These can be the building blocks of the macromolecules or they can have independent roles, such as signal transmission or being a source of energy or material for a cell. Some important examples besides water are sugars, fatty acids, amino acids and nucleotides. For instance, biological membranes are constructed from fatty acids, into which macromolecules are embedded. There are 20 different amino acid molecules, which are the building blocks for proteins (to be more precise, there are 19 amino acids and one which has a slightly different structure and therefore is called imino acid).
These are three examples of amino acid moleclues, there are 17 more. They differ by R side chains which determine their properties and the order of these different amino acids within the protein determines the three dimensional structure of the protein. There is a convention that each amino-acid is denoted by a letter in Latin alphabet, for instance arginine is denoted by R, histidine by H, lysine by L and there are 20 such letters .