Fig 2. Syn thylakoids transfer a direct photocurrent from QA while higher plants require the presence of an electron transfer mediator.
A. Schematic illustration of the photocell used for photocurrent measurement. A suspension of thylakoid membranes were allowed to settle onto a graphite anode placed at the bottom of a sealed container in a minimal amount of buffer B. The electrode was illuminated from the top by a xenon lamp calibrated to provide an equivalent spectrum of 1 sun. Currents were measured by a potentiostat connected in 3-electrode mode to an Ag/AgCl reference electrode and to a Pt cathode. B. Direct photocurrent produced by wild-type-Syn (solid lines) or spinach (dashed lines) thylakoids illuminated on the graphite anode in the absence (blue) or presence (red) of 50 μM DCMU under a bias of 0.05VAg/AgCl. Up and down facing arrows indicate light on and off, respectively. C. DCBQ mediated photocurrent produced by spinach thylakoids on the graphite anode in the absence (blue), the presence of 50μM (green) and of 500 μM DCMU (red) under a bias of 0.24 VAg/AgCl. D. Photocurrent produced by isolated spinach PSII on the graphite anode with (dotted lines) or without (solid lines) 0.3mM DCBQ, in the absence (blue) or presence (red) of 50 μM DCMU as in panel B. The inset schematically depicts the role of DCBQ as electron mediator. DCBQOX diffuses into the QB pocket, and is reduced by QA - (DCBQred). DCBQred then diffuses to the anode where it is re-oxidized to become available for another electron transfer cycle.