Introduction

Abstract

A biologically-active substance is said to be selective if it strongly affects certain cells without causing any change in others, even when the two kinds of cells are close neighbours. In living organisms, there are many substances, often quite small molecules, which have been chosen for their specificity. This choice has been made under the strong pressure of natural selection, unhurried by any consideration of time. Such chemical compounds operate the metabolism of the cells and tissues, and ensure their health, survival, and reproduction. Important among the smaller of these selective molecules are vitamins, coenzymes, hormones, neurotransmitters, inorganic ions, nutritional fragments, respiratory and photosynthesizing pigments; also adenosine triphosphate (ATP), which is the energy store of every living creature; and the purine and pyrimidine components of deoxyribonucleic acid (DNA) on which is encoded the genetic information that guides and controls each organism. The interaction of each of these small molecules with its complementary bio- polymer (most often an enzyme) generates the needed physiological response and affords a typical example of what is becoming known as cellular recognition [1].