The genetic analysis of Drosophila neurobiology has expanded into a considerable field. A host of genetic variants is now available for analyzing the development, structure, physiology, and chemistry of the fly's excitable cells and tissues and how they control the animal's behavior.

The plant root is a complex system that has evolved under the constraints of a number of functions. It is a pressure-probe that can penetrate the soil; it is a scavenger of nutrients that may be either tightly bound to soil particles or in low concentrations in the soil solution; it is an absorber of water from the soil. The tip of the root contains the region of dividing cells from which the root is formed, and is pushed through the soil by the extension of these cells some 10–20 times as they develop and differentiate.

About ten years ago, one of the authors participated in a review of haemocyanin structure and function (van Holde & van Bruggen, 1971). At that time, it was possible to describe the field in terms of a limited amount of exciting new structural information, and a long list of unanswered questions. While the stoichiometry of oxygen binding was understood, virtually nothing was known about the active site. Even the oxidation state of the copper was a matter of conjecture. The size of the haemocyanin polypeptide chains was the subject of intense debate, with very little substantive knowledge available. While the haemocyanins were known to be allosteric proteins, there were virtually no experimental studies of oxygen binding on a level that could be meaningfully interpreted in terms of extant theories.

Developing cells have constantly to make decisions: when to proliferate and divide, when and how to differentiate. It is an increasingly attractive idea that these decisions involve changes in intracellular cation concentrations. Our ideas about the mechanisms of changes in intracellular cations come largely from the application of biophysical techniques in the study of excitable tissues. These ideas are proving very valuable to the investigation of the control of proliferation and cell development and it is evident that the ionic mechanisms which pertain in nerve and muscle have their counterparts in other cells. Just as alterations in intracellular ion concentrations serve a signalling function in excitable tissue, so too they act as signals during development. Since almost all the quantitative data on the ionic mechanisms of fertilization come from work on sea urchins we have confined our review to sea urchin eggs.

This review represents a personal view of membrane thermodynamics. I do not intend to deal at all with the irreversible thermodynamics of membrane mass transfer processes. This aspect has been covered far more competently and completely by other people (Bittar, 1970; Paterson, 1970; Rottenberg, Caplan & Essig, 1970; Mitchell, 1970; Rothschild et al. 1980; Oster, Perelson & Katchalsky, 1973; Kedem & Katchalsky, 1958; Schwartz, 1971). The recent review on osmosis by Hill (1979) is a particularly succinct appraisal of a facet of irreversible membrane thermodynamics. Arata & Nishimura (1980) have considered the coupling of electron transfer to vectorial processes in biological membranes.

The sensitivity of biological systems to changes in environmental stimuli is connected with their regulatory properties. In order to achieve efficient control, these systems must respond to minute environmental variations by amplifying external stimuli to yield a significant response. To that end, biochemical systems have often evolved to a cascade organization in which the product of the nth reaction in a chain acts as a catalyst in subsequent transformations. The amplification properties of such cascades were first noticed in the process of blood clotting (MacFarlane, 1964, 1969) and visual excitation (Wald, 1965). Later on, a similar organization was noticed in hormonal control of metabolism (Bowness, 1964; Stadtman & Chock, 1977, 1978; Chock, Rhee & Stadtman, 1980).

Information on the swimming motion of microscopic calls and the factors which affect it is important to a wide range of disciplines. The functions of the motile apparatus of simpler organisms such as bacteria or algae can provide useful clues on the operation of more complex contractile systems such as muscles. Often celluar motility is a response to conditions external to the cell (e.g. chemotaxis) and, hence, can lead to increased understanding of the cell's sensory capability. On the medical front there is a physical similarity between the flagellar beat of the motile spermatozoa and the ciliary activity of epithelial lining of respiratory and reproductive tracts. Impairment of the motile apparatus can lead to sterility and to a variety of pathological conditions. In animal husbandry, in the artificial insemination industry and in sperm banking estimates of the extent of cellular motility and the fraction of cells which are motile are key quantities which can determmine the fertilization capability of a semen sample.