The effects of p-benzoquinone (BQ) on photosynthetic and respiratory electron transport in a single algal protoplast (radius, 100 μm) was investigated quantitatively by amperometric measurements using microelectrodes. Under light irradiation (25 kLx) in the presence of 1.00 mM BQ, a single protoplast consumed BQ by (2.9 ± 0.2) x 10-13 mol/s and generated p-hydroquinone (QH2) by (2.7 ± 0.3) x 10-13 mol/s, suggesting that BQ was quantitatively reduced to QH2 via the intracellular photosynthetic electron-transport chain. The generation of QH2 increased with light intensity and with concentration of BQ added to the outside solution but became saturated when the light intensity was above 15 kLx or the BQ concentration was higher than 0.75 mM. The addition of 3-(3,4- dichlorophenyl)-1,1-dimethylurea, a photosynthetic electrontransport inhibitor, decreased the generation of QH2 upon light irradiation, suggesting that BQ accepts electrons from a site in the photosynthetic electron-transport chain after the photosystem II site. The presence of 1.00 mM BQ increased the generation of photosynthetic oxygen by ~(2.6 Π 1.0) x 10-13 mol/s, which was ~1.5-2 times larger than that expected from the consumption of BQ. The electrons produced by the additional generation of oxygen is used to reduce intracellular species as well as to reduce BQ.
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