Abstract
Voltage-clamp and tracer techniques, applied simultaneously or separately to individual cells, have been used to show that K+-starved internodal cells ofChara australis can develop an electrogenic transport system, which requires and transports K+ with high affinity (K 1/2 about 30 μm) and Na+ with lower affinity (K 1/2 about 500 μm). The most likely mechanism is symport of K+ with Na+, with a stoichiometric ratio of 1∶1. In simultaneous measurements of quantities of charge and of ions entering individual cells, the quantity of K+ was consistently half the quantity of electric charge, while that of Na+ was consistently somewhat lower than that. Possible reasons for this discrepancy are discussed. The electrogenic symport of K+ with Na+ has not previously been reported for any cell. Its functional significance inChara is apparently the active uptake of K+ at the expense of the electrochemical potential difference for Na+. This new symport reveals the unexpected presence inChara of a Na+-linked chemiosmotic circuit alongside the known H+-linked circuit.
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