Why natural fatty acids almost always have an even number of carbons
Cells build fatty acids two carbon atoms at a time, adding pairs in a repeating cycle, so nearly every natural chain ends up even-numbered. The kinks in those chains matter as much as their length: a single bent double bond can change how fluid a cell membrane is, and may even help explain warm-bloodedness.
A fatty acid is a carboxylic acid attached to a hydrocarbon tail, typically unbranched and 4 to 28 carbons long. The French chemist Michel Eugene Chevreul introduced the idea in 1813, at first calling it acid fat or oily acid. In living things fatty acids rarely float free; they are usually bound up in triglycerides for storage, or in phospholipids that form every cell membrane. Neutralise stearic acid, an 18-carbon saturated chain, with sodium hydroxide and you get the most common form of soap.
Saturated chains have no carbon-carbon double bonds and lie straight. Unsaturated chains have one or more, and nature overwhelmingly uses the cis form, where hydrogen atoms sit on the same side and force a bend. Oleic acid has one kink, linoleic acid a sharper bend, and alpha-linolenic acid curls into a hook. Bent chains pack loosely, lowering melting points and making membranes more fluid. Trans double bonds leave the chain nearly straight; most trans fats come from industrial hydrogenation, though cattle and sheep produce small amounts by fermentation in the rumen.
Naming follows two conventions. Chemists count from the acid end, while nutrition labels count from the far end, the omega carbon, which is why linoleic and arachidonic acids are both called omega-6 despite different lengths. Synthesis starts in the mitochondria with acetyl-CoA, which must be smuggled out as citrate, then carboxylated to malonyl-CoA before the two-carbon additions begin. Rare odd-chain acids with 15 or 17 carbons turn up in dairy.
Fatty acids are burned only inside mitochondria. Brain cells cannot take most free fatty acids from the blood because the blood-brain barrier blocks them, so they make their own for membranes. Across species, mammal membranes hold more polyunsaturated fat than reptile membranes; that makes them leakier to ions and costly to maintain, a burden some researchers link to high metabolic rates and warm blood. Cold-water fish likewise add unsaturated fats to keep membranes supple.
Source: Fatty acid
