Lipids are biomolecules grouped together not because of chemical structure, but on the basis of their insolubility in water and solubility in organic solvents. Lipids can have very different chemical structures and also diverse functions. They are the primary component of most cell membranes, energy storage molecules, signaling compounds, and enzyme cofactors.
Lipids are "built" from fatty acids and can be subdivided into two groups: 1) open chain structures including fatty acids, triacylglycerols, glycerophospholipids, sphingolipids, and glycolipids and 2) closed or fused chain structures: the steroids (including cholesterol).
Fatty Acids: Fatty acids have two major roles in the body: as components of more complex membrane lipids and as the major components of stored fat in the form of triacylglycerols. Triglycerides, sometimes called oils, are triesters of glycerin with three equivalents of fatty acid. Fatty acids are formed from two carbon units, therefore they usually have even numbers of carbon atoms. Hydrocarbon chains contain from 4-36 carbons. Although they are hydrophobic, they are amphipathic because of the -COOH group which can ionize so that shorter chains are slightly soluble in water. Fatty acids can be fully saturated or partially unsaturated; double bonds are usually in the cis configuration.
The properties of fatty acids are determined by the length of the chain and the degree of saturation. Fatty acids tend to aggregate with the extended chains packed next to one another. This close association promotes many interactions.
Saturated Fatty acids (e.g. animal fats, like butter) tend to be flexible due to rotation around the carbon single bonds. They have relatively high melting points due to the high degree of packing between adjacent unsaturated chains.
Unsaturated Fatty acids (many vegetable oils) tend to be more rigid and have relatively low melting temps due to the disruption of packing by the introduction of double bonds (fewer van der Waals interactions).