Proton Translocation, Electron Transport and Phosphorylation in Isolated Chloroplasts

Abstract

The steady-state rates of proton translocation (ΦH) associated with rates of light-induced electron transport (Φe) have been studied under a variety of conditions, with and without phosphorylation, in both spinach and pea chloroplasts, and using the absorbance changes of phenol red as a pH indicator. The method is discussed critically and compared with others. Using values of ΦH estimated from the initial efflux of H+ after continuous illumination for 17 s, it has been found that ΦH/Φe increased to a maximum value of 2 under conditions which enhanced the permeability of membranes to H+ and a second ion. The maximum value of about 2 was observed over the whole pH range for which phenol red can be used as a pH indicator, namely pH 7.0 - 8.7. The stimulation of electron transport by phosphorylation was also studied with a view to enquiring into the ratio ΦHH/RATP, where ΦH is the proton flux via an ATPase giving rise to a phosphorylation rate RATP. A scheme of the pathways of protons in the thylakoid membrane is proposed to account for the evariation of ΦH/Φe under a large range of conditions. This scheme is in line with the 'bobbing up and down' hypothesis for the mechanism of active proton transport in energy-transducing membranes.