The science of biochemistry, the chemistry of life, has made rapid progress in recent years. Knowledge of how living systems operate at the molecular level has increased dramatically in just the past few decades. One branch of biochemistry studies genetic information and how it is coded and used by organisms. Genetic information deals with “blueprints” within genes—the designs that determine the origin and natural growth of organisms. All living cells contain these genes. This branch of biochemistry has grown so fast that it has come to be regarded as a separate science-molecular biology—which in turn finds practical application in the field of genetic engineering.

Biochemistry also studies the production of biological molecules, the changes they undergo in living cells, their interaction with different parts of an organism, the chemical events that underlie their effects, and what eventually happens to them. Thus, the process of digestion—from the kinds of molecules taken in by an organism in its food and how they are broken down, to what the organism does with the end products—is also a biochemical subject

In studying digestion, biochemists are also concerned with an important intermediate product—energy. Organisms obtain energy from food in order to sustain themselves. How they extract this energy from the food they eat and how it is used in the energy-requiring processes of life are questions biochemists investigate. Another fundamental question is how the energy of sunlight is trapped by green plants when they create food by photosynthesis.

Digestion breaks down the food we eat into very small pieces. These can then be absorbed through the walls of the alimentary tract. The main sites of digestion in humans are the mouth, stomach, and small intestine. The table at left gives the principal digestive reactions that occur at each site.

Large and small molecules

Biochemistry can be divided in two parts, one concerned with large molecules, the other with small ones. Most of the familiar polymers (large molecules) of organic chemistry are made up of hundreds of repeating units of one or two different small molecules. Living systems contain three main types of polymers: polysaccharides, proteins, and nucleic acids. Polysaccharides, like the industrially important polymers, are generally made up of only one or two different monomers (small molecules). Proteins, however, are made up of a much greater number of monomers, known as amino acids. Although the basic core of each amino acid is the same, differences in the side chains give rise to the immense variety of proteins in living organisms.

The blueprints for the structures of proteins are stored in coded form by the nucleic acid polymers—ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Although the nucleic acids also have a simple, repeating backbone, they use only a few different side chains arranged in an extremely precise way. By contrast, most commercial polymers that incorporate more than one monomer have only an average structure. Thus, two molecules of monomer A may be incorporated for each three molecules of monomer B in a batch of a polymer. Nevertheless, a short length of polymer chain might have a structure in which this ratio is reversed. Only the overall composition of the batch, which contains a large number of individual molecules, has the correct ratio of A and B.

However, the control of living systems must be much more exact than a statistical average, and the natural synthesis of nucleic acid polymers is carefully controlled. For example, the DNA strand that makes up a given gene in an organism always has the same structure because it is made by copying an existing molecule-one monomer at a time. When a gene ceases to be a precise copy, a genetic mutation occurs. Many mutations are harmless, but some, such as cancer cells, may be fatal.

Biochemistry is the chemistry of life. Some of its most significant features are illustrated in the photograph above. The starling (Sturnus vulgarisl is feeding. The food will have to go through the biochemical process of digestion. Flight requires energy, which comes from the biochemical processes of metabolism. Green plants are the means by which solar energy is biochemically converted into an edible energy source. The pigments that produce plant and animal colors are also created biochemically.
Two porphyrins are the
plant pigment chlorophyll (left) and the blood pigment heme (right). These porphyrins play vital roles in the biochemical processes of photosynthesis and respiration respectively. Porphyrins are organic pigments that form complexes with metal radicals. The radicals account for the different properties of otherwise similar complex molecules.

Control mechanisms

The maintenance of life frequently depends on a complex series of controls. Many of the small molecules important in biochemistry, such as hormones, carry out control functions. Small molecules also play other roles vital to life. Certain metals, for example, catalyze (bring about or speed up) chemical reactions. Life would be impossible without these catalysts. A plant cannot capture the energy of sunlight without the catalytic effect of magnesium, and we cannot survive without the oxygen captured for us with the help of iron atoms in our blood. Both the magnesium and the iron are held in small but complex molecules, themselves associated with an even more complex biochemical system.

We have known for hundreds of years that certain diseases can be cured by changes in diet, but only in this century have the relatively small molecules responsible—vitamins—been analyzed chemically. Many have been shown to be low-molecular-weight substances that can combine with specific proteins to form powerful catalytic agents called enzymes (though many enzymes are composed of just protein, with no vitamin-derived “coenzyme”).

All catalysts speed up reactions by providing a more favorable environment for a chemical reaction. Enzymes also require a favorable environment to work effectively. Biochemistry studies reactions in living organisms, including the environment in which molecules in living systems are found and the ways that this affects their behavior. Thus, although a study of proteins and lipids (fats) in isolation can provide considerable information about them, only a biochemical study of how they interact can help explain the properties of membranes in living cells. These membranes are made up of an intimate and organized physical combination of proteins and lipids.