Ion Activities in Solution in Relation to Na+−Ca2+Interactions at the Plasmalemma

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Abstract
Cramer, G. R and Lauchli, A. 1986. Ion activities in solution in relation to Na+ -Ca2+ interactions at the plasmalemma.—J. exp. Bot. 37: 321–330. Data from a previous study (Cramer, Läuchli, and Polito, 1985) were assessed using a computer program, GEOCHEM, to calculate various thermodynamic parameters Ion activity is a more appropriate parameter to express the chemical potential of anion than concentration, and was used in the present analysis of thc previous data instead of concentration. The displacement of Ca2+ by Na+ from the membranes of intact cotton root hairs was related to thc (Ca2+)/(Na+)2 ratio in solution using a mass-action, cation-exchange equation (with parentheses denoting ion activity). From this relation, displacement of Ca2+ by Na+, in the presence of low and high calcium activities in thc external solution, appeared to take place at two different classes of membrane binding sites, one of which had the properties of a high affinity Ca2+ binding site, possibly being a protein.

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