Our concern here will be with energetic rather than kinetic aspects of how a macromolecular enzyme molecule or enzyme complex in a biological membrane accomplishes the transfer or transduction of one kind of small-molecule free energy into another. An example is the use of the free energy of ATP hydrolysis (to form ADP and P1) to move Ca2+ across a membrane against its own electrochemical potential gradient (Stryer, 1981). In this case, the free energy decreases in the hydrolysis reaction but much of this is recovered in the form of an increase in free energy of the Ca2+ ions (the remainder is dissipated as heat). There is, of course, no net change in the free energy of the enzyme itself (Ca-ATPase).

Relaxation techniques have been widely used in kinetic studies of chemical reactions in homogeneous solution (Eigen & DeMayer, 1963). The principle of this method is well known: an external variable such as temperature or pressure is suddenly changed and the time course of a state parameter of the system such as concentration is recorded as it approaches a new steady value. Relaxation techniques can also be used for studying the rate of elementary processes in membranes. This method has proved particularly useful for the investigation of ion transport systems (ion carriers, channels, pumps) in artificial planar bilayer membranes. In this review we describe different relaxation techniques which have been developed for this purpose during the last years, as well as applications to a number of ion transport systems.

When light impinges on photosynthetic material – a plant leaf, an alga or a photosynthetic bacterium – it is absorbed by an array of lightcollecting pigments. Through resonant energy transfer the absorbed quantum of light is transported to a trap, the reaction centre. Within such a trap, a specialized (bacterio)chlorophyll complex is able to eject from its excited state an electron. This electron is ‘captured’ by an adjacent acceptor, which in turn donates the electron to a second acceptor, and so on. Thus, light energy is converted into chemical energy which is ultimately used in the metabolic processes of the cell.