Chemical Constituents of Living Cells: Macromolecules and Micromolecules

Every living cell is a highly organised chemical system, built from a mixture of very small molecules and very large ones. If a tissue sample is ground up and treated with cold trichloroacetic acid (TCA), the resulting mixture separates into two distinct fractions on centrifugation. The acid-soluble pool contains a large number of small molecules of relatively low molecular weight, generally under about 1000 daltons -- amino acids, monosaccharides and other simple sugars, nucleotides, various lipids, vitamins and a range of inorganic ions. These small molecules are collectively called micromolecules, and while individually simple, together they may number in the thousands of distinct chemical species within a single cell. The acid-insoluble fraction, by contrast, consists of a comparatively small number of very large molecules -- proteins, polysaccharides and nucleic acids, together with some lipids that behave similarly in this test -- built up by the polymerisation of the very micromolecules found in the soluble pool. These giant polymers are called macromolecules or biomacromolecules, and it is this insoluble fraction that forms the structural and functional machinery of the cell.

Elemental analysis of any living tissue shows that only a handful of elements make up the overwhelming bulk of its dry weight -- principally carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur, with smaller but essential amounts of elements such as sodium, potassium, calcium, magnesium and iron. Water remains the single most abundant compound in any cell, typically accounting for 70 to 90 percent of its wet weight, and acts as the medium in which every one of these chemical reactions and interactions takes place. The sections that follow examine, in turn, each of the four principal classes of biomacromolecule -- proteins, carbohydrates, lipids and nucleic acids -- built from their respective micromolecule subunits, and close with a detailed look at enzymes, the specialised proteins that catalyse the chemical reactions of life.