Summary

Living organisms are astonishingly varied, yet at the chemical level they are strikingly alike, both in what they are built from and in the reactions they run. If we simply list the elements present, living tissue and non-living matter look much the same; but on a closer, quantitative look, living systems are noticeably richer in carbon, hydrogen and oxygen than lifeless matter is. The single most abundant substance in any organism is water.

The chemicals of life fall into two broad size groups. There is a large collection of small molecules, each with a molecular weight below about one thousand daltons, and there are the giant molecules built from them. Among the small biomolecules are amino acids, single sugars and double sugars, fatty acids, glycerol, and the nucleotides, nucleosides and nitrogen bases. Living cells use only about twenty kinds of amino acids and five kinds of nucleotides as their standard building units. Fats and oils are glycerides, formed when fatty acids are esterified to glycerol, and phospholipids are like these but carry an added phosphorus-containing nitrogenous group. Some of these compounds are primary metabolites that take direct part in normal growth and function, while many organisms also make secondary metabolites, such as pigments, alkaloids and gums, whose roles are more specialised.

When the acid-insoluble, high-weight fraction of a cell is examined, only a few types of true macromolecule are found: proteins, nucleic acids and polysaccharides. Lipids, although individually small, are pulled into this heavy fraction because they cluster together in membranes. Each biomacromolecule is a polymer assembled from smaller building blocks strung together, but the identity of those blocks differs from one polymer to the next. Proteins are heteropolymers of amino acids; nucleic acids, meaning RNA and DNA, are chains of nucleotides; and polysaccharides are chains of sugars.

These large molecules are organised in a hierarchy of structural levels. A protein's primary structure is simply the sequence of its amino acids. Local coiling and folding of the chain gives the secondary structure, the whole chain's overall three-dimensional shape is its tertiary structure, and where several folded chains come together the arrangement of the subunits forms the quaternary structure. This layered architecture is what allows each macromolecule to do its particular job.

The macromolecules divide the labour of the cell. Nucleic acids carry the genetic information and pass it faithfully from one generation to the next. Polysaccharides build supporting structures such as the cell walls of plants and fungi and the exoskeletons of arthropods, and they also store energy in forms such as starch and glycogen. Proteins are the most versatile of all, serving as enzymes, antibodies, receptors, hormones and structural elements. Collagen is the most abundant protein in the animal world, and the carbon-fixing enzyme RuBisCO (ribulose bisphosphate carboxylase-oxygenase) is the most abundant protein across the whole biosphere.

Enzymes deserve special mention because they make the chemistry of life possible at the speeds cells require. Most enzymes are proteins that catalyse the biochemical reactions of the cell, although some nucleic acids, the ribozymes, also possess catalytic power. An enzyme works by binding its substrate at an active site to form a short-lived enzyme-substrate complex, guiding the substrate through a high-energy transition state, and then releasing the product while itself emerging unchanged and ready to act again. In this way enzymes greatly lower the activation energy of a reaction and enormously speed it up.

Because enzymes are folded proteins, their activity depends on conditions. Each shows greatest activity at an optimum temperature and an optimum pH, and each is specific for its substrate. Cold merely slows and preserves them, but heat denatures them and destroys their activity. Raising substrate concentration speeds a reaction only until the enzymes are saturated, at which point a maximum velocity is reached. Enzymes can be blocked by inhibitors, and a competitive inhibitor works by mimicking the substrate and vying for the active site, a principle used in controlling pathogens and designing drugs. …