Light requirements for proton movement by isolated chloroplasts as measured by the bromocresol purple indicator

Abstract

ATP formation by isolated chloroplasts is due to the proton gradient phenomena, according to Mitchell. The number of protons moved per electron pair transported and per photon absorbed is related to the number of protons required to produce each ATP. Thus, a critical test of the Mitchell hypothesis is the quantum yield of H + transport. Bromocresol purple, a pH indicator, can be used to measure the pH external to isolated chloroplasts accurately and rapidly. The action spectrum (with pycocyanine as the electron acceptor) appears to be that of a System I-linked reaction (high above 700 nm). The quantum yield has been calculated to be 3.5 ± 0.1 H + hv from 640 nm to 690 nm and 6.7 ± 0.4 H + hv above 700 nm. The action spectrum of the efflux of H + occurring in the dark, which is usually identified as being equivalent to the steady-state influx, has the same shape as that of the influx. The quantum yield, however, is reduced by 0.5. Therefore, Photosystem II seems to affect both the initial influx and dark efflux. The H + photon and H + e 2 for the initial influx are too high for the Mitchell hypothesis. Only the H + efflux in the dark from 640–690 nm has an H + hv of 1.6 which agrees with the theory of Mitchell.