This work investigates the photocurrent harvested from the isolated thylakoids. Several tests have been used to verify that the photocurrent measured is indeed from the photosynthesis on the thylakoid membranes. The photocurrent has a linear dependence on light intensity; the photocurrent shares similar frequency dependence as that of absorption spectrum of chlorophyll; the photocurrent decreases or disappears with the application of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea as an inhibitor. The new finding of this report is the observation of a peak in the photocurrent as a function of the concentration C-BQ of electron mediator para-benzoquinone (p-BQ). It is found that the photocurrent measured increases at small C-BQ,C- and a maximum current is obtained at C-BQ approximate to 1.8-2 mM and decreases with further increase in C-BQ. A simplistic model has been proposed to explain the peak. The effect of bias voltage applied between the electrodes on photocurrent is studied as well. (C) 2013 Elsevier Ltd. All rights reserved.