Life and lipids
All forms of life as we know it are made from the same four classes of small organic molecules: the amino acids, the nucleotides, the carbohydrates, and the fatty acids. Molecules from the same class or from different classes combine chemically to form larger macromolecular entities, specifically the poly-saccharides (sugars), the proteins (poly-peptides), the nucleic acids (poly-nucleotides like DNA and RNA), and the fats (lipids). Whereas poly-saccharides, nucleic acids, and proteins are all polymers, i.e. long-chain and possibly branched (in the case of sugars) molecules bound by strong covalent forces, the fats do not normally polymerize. There are no poly-lipids in nature. In contrast, the lipids organize into different types of macro-molecular assemblies such as micelles and membranes bound by weaker physical forces. The assembly process takes place spontaneously in the presence of the biological solvent, water.
Whereas the genes encode the information for constructing the proteins, and the proteins perform most of the functions of living systems, the lipids are foremost structure builders whose properties are characterized by fuzzy terms like adaptability, diversity, plasticity, and softness. This characterization implies that lipids and lipid assemblies have considerable elements of disorder whereas proteins and poly-nucleotides are characterized by well-defined molecular structure and a high degree of order. The relationships between structure and function for lipids are therefore far more subtle than for proteins and genes. As a consequence, lipid systems have obtained the doubtful status as the grease in which all the beautiful machinery of life controlled by genes and proteins are imbedded.