Multiphasic uptake of potassium by barley roots of low and high potassium content: Separate sites for uptake and transitions [diffusion, Hordeum, multiphasic kinetics, potassium influx, DNP, 2,4-dinitrophenol]
1980
Nissen, P. (Bergen Univ. (Norway). Dept. of Microbiology and Plant Physiology)
In the range 0.000001M - 5 X 0.01M, uptake of K+ in excised roots of barley (Hordeum vulgare L. cv. Herta) with low and high K content could in both cases be represented by an isotherm with four phases. Uptake, especially in the range of the lower phases, was reduced in high K roots through decreases in V-max and increases in K-m. Similar data for other plants are also shown to be consistent with multiphasic kinetics. The concentrations at which transitions occurred were not affected by the K status, indicating the existence of separate uptake and transition sites. Uptake was markedly reduced in the presence of 0.00001M 2.4-dinitrophenol, especially at low K+ concentrations, but the isotherms remained multiphasic. This contraindicates major contributions from a non-carrier-mediated, passive flux. A tentative hypothesis for multiphasic ion uptake envisions a structure which changes conformation as a result of all-or-none changes in a separate transition site. The structure is "tight" at low external ion concentrations (low V-max, low K-m, active uptake, allosteric regulation) and "loose" at high concentrations (high V-max, high K-m, facilitated diffusion, no regulation).
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