Lipids: Structure and Types

Lipids are a chemically diverse group of biomolecules united chiefly by their common physical property of being insoluble in water but soluble in non-polar organic solvents such as ether, chloroform and benzene. Most lipids are esters -- or contain ester linkages -- formed between fatty acids and an alcohol, most commonly glycerol.

A fatty acid is a long, unbranched hydrocarbon chain, typically containing an even number of carbon atoms, with a carboxyl group at one end. Fatty acids are described as saturated when every carbon-carbon bond in the chain is a single bond (such as palmitic acid and stearic acid), and these tend to pack tightly together and remain solid at room temperature; they are described as unsaturated when the chain contains one or more carbon-carbon double bonds (such as oleic acid and linoleic acid), and the resulting kinks in the chain prevent tight packing, so these fatty acids and the fats built from them tend to remain liquid at room temperature.

Simple lipids, chiefly the triglycerides (also called neutral fats or triacylglycerols), are formed when one molecule of glycerol is esterified with three fatty acid molecules, each ester bond forming through a condensation reaction that releases one molecule of water, for a total of three water molecules released per triglyceride formed. Triglycerides are the principal form in which the body stores chemical energy, chiefly in adipose tissue, since they yield considerably more energy per gram on oxidation than either carbohydrates or proteins.

Compound lipids incorporate an additional group beyond fatty acid and alcohol. Phospholipids are built on a glycerol backbone esterified with only two fatty acid chains, with the third position instead carrying a phosphate group, itself typically linked to a small polar head group. This gives the phospholipid molecule a distinctly amphipathic character -- a hydrophilic (water-attracting) phosphate head and two hydrophobic (water-repelling) fatty acid tails -- which is exactly the property that allows phospholipids to spontaneously assemble into the bilayer that forms the structural basis of every biological membrane. Glycolipids similarly combine a lipid portion with an attached carbohydrate group. …