J. B. Harborne’s research while affiliated with University of Reading and other places

The macromolecules of plants are distinguished from all other constituents by their high molecular weight. This may vary from 10000 to over 1000000, whereas in other plant metabolites the molecular weight is rarely above 1000. Chemically, macromolecules consist of long chains of small structural units or ‘building blocks’, linked covalently in a number of different ways. Chemical characterization in the first instance therefore depends on identifying these smaller units. Proteins, for example, are long chains of amino acids (up to twenty different ones) joined together through peptide (—CO—NH—) links. Polysaccharides are similarly derived from the union of simple sugar units, such as glucose, joined through ether (—O—) links. The nucleic acids, by contrast, are more complex and have three types of structural unit: purine and pyrimidine bases, pentose sugars and phosphate groups. The three main classes of macromolecules found in plants are thus proteins, polysaccharides and nucleic acids. However, mixed polymers are also known. such as the glycoproteins, which contain both sugars and amino acids in covalent linkage.