Illuminated chloroplast fragments, which can convert light into chemical energy (NADPH2 and ATP), contain a number of soluble and insoluble electron carriers that have been arranged, on the basis of their redox potentials and on kinetic and other evidence, in sequences (analogous to those in mitochondria) to describe the events involved in the light reactions of photosynthesis. Fractionation of chloroplasts allows separation of two light-dependent partial reactions: the evolution of oxygen and the reduction of pyridine nucleotide, accompanied by ATP synthesis. The stoichiometry of the latter reaction is still uncertain. Chloroplasts contain a directional proton-translocating ATPase (CF1) needed for the phosphorylation of ADP in the light. The CF1 also catalyses synthesis of ATP from ADP. When a pH gradient is applied in the dark across the phosphorylating membranes, the amount of ATP synthesized is related to both the change in pH and the electrical potential. During its catalytic activity, the CF1 protein undergoes reversible conformational changes, but this is not the source of the driving force for ATP synthesis.