Fatty acids and lipids

Lipids are water-fearing (hydrophobic): they do not dissolve in water but dissolve in organic solvents like chloroform. Breaking down fats gives fatty acids. Most fatty acids have an even number of carbons, because they are built up two carbons at a time.

Carbons in a fatty acid are numbered from the carboxyl carbon, which is carbon 1. A short code describes each fatty acid: 18:2 (Δ9,12) means 18 carbons, 2 double bonds, at carbons 9 and 12: this is linoleic acid.

• Saturated (no double bonds): lauric, palmitic, stearic acids.
• Monounsaturated (one double bond): oleic acid.
• Polyunsaturated (more than one): linoleic, linolenic, arachidonic acids.

Simple lipids: triacylglycerols (glycerol joined to three fatty acids; all three the same in a simple one, different in a mixed one) are the body's energy store, kept in adipose (fat) tissue. Waxes are a fatty acid joined to a long-chain (high-molecular-weight) alcohol; they form protective coatings, like on leaves and feathers.

Phospholipids are amphipathic: one end loves water, the other avoids it. They have two water-fearing tails and a water-loving phosphate head. This is exactly how a membrane forms: heads face the water, tails hide inside. Glycerophospholipids (like phosphatidylcholine and phosphatidylserine) are built on glycerol. Sphingolipids (like ceramide and sphingomyelin) are built on a different base, sphingosine, with the fatty acid attached by an amide bond.

Steroids have four rings fused together. Cholesterol, made mainly in the liver, stiffens cell membranes and is the starting material for steroid hormones such as testosterone and estradiol. Plants have phytosterols; fungi have ergosterol, which can be turned into vitamin D.