Lipids: The Building Blocks That Refuse to Polymerize

One cis double bond is the difference between lard and olive oil. Same number of carbons in the chain, the same carboxyl head, the same chemistry on paper. Add a single kink and a fat that was solid at room temperature pours like a liquid. That observation is the whole chapter in miniature, because the rule it obeys runs through everything lipids do: shape is destiny. Get the shape right and the rest follows — how it packs, what it does, and where in your body it shows up.

Lipids break the rules

Proteins are polymers of amino acids. DNA is a polymer of nucleotides. Lipids are neither. There is no “the lipid polymer,” and that absence is the point. Lipids are assembled combinatorially from a small kit of modular parts: fatty acids, glycerol, sphingosine, and sterols, mixed and matched. The only property that unites all of them is solubility in organic solvent. That is a definition by behavior, not by structure, and it is unusual enough to be worth pausing on.

Lipids sort into two big jobs. Some store energy. Others build membranes and carry signals. The same handful of parts does both.

Fatty acids and the geometry of melting

A fatty acid is a long hydrocarbon tail with a carboxylic acid head. At physiological pH the head matters less than you might think: the carboxyl pKa is about 4.8, so at pH 7.4 it is more than 99% deprotonated. That is why the correct name is palmitate, not palmitic acid. The molecule in your blood is the ion.

Two competing rules set a fatty acid’s melting point. Each CH₂ you add to the chain raises the melting temperature by roughly 2 °C, because a longer chain has more surface area for van der Waals contact with its neighbors. Each cis double bond lowers it sharply. The double bond is not weaker — double bonds are stronger than single bonds. It is the geometry that matters: a cis double bond puts a fixed kink in the chain, and kinked chains cannot pack tightly, so the solid falls apart at a lower temperature. Palmitate (16:0, saturated, straight) is a waxy solid. Oleate (18:1 cis-Δ9, one kink) is a liquid. Lard versus olive oil.

Chemists number a fatty acid from the reactive carboxyl end, where reactions happen, and mark double bonds with Δ. Nutritionists count from the methyl, or omega, end. The omega convention earns its keep because human desaturases cannot install a double bond beyond Δ9. We cannot make omega-3 or omega-6 fatty acids, which is exactly what makes them dietary essentials. One of them, arachidonate, feeds into cyclooxygenase to make prostaglandins, the signaling molecules behind inflammation, fever, and pain. NSAIDs block that enzyme, which is also why they irritate the stomach: the same prostaglandins protect the gastric lining.

Trans fats are the cautionary case. A trans double bond has the same molecular formula and the same number of double bonds as its cis twin, but the trans geometry is nearly linear, so it packs like a saturated fat while resisting normal beta-oxidation. The body handles it poorly. Artificial trans fats were banned by the FDA in 2018.

The payoff of all this chemistry is energy. Palmitate yields about 106 ATP; glucose yields about 30-32. Fat carries roughly 9 kcal/g against carbohydrate’s 4, and the real-world gap is larger still: glycogen is stored hydrated, binding about 2 grams of water per gram, while fat is stored anhydrous. Effective storage runs near 9 versus 1.5 kcal/g, a sixfold advantage. A migrating bird crossing an ocean carries fat, not glycogen, for the same reason you do.

Triacylglycerols: the savings account

Stored fat is a triacylglycerol: three fatty acids esterified to a glycerol backbone. Natural fats are mixtures. Saturated chains tend to occupy the sn-1 and sn-3 positions and an unsaturated chain the sn-2, which gives natural fats an intermediate melting point rather than the high one a pure saturated fat would have. The “sn” is stereospecific numbering; sn-2 is the chiral center, and pancreatic lipase cleaves at sn-1 and sn-3 to release fat for absorption.

We need fat to be dense, and that is pure thermodynamics. A 70 kg person stores about 135,000 kcal as fat but only about 1,600 usable kcal as glycogen. Fat is the savings account: dense, slow, patient. Glycogen is the checking account: fast to draw on but heavy with water. Replace your fat with the energy-equivalent mass of hydrated glycogen and you would be hauling tens of extra kilograms.

The lipid droplets that hold all this were drawn for decades as inert yellow blobs. In fact they are perilipin-coated organelles that physically dock onto mitochondria to hand off fatty acids, and perilipin gates a lipase’s access to the stored fuel. When that regulation fails, the consequences are clinical: perilipin mutations cause lipodystrophy, and droplet dysfunction contributes to fatty liver disease and type 2 diabetes.

