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Fatty acid oxidation, synthesis, and ketone body basics
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Triacylglycerols (glycerol plus three fatty acids) are the storage form; phospholipids build membrane bilayers; cholesterol tunes membrane fluidity and is the precursor of steroid hormones, bile acids, and vitamin D. Saturated fatty acids pack tightly; cis double bonds kink chains and increase fluidity. Lipoproteins ferry lipids through blood: chylomicrons carry dietary fat, LDL delivers cholesterol to tissues, HDL returns it to the liver.
Fatty acids are activated to acyl-CoA (costing 2 ATP equivalents), carried into the mitochondrial matrix by the carnitine shuttle, then cleaved two carbons at a time. Each round yields A fatty acid with carbons needs rounds. Fats store roughly 9 kcal/g versus 4 kcal/g for carbohydrates because their carbons are more reduced and stored anhydrously.
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In prolonged fasting, the liver converts excess acetyl-CoA to ketone bodies (acetoacetate, beta-hydroxybutyrate) that fuel brain and muscle. Fatty acid synthesis is the cytoplasmic mirror image: acetyl-CoA carboxylase commits acetyl-CoA to malonyl-CoA, using NADPH, when insulin signals the fed state.