The effects of p-benzoquinone (BQ) on photosynthetic and respiratory electr
on transport in a single algal protoplast (radius, 100 mu m) was investigat
ed quantitatively by amperometric measurements using microelectrodes. Under
light irradiation (25 kLx) in the presence of 1.00 mM BQ, a single protopl
ast consumed BQ by (2.9 +/- 0.2) x 10(-13) mol/s and generated p-hydroquino
ne (QH(2)) by (2.7 +/- 0.3) x 10(-13) mol/s, suggesting that BQ was quantit
atively reduced to QH(2) via the intracellular photosynthetic electron-tran
sport chain. The generation of QH(2) increased with light intensity and wit
h concentration of BQ added to the outside solution but became saturated wh
en the tight 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 ph
otosynthetic electron-transport inhibitor, decreased the generation of QH(2
) upon light irradiation, suggesting that BQ accepts electrons from a site
in the photosynthetic electron-transport chain after the photosystem II sit
e. The presence of 1.00 mM BQ increased the generation of photosynthetic ox
ygen by similar to(2.6 +/- 1.0) x 10(-13) mol/s, which was similar to 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 intracell
ular species as well as to reduce BQ.