Membrane lipids: barrier, fluidity, identity

A membrane has to do three things. It must be a barrier that keeps the inside in. It must stay fluid enough for its proteins to move and work. And it must carry an identity the cell and its neighbors can read. Three lipid classes solve those three problems.

The trick that makes a membrane possible is amphipathicity. A phospholipid has a water-loving head and water-fearing tails, so in water it self-assembles into a bilayer with no help required. Most membrane lipids are phospholipids, and a phospholipid has four parts: two fatty-acyl tails, a platform (glycerol or sphingosine), a phosphate, and a variable head alcohol.

That variable head is where the function lives. Head-group identity sets charge, charge sets which leaflet the lipid sits in, and location sets what it does. Phosphatidylcholine is uncharged and faces outward. Phosphatidylserine is negative and is held on the inner leaflet, where positively charged cytoplasmic proteins meet it. Phosphatidylinositol is also inner and is the signaling lipid: PIP₂ is cleaved by phospholipase C into DAG and IP₃. Cardiolipin, with two phosphates and four tails, lives only in the mitochondrion.

The most striking case is the death flag. A healthy cell keeps phosphatidylserine hidden on the inner leaflet using ATP-dependent flippases. When a cell undergoes apoptosis it flips PS to the outer surface, and a macrophage reads externalized PS as an “eat me” signal, engulfing the dying cell before it bursts and inflames the tissue. Some hemolytic anemias come from flippase mutations that expose PS on red cells, which the immune system then attacks.

One structural diagnostic is worth memorizing. Glycerophospholipids hang their tails off a three-carbon glycerol by ester bonds. Sphingolipids use an 18-carbon sphingosine backbone with one amide bond. Ester versus amide tells the two families apart. Sphingomyelin, a sphingolipid, is enriched in the myelin that insulates nerves; when the enzyme that breaks it down is missing, sphingomyelin accumulates and produces Niemann-Pick disease, with seizures and cognitive loss. Disrupt lipid catabolism and you wreck the nervous system.

Glycolipids are the cell’s name tag. Their sugars face outward only, advertising the cell’s identity to the immune system. The A and B blood-group antigens are carried on cell-surface glycolipids and glycoproteins, and differ by a single terminal sugar — N-acetylgalactosamine for A, galactose for B — which is why transfusion matching can be a matter of life and death.

Then there is cholesterol, a rigid tetracyclic steroid about as long as a 16-carbon fatty acid, spanning a single leaflet. Its hydroxyl hydrogen-bonds at the head-group interface, its ring system pins the upper part of neighboring chains, and its tail reaches into the core. It is a thermostat, not a heater or an air conditioner. When the membrane is hot and loose, cholesterol restricts movement and adds order. When the membrane is cold, it wedges between chains and prevents them from crystallizing, raising fluidity. It buffers the membrane in both directions, and it is the precursor to every steroid hormone you make.

That two-way buffering, and the way chain length and double bonds push a membrane’s melting temperature around, is best felt rather than read. Try the bench.

Holding the membrane together

Every membrane has a melting temperature, Tm: a gel below it, a fluid above. Saturated lipids raise Tm, unsaturated lipids lower it, and organisms tune their fatty-acid composition to hold viscosity constant as the environment changes. This is homeoviscous adaptation. Cool E. coli and it makes more unsaturated fatty acids. A salmon near freezing loads its membranes with omega-3 chains to stay fluid, while a warm-blooded mammal can afford saturated fat at 37 °C. Same physics, opposite solutions, dictated entirely by habitat temperature.

Proteins associate with membranes in three ways, and the extraction test tells them apart. Integral proteins sit in the hydrophobic core and need detergent to remove. Peripheral proteins cling to the surface by ionic and hydrogen bonds and come off with a salt or pH change. Lipid-anchored proteins are tethered by a covalent fatty acid (palmitoyl, farnesyl) or a GPI anchor and never enter the core. A transmembrane segment has a problem to solve: its backbone amides need hydrogen-bond partners, and the lipid core offers none. An alpha-helix solves this by satisfying its own backbone internally, taking about 20 hydrophobic residues to cross the roughly 30-angstrom core; a beta-barrel solves it by hydrogen-bonding strand to neighboring strand.

Lipidation has its own clinical edge. The Ras oncoprotein must be farnesylated to reach the membrane, which made farnesyltransferase a cancer drug target. And in Hutchinson-Gilford progeria, a lamin A mutation leaves a farnesyl group stuck in place, anchoring the protein in the nuclear membrane, deforming the nucleus, and driving premature aging. Geometry, again, deciding fate.

Next chapter takes the membrane’s barrier function and asks the obvious follow-up: if the bilayer blocks ions, how does a channel let potassium through while turning sodium away?