Passive Absorption

It is the absorption of minerals without direct expenditure of metabolic energy.

Passive absorption of ions by root system was demonstrated by Briggs and Robertson (1957).

(i) Mineral salt absorption is not affected by temperature and metabolic inhibitors.

(ii) Rapid uptake of ions occurs when plant tissues are transferred from a medium of low concentration to high concentration.

Theories of Passive Absorption

Mass flow theory

According to this theory ions are absorbed by the root along with mass flow of water under the influence of transpiration. This theory failed to explain the salt accumulation against osmotic gradient.

An increase in transpiration pull increases the uptake of ions by the roots. The uptake of ions by free diffusion.

Thus, mass flow of ions through the root tissues occurs due to transpiration pull in the absence of metabolic energy.

Ion exchange theory

Both cations and anions have a tendency to get adsorbed on the surfaces of the cell walls, and exchange with ions present in the soil solution. This process of exchange between the adsorbed ions and ions in solution is known as ion exchange.

Mineral elements can be absorbed in the form of molecules or as ions. When a positively charged ion as K⁺ is absorbed by the cell, either a positively charged ion as H⁺ is displaced from the cell (ion exchange) or a negatively charged ion must accompany it. Similarly anions can exchange with free hydroxyl (OH^-) ions. Likewise Cl^- and Br^- are also exchanged without disturbing the electrical neutrality.

Ion Exchange Theory

Negatively charged Cl^- and Br^- are exchanged without disturbing the electrical neutrality

Donnan equilibrium

This theory describes the effect of fixed or non-diffusible ions which mostly accumulate on the inner surface of the outer membrane. Process is named after its discoverer F.G.Donnan. If there is a negative non-diffusing charge on one side of a membrane, it will create a potential gradient across the membrane from which ions will diffuse. The result will be an electrochemical equilibrium. The concentration (chemical potential) of ions will not necessarily be the bsame inside and outside. Thus, as an electrical disequilibrium is maintained because of diffusing charges. A concentration disequilibrium is established.

Donnan Equilibrium

Therefore, according to Donnan, Donnan equilibrium is attained if the product of anions and cations in the internal solution becomes equal to the product of anions and cations in the external solution.

Ci⁺ = Cations inside

Ai^- = Anions inside

Co⁺ = Cations outside

Ao^- = Anions outside

[Ci⁺][Ai^-] = [Co⁺][Ao^-]

or