The generation of latent-ion-transport capacity

Of the various hypotheses designed to explain the movement of ions from the environment into the plant cell, the most attractive suggests that ions traverse cell membranes in combination with an endogenously produced carrier substance.(1,2) The hypothesis stems from the recognition that absorption kinetics are not first-order, but are, rather, best represented by a formulation analogous to that which describes the reversible combination of substrate and enzyme.(3) The carrier hypothesis is at least consistent with the known characteristics of the absorption process. Thus the remarkable specificity of the absorption process may be imputed to specificity on the part of the carriers; the requirement for respiratory energy may be related either to the production of carrier or to the degradation of ion-carrier complex; and the ability of ions to penetrate ion-impermeable cellular membranes may be ascribed to the formation of a permeating ion-carrier complex

The Prevalence of Carbon-13 in Respiratory Carbon Dioxide As an Indicator of the Type of Endogenous Substrate. The change from lipid to carbohydrate during the respiratory rise in potato slices

Isotope discrimination is a common feature of biosynthesis in nature, with the result that different classes of carbon compounds frequently display different 13C/12C ratios. The 13C/12C ratio of lipid in potato tuber tissue is considerably lower than that for starch or protein. We have collected respiratory CO2 from potato discs in successive periods through 24 hr from the time of cutting—an interval in which the respiration rate rises 3–5-fold. The 13C/12C ratio of the evolved CO2 was determined for each period, and compared with the 13C/12C ratios of the major tissue metabolites. In the first hours the carbon isotope ratio of the CO2 matches that of lipid. With time, the ratio approaches that typical of starch or protein. An estimation has been made of the contribution of lipid and carbohydrate to the total respiration at each juncture. In connection with additional observations, it was deduced that the basal, or initial, respiration represents lipid metabolism —- possibly the alpha-oxidation of long chain fatty acids -— while the developed repiration represents conventional tricarboxylic acid cycle oxidation of the products of carbohydrate glycolysis. The true isotopic composition of the respiratory CO2 may be obscured by fractionation attending the refixation of CO2 during respiration, and by CO2 arising from dissolved CO2 and bicarbonate preexisting in the tuber. Means are described for coping with both pitfalls