Despite major successes of this approach, a discrete biological function can only rarely be attributed to an individual molecule. Cellular functions, such as signal transmission, are carried out by 'modules' made up of many species of interacting molecules. Understanding how modules work has depended on combining phenomenological analysis with molecular studies. General principles that govern the structure and behaviour of modules may be discovered with help from synthetic sciences such as engineering and computer science, from stronger interactions between experiment and theory in cell biology, and from an appreciation of evolutionary constraints. Although living systems obey the laws of physics and chemistry, the notion of function or purpose differentiates biology from other natural sciences.
Organisms exist to reproduce, whereas, outside religious belief, rocks and stars have no purpose.
Selection for function has produced the living cell, with a unique set of properties that distinguish it from inanimate systems of interacting molecules. Cells exist far from thermal equilibrium by harvesting energy from their environment. They are composed of thousands of different types of molecule. Their interactions with the environment depend in a byzantine fashion on this information, and the information and the machinery that interprets it are replicated by reproducing the cell. This are the fundamental questions of biology in the next century.