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

1. A) Donan equilibrium:-

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

1. B) Ion exchange theory:

                   This theory was purposed by Jenny and over-street (1938). ion exchange theory is two type:

a) Contact exchange theory :

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

b) Carbonic exchange theory:

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

c) Mass flow theory:

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