On the steady-state electrical potential difference across the thylakoid membranes of chloroplasts in illuminated plant cells.

The potential defference across the thyladoid membranes under steady-state saturating light conditions, measured with microcapillary glass electrodes, was found to be small as compared to the potential initially generated at the onset of illunimation. This result is discussed to be in agreement with quantitative estimates on the approximate magnitudes of the potential generating electron flux through the photo-synthetic electron transport chain and of the potential dissipating ion fluxes across the thylakoid membrane under steady-state conditions. It is concluded that a pH gradient of approx. 3-3.4 units is built up in the light across the membrane. The negative diffusion potential associated with this gradient is suggested to cause the transient negative potential observed in the dark after illumination.