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© 1974 Oxford University Press

RESEARCH-ARTICLE

The Kinetics of Bicarbonate and Malate Exchange in Carrot and Barley Root Cells

W. J. CRAM and G. G. LATIES

School of Biological Sciences, The University of Sydney Sydney, N.S.W. 2006, Australia
Department of Biology and Molecular Biology Institute, University of California Los Angeles, U.S.A.

The time course of loss of14C from H14CO3-labelled carrot tissue has been measured. The graph of log (14C remaining in the cells) versus time can be fitted by two exponential components. The graph of log (rate of 14C loss over successive periods) versus time can also be fitted by two exponential components using the same fitting procedure. However, the half times of the components fitting the two types of graph are not the same, and therefore the apparently good fit is not valid. Three exponential components can be fitted to the two types of plot such that their rate constants are equal and their intercepts in the correct theoretical relationship. Their rate constants are about 7 h–1, 1.5 h–1, and less than 0.1 h–1, and they probably correspond to the total -CO2 in the cell, the malate in the cytoplasm, and the malate in the vacuole, respectively. From these data it is shown that one can calculate the influx of -CO2 across the plasmalemma, and the influx of malate across the tonoplast during accumulation of endogenously produced malate.

The time course of uptake of 14C-malate is estimated as the sum of 14C accumulated in the tissue and 14C evolved as CO2. At 1 mM external malate total uptake is linear with time, suggesting that uptake is limited by and equal to the influx across the plasmalemma. At higher external malate concentrations the evolution of 14CO2 saturates but accumulation in the tissue continues to rise. Under these conditions it is concluded that the tonoplast influx of malate can be calculated and that this influx does not saturate at high external malate concentrations.

The net efflux of malate is very small but measurable. The efflux of 14C-labelled malate is not stimulated by external malate–, Cl–, or . There are therefore no plasmalemma systems exchanging internal malate for these anions.


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