The Determination of the Membrane Potential of Chlorella vulgaris. Evidence for Electrogenic Sugar Transport

Abstract

From data on the accumulation of tetraphenylphosphonium within Chlorella vulgaris cells, it can be estimated that these cells possess a membrane potential of −120 to −I50 mV (inside negative). Under anaerobic conditions as well as in the presence of uncoupling agents the membrane potential drops to about −60 to −80 mV. Nystatin (50 μg/ml) abolishes it almost completely. Since it took more than 1 h before the tetraphenylphosphonium equilibrium was reached, this method could not be used to measure relatively fast transient changes in membrane potential. However, the rate of influx of tetraphenylphosphonium is also directly dependent on membrane potential and can be followed within minutes. Using this phenomenon as an indicator for membrane potential a brief transient depolarisation was detected after the addition of sugars taken up by Chlorella via the proton cotransport system. The depolarisation was absent from cells not induced for sugar uptake and induced cells did not show it with substances not transported, like mannitol. The maximal depolarisation observed amounted to about 70 mV; after 1 min, however, the membrane potential returned to a value about 25 mV less negative than the one before sugar was added. The results demonstrate that sugar uptake in Chlorella is electrogenie. The ΔpH plus membrane potential measured for Chlorella completely cover the energy required to explain the 1600-fold accumulation of' 6-deoxyglucose experimentally observed.