Each of these major components has one or more well-defined roles. Water is the solvent medium for all chemical reactions. Calcium sulfates and phosphates are the rigid framework materials of bone, teeth, and shells. Proteins and lipids provide the more dynamic framework materials for membranes, connecting fibers, tendons, and muscle. Fats contribute mechanical protection and thermal insulation. Proteins and fats each have a second role: Fats are the main energy reservoirs in animals, and globular proteins serve as enzymes (catalysts), regulators, carriers, and recognition and protective molecules. Carbohydrates are the structural materials in plants; they also are the rapidaccess energy-storage molecules in animals, and the only energy reservoirs in plants. Nucleic acids have a very special role: the storage and transmission of genetic information. Deoxyribonucleic acids (DNA) are the permanent repository of information in the nucleus of a cell, and ribonucleic acids (RNA) are involved in the transcription and translation machinery that interprets that information and uses it to synthesize proteins. A small cousin of nucleic acids, ATP, is the central shortterm energy-storage molecule for all life processes.

The small organic molecules act mainly as carriers of energy (ATP), electrons or reducing power (NADH), chemical groups (other ATP-like molecules), or information (hormones). Most vitamins, such as vitamin A, the precursor of retinal and ß-carotene, are essentially synthetic precursors of these molecules that we no longer can make metabolically for ourselves. Of the many inorganic ions and metals in living organisms, K⁺ is the principal cation within a cell and Na⁺ in the extracellular fluids. Calcium has been mentioned for its role in bones, teeth, and shells. Other metal atoms, such as Mg, Mn, Fe, Co, Cu, Zn, and Mo, are essential for the functioning of enzymes, with which they act in electron rearrangement during catalysis, electron transfer, and the binding of 0₂ and other small molecules.

All of these chemical components are only the trees, when what we really want to see is the forest. If we say that a mammal is nothing but water, salts, proteins, lipids, carbohydrates, nucleic acids, and small organic molecules, we are only perpetuating a more involved version of the cliché that a man is made of nothing but $1.98 worth of chemical elements. What must be added to these chemical components, or how must they be arranged, to produce a living organism? This is what the last five chapters of this book are really about.