Theories of mineral absorption

                       There are two theories:

1. Passive absorption:-

                   Absorption of ions through the concentration gradient without utilizing metabolic energy is called passive absorption. There are many theories of passive absorption. Some are as follows:-

A) Donan equilibrium:-

                   This theory was given by F.G. Donnon in 1911. According to this theory, some anions and cations are present in the inner surface of the plasma membrane called fixed ions. To balance them, ions of opposite charge enter the cell without utilizing bergy i.e. if the fixed ion is a cation, more anions diffuse inside the cell.

B) Ion exchange theory:

                   This theory was proposed by Jenny and over-street (1938). ion exchange theory has two types:

a) Contact exchange theory :

                   According to this theory, ions are not completely static, they are always oscillating around their absorption surface and when the oscillation volume of the ions on the root and on the colloidal particles overlap each other, ion exchange takes place.

Fig:  Contact exchange theory

b) Carbonic exchange theory:

                               In this theory, CO₂ is liberated from root cells during respiration which is mixed with water to form carbonic acid. Carbonic acid dissociates into hydrogen and bicarbonate ions. The exchange of ions of the minerals occurs within singular ions of carbonic acid and finally reaches on cell’s surface. Exchange of ions of H⁺ & HCO₃^– with similar charge ions of soil particles takes place.

Fig: Carbonic exchange theory

c) Mass flow theory:

                   This theory was proposed by Kramer in 1956. According to this theory, when solvent moves fastly into the root of the cell from soil then along with them, a few amount of ions of minerals also move into the root cells of the plant.

2. Active absorption:

               The absorption of ions of minerals by utilizing energy is called active absorption. It is explained by the carrier concept, proposed by Van Den Ho Nert in 1937. According to this concept, no proteinous substance acts as a carrier for ions and helps in transporting ions through the cell membrane. Such substances are known as carriers. By utilizing the energy of root cells, they pick up the ions of minerals and form an ion carrier complex. This complex returns to the inner surface and reduces or releases the ions into the cell. The carriers again become excited using the energy of the cell and bring the ions of minerals as like previous condition or theory.

Fig: Active absorption