Absorption processes that do not require metabolic energy are called passive absorption. Very small amounts of mineral salts are passively absorbed. The principles of passive absorption are:
1. Donnan equilibrium
Donnan introduced this theory in 1911-1914. According to this theory, there is a fixed anion or anion charge inside the cell membrane that cannot be absorbed. To neutralize this fixed negative charge, the cation or cationic charge moves in from the outside of the cell membrane in the diffusion process. This process continues until there is an equilibrium of cations and anions inside the cell. Donnan calls this phenomenon equilibrium. In Donnan equilibrium, the ratio of positive charges inside and outside the cell is equal to the ratio of negative charges inside and outside the cell.
2. Diffusion theory
Diffusion theory was introduced by Hope (1953) and Heilmo (1955). Cells of absorptive organs of plants have lower concentrations of ions and higher concentrations of ions in soil solutions. Therefore, few ions enter the cell during the diffusion process. Absorbed ions enter various cells from the cortex. The absorbed ions are then used for metabolic purposes in various cells.
3. Ion exchange theory
Proponents of ion exchange theory are Devlin (1969) and Pandey and Sinha (1972). Plant root cells contain H+ and OH-. H+ ions are released and K+ ions from the outside solution enter the cell. Similarly exchange between OH- ions and Cl- ions takes place. In this method anion exchange is least and cation exchange is most. Hence it is called cation exchange method. However, cations and anions are not adsorbed simultaneously.
4. Mass flow theory
Some scientists Hylmo (1955) and Kramen (1956) think that when water is absorbed at a large rate during respiration, mineral salt ions are also absorbed with water. Lopushinsky observed in 1964 that the rate of ion absorption increased when the rate of transmission increased.