The movement of minerals into the cell or tissue without the expenditure of energy is called passive absorption. This is a physical phenomenon. Passive absorption takes place by following ways:
The concentration of ions inside the root cells is several times more than in the soil solution. Secondly the mineral has to cross several barriers before entering (he cells of the root.’ Therefore, mineral ions cannot directly move from the soil solution into the cells of the root by simple diffusion. They can pass through only from the free space by diffusion.
The first barrier is the cell wall. Cell wall is permeable for the entry of ions. Therefore, they can freely pass through it by simple diffusion. After crossing the cell wall, the,ions freely move. They come across the cell membrane called ectoplast. It prevents further movement of ions by diffusion. It means ions can free)/ move from the soil solution up to this barrier. The term free space is used for the part of the plant cell and tissue.where ions can freely move by diffusion. It is also called apparent free space (AFS).The inner boundary of free space is the ectoplast. Entry of ions into free space is very fast and it takes place by diffusion. Ions can also easily diffuse out.
Therefore, ions can pass through free space area by diffusion. Ions cannot cross the barrier of ectoplast by this process and enter the protoplasm.
(b) MASS FLOW
The movement of ions along with bulk amount of water is called mass flow of ions. The increase of transpiration also increases the absorption of ions from the soil. Cohesion tension theory explains the role of transpiration in the mass movement of water. It is believed that mineral ions also move into the root along with this mass flow of water. Some physiologists believe that the effect of transpiration on mineral absorption is indirect. Transpiration removes the minerals from the root xylem with the transpiration stream. So it reduces its concentration in the root cells. As a result, root absorbs more minerals from the soil.
(c) MECHANISM OF IONIC EXCHANGE
The exchange of anions or cations of the cells with the ions of the same and equivalent charge of outer solution is called ion exchange. The ion er.change does not require metabolic energy. Therefore, it is a
passive phyi process. Thus it is not affected by the rate of aerobic
respiration. ‘1 fs:., ace two theories explaining the mechanism of ion exchange. These, tz.e fl) contact exchange theory and (ii) Carbonic acid exchange theory
(i) Contact exchange theory
According to this theory, ions are not dissolved in water. The roots are Present near the clay particles in the soil. Cations are adsorbed on surface of the clay colloids. The roots releases I-I ion. These H ion are directly exchanged by the cations of clay particles.
(ii) Carbonic acid exchange theory
According this theory, ions first dissolve in the soil solution. Carbon dioxide released during the respiration in roots. It dissolves in soil water and form carbonic acid (II,C0)). Carbonic acid is a weak acid. It dissociates into IT ions ad HCO3‘ ions. IT ion ions reach the clay particles. They are exchanged with cation like K’ present in the soil. The released cation goes into the soil solution. Finally they reach the root surface.
(d) DON-NAN EQUILIBRIUM
The equilibrium of ions between root cells and soil solution in the presence of fixed ions in the root cells issalled Dorman equilibrium, Suppose a eel! contains potassium proleinate (K and Pr protein ions). it is itun,er,01 in an external solution. This solution contains potassium
chloride. Inside the cell are. K and Pr– ions. The outer solution contains K and Cf ions. The plasma membrane is freely permeable to Cl– and K.’ ions. But it is impermeable to Pr ions. Pr ion is thus called fixed anions. Cl– will diffuse into the cell across the membrane due to difference of concentration. An equal number of K+ ions accompany them to neutralize them. This will continue till equilibrium is reached. Additional cations (K) are needed to neutralize the negative charges of the fixed anions (Pr ). Therefore, the–concentration of K. inside the cell is greater than the external solution at equilibrium point. Thus cations are accumulated in the cell at a higher concentration than in the external solution. It does not need expenditure of metabolic energy. This equilibrium is called Donnan equilibrium. Similarly, if there are fixed cations on the inside of the cell membrane it will accumulate anions in the cells.