The Salt Absorption Isotherm

It is shown that the absorption isotherm of rubidium by excised barley roots can be explained either by two uptake mechanisms following Michaelis‐Menten kinetics or by two mechanisms, one actively transporting salts into the tissue (the pump), the other one being more passive in nature (the leak), operating in either direction, depending on external and internal substrate concentration. Kinetic data are thus consistent with more than one transport model. It was further demonstrated for the pair K‐Na, that a competitor not only reduces salt uptake but can also reverse the direction of net flux. This observation cannot be explained by classical enzyme kinetics, it is, however, consistent with the pump and leak system. Just as Michaelis‐Menten kinetics, the pump and leak system can explain ion competition, in addition it offers a possible explanation of the Viets' effect and it can explain the time curve of absorption.