The Affinity of Onion Cell Walls for Calcium Ions

Cell walls prepared from onion bulbs were found to exhibit an affinity for Ca ++ . The adsorption of this ion was enhanced by the action of pectin methylesterase. It was confirmed that Ca ++ reacts with two COO“ groups and the corresponding affinity constant, K, was found to be: log K = 4.25. The action of pectin methylesterase had no effect upon K. The cell walls, as prepared, had 25 % of the total COO − groups occupied by Ca ++ , 14 % by Mg ++ , and 39 % by H + . Treatment with acidified ethanol removed all of the metallic cations. K + and Mg ++ could displace Ca ++ from the cell walls. At concentrations from 10 −3 to 3 times 10 −3 m it required from 4.9 to 13.2 moles of Mg ++ to displace one mole of Ca ++ . For K + it required 80 moles to displace 1 mole of Ca ++ at K + concentrations from 0.65 × 10 −2 to 1.6 × 10 −2 M.