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.