Pathway of Solutes from the External Solution into the Cells | Plants

After reading this article you will learn about the pathway of solutes from the external solution into the plant cells.

Movement of low molecular weight solutes (e.g. ions, organic acids, amino acids, sugars) from the external solution into the cell walls of individual cells or roots (free space) is a non-metabolic, passive process, driven by diffusion or mass flow. Nevertheless, the cell wall can interact with solutes and thus may facilitate or restrict further movement to the uptake sites of the plasma membrane of individual cells or roots.

In contrast to mineral nutrients and low molecular weight organic solutes, high molecular weight solutes (e.g. metal chelates, fulvic acids, and toxins) or viruses and other pathogens are either severely restricted or prevented by the diameter of pores from entering the free space of root cells.

In both the roots and in the cell wall continuum of other plant tissue, the so called apoplasm, the carboxylic groups (R.COCT) act as action exchangers. Therefore, in roots cations from the external solution can accumulate in a non-metabolic step in the free space whereas anions are repelled.

Because of these negative charges the apoplasm does not provide a free space for the movement of charged solutes. The term apparent free space (AFS) comprises the water free space (WFS), which is freely accessible to ions and charged and uncharged molecules, and the Donnan free space (DFS) in which cation exchange and anion repulsion take place.

Ion distribution within the DFS is characterised by the typical Donnan distribution which occurs in soils at the surfaces of negatively charged clay particles. Divalent cations such as Ca²⁺ are therefore preferentially bound to these cation exchange sites. Plant species differ considerably in CEC i.e., in the number of cation exchange sites (fixed anions, RCOO^–) located in cell walls.

Due to spatial limitations, only part of the exchange sites of the AFS is directly accessible to cations from the external solution. Exchange adsorption in the AFS of the apoplasm is not an essential step for ion uptake on transport through the plasma membrane into the cytoplasm.

Nevertheless, the preferential binding of di- and polyvalent cations increases the concentration of these cations in the apoplasm of the roots and thus in the vicinity of the active uptake sites at the plasma membrane. As a result of this indirect effect, a positive correlation can be observed between the CEC and the ratio of Ca²⁺ to K⁺ contents in different plant species.

The importance of cation binding in the AFS for uptake and subsequent shoot transport has been indicated with the same plant species but with different binding forms of a di-valent cation such as Zn.

Where Zn is supplied in the form of an inorganic salt (i.e., as free Zn²⁺) the Zn content not only of the roots but also of the shoots is several times higher than when zinc is supplied as a chelate (Zn—EDTA) that is, without substantial binding of the solute in the AFS. In addition, restricted permeation of the chelated Zn within the pores of the AFS may be a contributing factor.

While these differences in uptake rate between metal cations like Zn²⁺ (and also Cu²⁺ and Mn²⁺) and their complexes with synthetic chelators in so called chelator-buffered solutions calculations can be made of the concentrations of free metal cations in the external solution, required for optimal plant growth.

The root apoplasm may also serve as transient storage pool for heavy metals such as Fe and Zn which can be mobilized, for example by specific root exudates such as phytosiderophores, and trans-located subsequently into the shoots.