Lipid Metabolism

Lipid Classes

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.

Beta-Oxidation

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 $1\,\text{FADH}_2 + 1\,\text{NADH} + 1\,\text{Acetyl-CoA}$ A fatty acid with $n$ carbons needs $\tfrac{n}{2} - 1$ rounds. Fats store roughly 9 kcal/g versus 4 kcal/g for carbohydrates because their carbons are more reduced and stored anhydrously.

Ketones and Synthesis

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.

Key Takeaways

• Each beta-oxidation round: one FADH$_2$, one NADH, one acetyl-CoA.
• Carnitine shuttle in, citrate shuttle out — location separates breakdown from synthesis.
• Ketone bodies are exported liver acetyl-CoA for the fasting brain.
• Reduction state and anhydrous storage explain fat's energy density.