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Articles

HCO₃^– Influx Across the Plasmalemma of Chara corallina

Divalent Cation Requirement ¹

^a Department of Botany, University of Toronto, Toronto, Ontario, M5S 1A1, Canada

An absolute requirement for divalent cations is reported for H¹⁴CO₃^– influx in Chara corallina. Effective substitution of eluted Ca²⁺ by Mg²⁺ and Sr²⁺ was observed, but Mn²⁺ was completely ineffective in restoring H¹⁴CO₃^– transport activity. Similarly, La³⁺ could not substitute for Ca²⁺ in this system. Low concentrations of ethylenediaminetetraacetate (0.01 to 0.06 mM) significantly enhanced the rate at which H¹⁴CO₃^– transport capacity was lost.

Examination of the response of OH^– efflux, during Ca²⁺-free treatment, indicated that the cellular control over OH^– efflux remained unaffected until membrane integrity became severely affected. This conclusion was supported by the response of OH^– efflux to 10 mM K⁺. Therefore, assimilation of H¹⁴CO₃^– is not rate-limited by an effect of Ca²⁺ elution on the OH^– transport system. Kinetic experiments indicated that Ca²⁺ removal from the membrane resulted in noncompetitive inhibition of H¹⁴CO₃^– assimilation; the apparent Michaelis constant remained unaltered over a wide range of conditions. An hypothesis is presented which suggests that membrane integrity is necessary for HCO₃^– transport to occur, but Ca²⁺ (Mg²⁺, Sr²⁺), per se, must be bound to the transport complex before activity is established.

¹ This work was supported jointly by grants from the National Research Council of Canada (No. A6459) and the Connaught Fund (University of Toronto).