Membrane Lipids and Lipoproteins

Core Idea

Cell membranes are composed of phospholipid bilayers (glycerophospholipids and sphingolipids) with embedded and peripheral proteins. Lipoproteins (VLDL, LDL, HDL, chylomicrons) are complexes of lipids (cholesterol, triglycerides) and proteins that transport lipids through the bloodstream. VLDL and chylomicrons carry triglycerides to tissues; LDL delivers cholesterol to cells; HDL removes excess cholesterol from peripheral tissues and transports it to the liver for excretion. Dysregulation of lipoprotein metabolism is a major risk factor for atherosclerotic cardiovascular disease.

Explainer

You already understand that fatty acids can be saturated or unsaturated and that their chain length and degree of unsaturation determine their physical properties. Membrane lipids are built from these fatty acids: a glycerol backbone esterified with two fatty acid tails and a polar head group containing a phosphate and an alcohol (choline, serine, ethanolamine, or inositol). This amphipathic structure — hydrophobic tails, hydrophilic head — is what drives the spontaneous formation of the lipid bilayer, the fundamental architecture of all cell membranes. The two leaflets of the bilayer face their hydrophobic tails inward, creating a barrier that is permeable to small nonpolar molecules but impermeable to ions and most polar molecules.

Membrane fluidity is not fixed — it depends on the composition of the fatty acid tails. Unsaturated fatty acids introduce kinks (from cis double bonds) that prevent tight packing and increase fluidity. Saturated fatty acids pack tightly and decrease fluidity. Cholesterol — whose synthesis pathway you have studied — inserts between phospholipids with its hydroxyl group near the polar heads and its rigid steroid ring system alongside the fatty acid tails. At physiological temperatures, cholesterol acts as a fluidity buffer: it restricts movement of neighboring tails (reducing fluidity when it would otherwise be too high) and prevents tight crystalline packing (maintaining fluidity when it would otherwise be too low). Sphingolipids, built on a sphingosine backbone rather than glycerol, tend to have longer, more saturated tails and cluster with cholesterol into lipid rafts — thicker, more ordered membrane domains that organize signaling proteins.

Because lipids are insoluble in the aqueous environment of blood, they cannot travel freely through the circulation. Instead, they are transported in lipoproteins — spherical particles with a phospholipid monolayer on the outside, cholesterol esters and triglycerides in the hydrophobic core, and specialized apolipoproteins embedded in the surface that serve as addresses and enzyme activators. The four major classes differ in size, density, and cargo. Chylomicrons (largest, least dense) carry dietary triglycerides from the intestine to tissues. VLDL carries endogenously synthesized triglycerides from the liver. As VLDL delivers its triglyceride cargo via lipoprotein lipase, it shrinks into LDL, which is cholesterol-rich and delivers cholesterol to peripheral cells via the LDL receptor. HDL (smallest, densest) performs reverse cholesterol transport, picking up excess cholesterol from peripheral tissues and returning it to the liver for excretion into bile.

The clinical significance of this system centers on LDL. When LDL particles accumulate in the blood — due to genetic defects in the LDL receptor (familial hypercholesterolemia), dietary excess, or other causes — they infiltrate the arterial wall, become oxidized, and trigger an inflammatory cascade that produces atherosclerotic plaques. HDL counteracts this by removing cholesterol from arterial walls. This is why LDL is colloquially called "bad cholesterol" and HDL "good cholesterol," though the reality is more nuanced: it is the balance between delivery and removal, and the particle number and size, that determine cardiovascular risk.