Transport of Ions from the Epidermis to Xylem | Plants

After reading this article you will learn about the passive and active transport of ions from the epidermis to xylem in plants.

Passive Transport of Ions:

Transport of ions from the epidermis to the xylem without any consumption of metabolic energy merely in consequence of the spontaneous disappearance of non-equilibria driven by a decrease in free enthalpy is known as passive transport.

Thermodynamically passive transport is the transport down or anion along electro-chemical gradient. Passive transport processes can occur in dead or non-respiring tissues and are not sensitive to an inhibitor. There is no selectivity of ions in this transport processes.

Extracellular spaces exist in the mesophyll cells of leaves where ions are able to diffuse and exchange. Most of the nutrient ions reach the “outer” space of leaves via the xylem, from the roots. Mineral ions in rain, irrigation water and in foliar applications penetrate leaves through the stomata and cuticle to reach the interior of leaves where they become available for absorption by mesophyll cells.

Active Transport of Ions:

Active transport is the process by which ions cross the root from the epidermis to the xylem with the expenditure of metabolic energy against the gradient of their concentration. Thermodynamically it is the transport process against an electrochemical gradient.

Active transport processes require the presence of living aerobically respiring cells and can, therefore, mostly be inhibited by metabolic inhibitors. Active processes accumulate ions against concentration gradients behind a physiological barrier.

This barrier is assumed to be permeable to the in-going ions only at the expenditure of energy and is impermeable to them as soon as they have been accumulated inside.

Thus the energy involved in active transports is not only required to move ions against a concentration gradient, but also forces them through barrier which otherwise would be much less permeable for them, even if the ion movement occurred along the concentration gradient. The selectivity of ions is attributed to the active transport processes.

The various carrier like ribonucleoproteins (nucleic acids bind cation and proteins bind anions), cell mitochondria etc. bind ions selectively and after formation of such carrier-ion complexes, they cross membranes and other barriers not permeable to free ions. After the transfer is accomplished, the ion—carrier complex is broken, the ion is released into the inner space of the cell and the carrier becomes free.

Two different mechanisms are involved in transport of ions into the inner space. For some ions a “mechanism 1” operates at very low concentrations, while at high concentrations above about 1 m Ma “mechanism 2” with different properties comes into play. The plasma lemma or external cytoplasmic membrane is implicated as the locale of these dual mechanisms.

Ion translocation through long distances across the roots is perhaps through a “free space” and after overcoming a physiological barrier continues to move through the symplast.

Ions in the solution are subjected to two main physical “forces”,

(a) Arises from the chemical potential gradient and

(b) The other from the electrical potential gradient i.e. from a higher to a lower concentration. For ions acted upon by an electrical gradient, cations are attracted to a negative electro-potential, whereas anions are attracted to a positive electro potential.

Ion movement is thus dependent on an electro-chemical potential gradient. Living cells are negatively charged as compared with the outer medium. For this reason the passage of ions through the plasma lemma or tonoplast must also be considered in relation to the prevailing electrical potential gradient as well as to the concentration gradient between the “outer solution” (soil medium) and inner solution (cytoplasm).

The net inward movement of cations terminates as soon as the equilibrium between the electrical and kinetic driving forces is attained. This equilibrium is described by the Nernst equation considering an aqueous solution of KCl separated by a membrane permeable to both ions K⁺ and CI^– based on the assumption that the electrical potential across the membrane E, equilibrium for K⁺ and CI^– is attained.

where, Ψ_i = electrical charge of the inner solution i.e. in the cytoplasm.

Ψ₀ = electrical charge of the outer medium i.e. in the nutrient solution.

R = gas constant

T = Absolute temperature

F = Faraday constant

Z = Valency of the ions.

[K⁺ and [CI^–] are concentrations of K⁺ and CI^– in the outer (o) and inner (i) solution.

From this equation, it can be derived that when E < 0 (the cell is negatively charged), the term [K₀⁺]/[K_i⁺] must be < 1 and this means that under equilibrium conditions an accumulation of K⁺ occurs in the inner solution. Again it follows that the term [CI_i^–]/[Cl₀^–] must be < 1 and this means that under equilibrium conditions the CI^– concentrations of the outer solution is higher than that of the inner solution.

It thus appears that the cation concentration in the cytoplasm can be several times higher than that of the outer solution without requiring an “Uphill transport of cations” i.e. a transport against an electro-chemical gradient.

If for an example, the K⁺ concentration of the inner solution is 10 times higher than that of the outer solution the term log [K₀^–]/[K_i^–] = -1 with the corresponding electrical potential difference is then -58 mV. Only where the concentration is higher than that of the equilibrium condition, an “Uphill transport” i.e. a transport against an electro-chemical gradient, must have occurred.

In order to test, whether an ion species has moved actively or passively into the cell, the concentrations of the particular ion species in the outer medium (soil solution) and in the cell must be measured as well as the electro potential (E_m) between the cell and the external solution.

This can be obtained using a micro-electrode substituting the measured ion concentrations—into the Nernst equation an electrical potential difference (E_cal) can be calculated. Where E_m denotes the measured potential, the difference between E_m and E_cal indicates where a passive or an active transport has occurred.

E_m– E_cal=E_d,

where E_d = driving force,

For the cations, a negative value of E_d indicates a passive uptake and a positive value on active uptake. Again, for the anions, the reverse is true, i.e. negative value of E_d indicates an active transport and positive value a passive transport.