Stimulation of ATP Synthesis by a Membrane Potential in Chloroplasts

Chloroplasts preloaded with protons by light‐induced proton uptake do not synthesize ATP in a following dark period if the proton concentration gradient (ΔpH) is lower than 2.5–3.0 pH units. At such suboptimal ΔpH values synthesis of ATP can be obtained by imposing a diffusion potential across the chloroplast membrane. This potential was realized by providing a high external concentration of either KCl in the presence of valinomycin or of NaCl in the presence of monactin. Diffusion‐potential‐induced stimulation of ATP synthesis was observed both in chloroplasts and in bacterial chromatophores. The stimulation could be abolished by the addition of a highly permeant anion, like picrate. It is proposed that the effect of the diffusion potential indicates that an increase in the electrochemical gradient of protons can be coupled to the synthesis of ATP.