The major constituent molecules in all living organisms are based on carbon. Carbon has versatility stemming from its four outer shell electrons, allowing the possibility of four covalent bonds with a variety of partners, including very stable carbon-carbon covalent bonds. Because of this, long carbon chains can form the backbone of more complex molecules and makes possible the great diversity of macromolecules found in the cell. The carbon chains themselves are not very reactive, but they often have reactive chemical groups attached to them.
Common groups are the hydroxyl (—OH), carbonyl (—CO), carboxyl (—COOH), and phosphate (—PO4). Carbon chains may even have other carbon chains attached to them. The smaller ones behave and are named as groups also: methyl (—CH3), ethyl, (—C2H5), propyl (—C3H7), and so forth. Figure 4B (below) depicts several functional groups that can be found in the simple molecule acetic acid (very dilute acetic acid is the primary component of vinegar).
Carbon is also the basis for the four major classes of biological molecules: sugars, nucleotides, amino acids, and fatty acids. The first three are classes of molecules that can be strung together by covalent bonds to make important large biomolecules: simple sugars can form large polysaccharides such as starch, cellulose, or glycogen, nucleotides can form RNA (ribonucleic acids) or DNA (deoxyribonucleic acids), and amino acids can form proteins. Fatty acids, on the other hand, are acid derivatives of long chains of carbons linked to one another, with hydrogens taking up most of the other bonding positions.