1. Donnan equilibrium is a process of passive absorption of minerals in plants which is without any expenditure of energy.
2. It is assumed that certain anions after their entry by diffusion into the cell get fixed on inner side of cell membrane.
3. Additional mobile cations are needed to balance this fixed anions as they cannot diffuse outside.
4. Concentration of cations thus increases due to accumulation.
5. This passive absorption of anions or cations from exterior against their concentration gradients so as to neutralize the effect of cations or anion is known as Donnan equilibrium.