Transport of potassium inChara australis: I. A symport with sodium

Summary 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 m) and Na⁺ with lower affinity (K _1/2 about 500 m). The most likely mechanism is symport of K⁺ with Na⁺, with a stoichiometric ratio of 11. 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.