How to lay out the answer. Follow the frame, then write it in your own words.
1 · Open in two lines. Fatty acid synthesis (lipogenesis) builds palmitate (16 C) from acetyl-CoA in the cytoplasm of liver, adipose tissue and lactating mammary gland. It runs in the well-fed state.
2 · Draw the pathway. [Diagram: citrate leaving the mitochondria, acetyl-CoA → malonyl-CoA, then the synthase cycle: condensation → reduction → dehydration → reduction, ×7 → palmitate]
3 · Supply of acetyl-CoA and malonyl-CoA.
Acetyl-CoA leaves the mitochondria as citrate; ATP-citrate lyase releases it in the cytoplasm.
Acetyl-CoA carboxylase (biotin) adds CO2 to make malonyl-CoA. This is the rate-limiting step.
4 · Reactions on fatty acid synthase.
Loading: acetyl and malonyl groups bind to the synthase (ACP).
Condensation: 4-carbon acetoacetyl-ACP forms and CO2 is released.
Reduction (NADPH), dehydration, reduction (NADPH) give butyryl-ACP.
Seven rounds in all give palmitoyl-ACP; thioesterase releases palmitate.
5 · Overall equation and regulation. 8 acetyl-CoA + 7 ATP + 14 NADPH → palmitate. NADPH comes mainly from the HMP shunt. Citrate and insulin activate acetyl-CoA carboxylase; palmitoyl-CoA and glucagon inhibit it.
6 · Significance. Stores extra carbohydrate as fat. Supplies fatty acids for membranes and milk. Palmitate is lengthened and desaturated to make other fatty acids.
7 · Nutrition link. Biotin for acetyl-CoA carboxylase, pantothenic acid in ACP and CoA, niacin in NADPH.
8 · Close in one line. Sum up fatty acid synthesis as the body's way of turning surplus food energy into stored fat.
Scoring tip: Write the overall equation in a box. Examiners look for biotin, malonyl-CoA and NADPH.
Write-up practice
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