Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Biochemistry is the molecular logic of living organisms
Cellular metabolic processes are under constant control

Growing Cells can simultaneously synthesize thousands of different PROTEIN AND NUCLEIC acid molecules in relative amounts that precisely match the requirements of the living, functioning protoplasm of a given Cell type. Consequently, enzyme-catalyzed metabolic reactions are precisely regulated to produce only as many simple molecules of various kinds as are necessary for the assembly of a strictly predetermined number of nucleic acid, protein, lipid, or polysaccharide molecules of each type. Furthermore, living cells are capable of regulating the synthesis of their own catalysts—Enzymes. For example, a cell can "turn off" the synthesis of an enzyme required to produce a given product from its precursors if it receives that product ready-made from the external environment. This capacity for self-adjustment and self-regulation enables living cells to maintain a stable steady state internally, even under conditions of significant environmental change. Hence follows another principle of the molecular logic of the living state:

Living cells are self-regulating chemical systems tuned to operate with maximum efficiency.