oxidation as the pathway that makes fat the most energy-rich fuel (9 kcal per gram).

Scoring tip: Name the ATP values you use (older 3 and 2, or newer 2.5 and 1.5) and show the working line by line.