Lipids are the one biomolecule class defined by a physical property rather than a functional group: they are the biological molecules that dissolve in nonpolar solvents and not in water. That makes the category structurally mixed, and it means the useful question about a lipid is usually what its shape does rather than what its functional group does.
Fatty acids, and the kink that changes everything
A fatty acid is a long unbranched carboxylic acid, typically 12–20 carbons. It is saturated if it has no C=C and unsaturated if it has one or more.
Naturally occurring unsaturated fatty acids have their double bonds in the cis configuration, and that single fact explains the physical behavior of every fat:
- A saturated chain is straight, so molecules stack closely, van der Waals contact is maximized, and the melting point is high. Saturated fats are solids at room temperature.
- A cis double bond puts a permanent kink in the chain, so molecules cannot pack as well, contact is reduced, and the melting point is low. Unsaturated fats are oils.
- More double bonds means more kinks and a lower melting point again.
Triglycerides are triesters
The storage form of fat is a triglyceride: glycerol, a three-carbon triol, esterified with three fatty acids. Nothing about it is a new reaction — it is three Fischer esterifications, and everything an ester does, a triglyceride does.
In particular, saponification is simply base hydrolysis of those esters. Heating a triglyceride with NaOH gives glycerol and three carboxylate salts, which are soap.
Soap cleans because a carboxylate salt has both halves at once: a long nonpolar tail that dissolves in grease and an ionic head that dissolves in water. Many of them assemble into a micelle — tails inward around a droplet of oil, heads outward facing the water — and the grease leaves with the rinse.
Phospholipids and the bilayer
Replace one of a triglyceride's three fatty acids with a phosphate-containing group and you have a phospholipid: two nonpolar tails and one charged head. That is the same amphipathic arrangement as soap, with two tails instead of one, and the geometry drives it to a different structure.
A single-tailed soap forms a spherical micelle; a double-tailed phospholipid forms a bilayer — two sheets of molecules tail-to-tail, heads facing the water on both sides. That bilayer is the cell membrane, and its existence is a direct consequence of one molecule having a polar end and a nonpolar end.
Steroids: the other kind of lipid
Steroids are lipids by solubility and have nothing structurally in common with the fatty acids above. They share a fused four-ring skeleton: three six-membered rings and one five-membered ring.
Cholesterol is the parent of the class in animals, sits within membranes moderating their fluidity, and is the biosynthetic precursor of the steroid hormones. That a rigid fused-ring system and a floppy fatty acid chain end up in the same category is the clearest sign that "lipid" is a solubility class rather than a structural one.
Stearic acid (C18, no double bonds), oleic acid (C18, one cis double bond) and linoleic acid (C18, two cis double bonds).
Same length, so chain length cannot be the variable. The only difference is the number of kinks.
Stearic is straight, packs best, melts highest. Oleic has one kink and melts lower. Linoleic has two and melts lower still — it is a liquid well below room temperature.
The reasoning is entirely about packing and intermolecular contact, which is the same argument used for boiling points back in Foundations. Biology did not introduce a new principle here; it supplied a case where the principle has consequences you can taste.
What carries forward
Two threads run through this section and neither is new. Ester chemistry — formation, hydrolysis, saponification — is Module 13 applied to a big molecule. And the shape argument, that cis alkenes cannot pack, is the intermolecular-forces reasoning from the very first chapter, arriving at a conclusion about whether something is butter or oil.