2.1—
Introduction

The control of metabolism and the development of cells frequently depends on the right substance being present in the right amount at a specific location in the cell at the right time. This may be achieved by regulation of the passage of materials from the external environment into the cell or from one compartment of the cell to another. All compartments of the cell, and its external surface, are bounded by membranes. It is clear, therefore, that any complete understanding of control mechanisms in metabolism or development must include a precise knowledge of the structure and composition of membranes and of the mechanisms whereby materials move through them. While it would not be true to suggest that all of this knowledge is available at present, the pace at which new information and insight has been gathered in the last decade is most impressive. In this short chapter it will not be possible to trace the history of the way in which ideas about membrane structure have developed, but it is worth mentioning that a (substantially correct) view of the basic structure of biological membranes was advanced in the 1930's, long before it was possible to visualize membranes in the electron microscope or to examine their detailed structure by X-ray deffraction techniques (Danielli & Davson, 1935). The simple trilaminar appearance of biological membranes in the transmission electron microscope (Fig. 2.1) is now familar to elementary students of biology and is known as the unit membrane; its occurrence is ubiquitous and this very fact has impressed on biochemists and others that this apparently uniform structure cannot explain the diverse properties of different membranes. This chapter covers the chemical composition of membranes and how these components are arranged. From this it will become apparent that the membrane is composed of a matrix, whose design is broadly similar in all cases, and a sub-structure on which many of the specific properties of the membrane probably depend. With this picture in mind it will then be possible to explore the basic types of transport which can occur across membranes and to relate them to the structures described.