A fatty acid contains a carboxyl group attached to a hydrocarbon chain. The carboxyl group can interact strongly with water, while the long hydrocarbon portion is largely nonpolar.

A fatty acid is saturated when its hydrocarbon chain contains no carbon-carbon double bonds. It is unsaturated when one or more C=C bonds are present.

Many naturally occurring unsaturated fatty acids contain cis double bonds. Because free rotation around C=C is restricted, a cis double bond introduces a persistent bend or kink in the chain. A trans double bond produces a straighter geometry more similar to that of a saturated chain.

These geometric differences affect how closely hydrocarbon chains can pack. Straight saturated chains usually pack more efficiently and therefore tend to have stronger intermolecular contacts and higher melting temperatures than otherwise similar cis-unsaturated chains.

Triacylglycerols

Glycerol is a three-carbon molecule with three hydroxyl groups. Each hydroxyl can form an ester linkage with the carboxyl group of a fatty acid. When all three positions are esterified, the product is a triacylglycerol, also called a triglyceride.

Schematically,

$$\mathrm{glycerol+3\ fatty\ acids\rightarrow triacylglycerol+3H_2O}$$

is the formal condensation relationship, although biological synthesis uses enzyme-catalyzed activated intermediates rather than uncatalyzed direct dehydration.

Triacylglycerols are largely nonpolar because their three long hydrocarbon chains dominate the molecule and the fatty-acid carboxyl groups are converted into neutral ester linkages. They therefore aggregate into hydrophobic droplets rather than dissolving freely in water.

This lets cells store a large amount of hydrocarbon-rich material in a compact, poorly hydrated form. Triacylglycerols are therefore widely used for long-term chemical energy storage in organisms.

Hydrolysis of the ester bonds releases fatty-acid and glycerol-derived products that can enter metabolic pathways.

Fatty acids also appear in other lipid classes. In phospholipids, for example, hydrophobic chains are combined with a strongly polar head group, producing amphipathic molecules that behave very differently from neutral triacylglycerols in water.