Plants absorb mineral salt from the soil in the form of cations and anions through different methods. The mechanism of mineral absorption can be understood under the following heads.
(1) Passive absorption
1. Mass flow hypothesis
2. Ion exchange theory
3. Donnan equilibrium theory
(2) Active absorption
1. Carrier concept
2. Ion pump or cytochrome pump concept
3. Electrochemical gradient hypothesis
(1) Passive absorption:
According to this theory, mineral elements enter the cell from the soil solution along their electrochemical potential gradient i.e. from their higher concentration to the place of their lower concentration without any expenditure of energy of the cell. Three theories have been put forwarded in support of passive absorption of minerals.
1. Mass flow hypothesis:
According to this theory, under the influence of transpiration pull, mineral ions are also absorbed along with the stream of water. Mass flow, also called as the bulk flow is the unidirectional movement of molecules or ions through the root along with a stream of water due to the suction force created by transpiration pull. An increase in the rate of transpiration results in to increase in the rate of passive absorption.
2. Ion Exchange theory:
Exchange of cations and anions in between the external solution (soil solution) and the surface of the root is called ion exchange. This hypothesis assumes that exchange of cations and anions takes place by ions of similar charge i.e. of the external solution can exchange with on the surface of the membrane and similarly the anion can also exchange with free hydroxyl ion.
3. Donnan equilibrium theory:
- This theory was proposed by Donnan (1927).
- According to this theory, there are some fixed ions in the cell which cannot pass out of the cell membrane. These are called stable ions, whereas both anions and cations can enter the membrane from outside.
- Normally there is an equilibrium of ions between external and the internal solution. This theory assumes that a cell has a concentration of fixed ions to which its membrane is impermeable. An equal number of anions and cations from the external solution will diffuse across the membrane until it reaches the state of equilibrium.
- This theory explains the accumulation of anions against a concentration gradient without the participation of metabolic energy
