Notes on the passive absorption of mineral salts


1. Ion exchange:

The exchange of ions (anions or cations) from within the plant cells to the ions of soil is called ion exchange. By this method, the plant cell can exchange the unwanted ions for the required ones. Ion ex­change is of two types. Contact exchange and Carbonic acid exchange.

In contact exchange, the ions that oscillate around the root hair overlap with the ions that oscillate around the soil particle and there will be mutual ex­change of ions. According to carbonic acid exchange theory CO2 released during respiration combines with water to form carbonic acid disassociates into H+ and HCO3 in soil solution. These H+ icons may be exchanged for K+ ions held by the soil particles. Since the exchange of ions here is mediated by carbonic acid, it is called carbonic acid exchange theory.


2. Mass flow:

According to this theory, when water enters into the plant passively through the influence of transpira­tion, salts also get in. Kramer (1956) is one of the supporters of this theory.

3. Diffusion:

Ini­tially when there is difference in concen­tration gradient (when the concentration of salts inside is less than outside) salts enter the plants through the simple process of dif­fusion.


4. Donan equi­librium:

Normally the total amount of cati­ons and anions present inside the cell are balanced with the ions outside and they would be in constant motion. But there are certain fixed ions in the plant cell which are immobile. In or­der to balance this more ions have to enter the plant cell to maintain the equilibrium. This is called Donan’s equilibrium.

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