Membranes are the most common cellular structures in both plants and animals. They are now recognized as being involved in almost all aspects of cellular activity ranging from motility and food entrapment in simple unicellular organisms, to energy transduction, immunorecognition, nerve conduction and biosynthesis in plants and higher organisms. This functional diversity is reflected in the wide variety of lipids and particularly of proteins that compose different membranes. An understanding of the physical principles that govern the molecular organization of membranes is essential for an understanding of their physiological roles since structure and function are much more interdependent in membranes than in, say, simple chemical reactions in solution. We must recognize, however, that the word ‘understanding’ means different things in different disciplines, and nowhere is this more apparent than in this multidisciplinary area where biology, chemistry and physics meet.

For the cells of an organism to act in the coordinated fashion necessary for complex functioning, they must be able to receive and transmit information. Information transfer is mediated by molecules released by the cells and may be local, as in the case of neurotransmitters, or long range, as in the case of hormones. It is apparent, however, that irrespective of the range of interaction, a cell must be able to distinguish, with a high degree of precision, the signals relevant to it from an enormous flow of background noise.

Molecular recognition at the cell surface is mediated by receptors: cell surface glycoproteins that usually form an integral part of the plasma membrane (see, for example, Cuatrecasas & Greaves, 1978). Typically, receptors bind the ligands they are designed to recognize with affinities of the order of 108 M-1, and they translate that interaction into a sequence of signals that ultimately lead to biological activity.

A bacterium incorporates amino acids into protein with an error frequency close to one in three thousand (Edelman & Gallant, 1977). Nevertheless, the structural differences between related amino acids are so small that it is difficult to see how they can be distinguished from each other with such accuracy (see, for example, Pauling, 1958).

Indeed, the selection of amino acids during protein synthesis is carried out twice: first by the aminoacyl-tRNA synthetases and then by the codon-programmed ribosome. Each of these substrate selections is in fact a double selection. In the case of the synthetase both a particular amino acid and a corresponding cognate tRNA must be chosen to form the aminoacyl-tRNA. On the ribosome, the aminoacyl-tRNA must be matched with a cognate codon, and then the mRNA must be advanced by exactly one codon length to position the next codon in the appropriate ribosome site so that it too can be translated.

The discovery of nucleosomes as a basic repeating unit of the chromatin structure organizing the major part of eukaryotic DNA greatly catalyzes the expansion of our knowledge on chromatin. Several lines of experimental evidence have led to the formulation of the nucleosome conception: the observation of chains of globular particles in electron micrographs of chromatin (Olins & Olins, 1974); the demonstration that DNA is released as a set of discrete sizes upon digestion of chromatin with endogenous nucleases (Hewish & Burgoyne, 1973); the isolation of discrete nucleoprotein particles upon digestion of chromatin with micrococcal nuclease (Rill & Van Holde, 1973).