ACS Distance Education UK
Biochemistry is the chemistry of organisms. An organism is anything that is alive, or if not, was once alive (a "dead organism"). What, then, is the condition we call life? We cannot offer a rigid, precise definition, but we do know that living things are characterised by metabolism, growth, and reproduction. Metabolism is the process by which a body introduces into itself ("ingests") various energy‑rich materials from its environment ("food"), and transforms these materials, with the release of energy, into other substances, some of which are retained by the body ("growth" or "repair") and some eliminated. Reproduction is the process by which one body produces another that is like itself in properties, structure, composition, and function, including metabolism and reproduction.
The distinction between an organism and a material is not always clear. A virus consists of particles several hundred angstrom units in length or diameter; these particles can reproduce themselves in a suitable environment but they do not ingest food, or grow, or carry on any other metabolic processes. Are viruses, then, living organisms or are they chemical materials that consist of large molecules capable of replicating themselves under suitable conditions? To include viruses among the living the definition of life must be modified. Most broadly, we may consider anything living if it can bring order out of disorder at the expense of energy and has the capability to preserve accidental variations (called mutations) that may occur in the process.
In an organism, the structure called the cell may be considered to be a biochemical reactor. An organism consists of one or more cells, and the various groups of cells in a multicellular organism may be sharply differentiated as to biochemical function. The reactions in the cell are said to occur in vivo (Latin, "in the living organism"); the corresponding reactions outside of the cell are said to occur in vitro (Latin, "in glass").
The living cell is not merely a tiny membranous beaker with homogeneous contents. It is, rather, an entity of great complexity, not yet completely understood as to structure and function. There are specific sites within the cell at which specific reacting systems, metabolic or reproductive, operate. The biochemist seeks to identify these sites, and to illuminate the course and mechanism of the reactions that occur there. Sometimes he tries to remove a chemically reacting system from its cellular environment and duplicate it in vitro. He does this because reactions are usually easier to study under the more controllable conditions of laboratory reactors than they are in vivo.
Biochemical Processes in the Cell
Several anatomical features are so small that they can be revealed only with the aid of an electron microscope. Some of these fine structures of the cell are nonessential inclusions, like blobs of fat, or particles of starch. Others, called organelles, perform essential functions and are reproduced when the cell divides. Some of these functions are well known; others still elude us.
The mitochondria are organelles shaped like elongated slippers; their
cross‑sectional diameters are about 1 micron. The highly differentiated structure of a mictochondrion contains some 40 enzymes, which control a complex series of redox reactions, including the conversion of diverse organic substances into ATP. The energy reservoir that is thus stored up is available for biochemical work such as muscle contraction, for electrical work like the action of nerve impulses, and for the activation of other biochemical reactions. Because of these functions, the mitochondria have been called, by an analogy that not all mechanical engineers would accept, the "furnace of the cell".
Chloroplasts are organelles that occur in plant cells and that contain the green pigment chlorophyll. Chlorophyll is the catalyst for the endothermic process of photosynthesis, in which glucose is synthesised from carbon dioxide.
The nucleus is a well defined structure which contains the genetic material of the cell; the nucleus thus is the site of the reproductive function. Each time a cell divides, it reconstitutes itself. This ability of self duplication is retained by new cells and is transmitted repeatedly through successive generations of cells. The reliability of this transmittal accounts for the continuity of species.