Photosynthesis begins with the absorption of light energy and this absorbed energy is transferred to special sites, termed reaction centres. At these sites, the light energy is transformed into chemical products through an oxidation-reduction reaction that generates the primary reactants, an oxidized pigment molecule (P+) and a reduced electron acceptor (A–) (Clayton, 1972). The subsequent reactions of these species in the dark ultimately results in the formation of chemical products required for the fixation of CO2. In this essay we will discuss the nature of the primary reactants generated in the light reactions of chloroplast photosynthesis, stressing recent advances in the identification and characterization of such reactants.

The current phase of axon physiology began with the invention of the voltage clamp by Cole (1949) and its use by Hodgkin & Huxley (1952d) to produce an astonishingly complete analysis of the ionic permeabilities that are responsible for the action potential. Their description did notcontain much in the way of molecular detail, and left open such questions as whether ions cross the membrane by way of pores or carriers, and the nature of the ‘gating‘ processes that increase ordecrease ion permeability in response to changes of the membrane potential. In the last few years our picture of the ionicchannels has grown considerably more tangible, though it still falls far short of a detailed molecular description. This article describes this sharpened picture and reviews the evidence for it. The viewpoint expressed is a very personal one, andno attempt has been made to review the literature of axonology comprehensively.

Commercially available electron microscopes routinely provide resolution of some 2–4 Å, as determined on the spacing of crystalline lattices of certain stable, small-molecular substances. On biological material either macromolecules or macromolecular assemblies— ‘biologically significant’ details below some 20 Å have hitherto not been observed.we consider as ‘biologically significant’ those structural details observed or contained in electronmicrographs which are consistent with, or confirmed by, other data obtained from biochemical or functional experiments or by other physical methods (optical, magnetic, electric).

A satisfactory understanding of the functions of the sodium pump, the system responsible for the active transport of sodium and potassium, require the isolation and characterization of its protein and lipid components which are integrated in the structure of the cell membrane. The enzyme system (Na+ + K+)-ATPase, is located in membrane fragments and behaves in the test tube like the transport system in the intact cell membrane (Skou,1957) Purified preparations of this enzyme will contain some, if not all, of the components of the sodium pump.