Absorption of Mineral Salts

Most minerals in the soil exist as charged particles (ions), and charged particles cannot simply diffuse across a cell membrane, so the majority of mineral uptake by roots requires an active, energy-consuming process -- though some mineral absorption can also occur passively, without any expenditure of energy. Mineral ion absorption is, notably, independent of water absorption; the two are governed by different mechanisms even though water and minerals both ultimately move together once inside the xylem.

Mineral ion absorption can occur in two ways:

a. Passive Absorption : Passive absorption is the movement of mineral ions into root cells purely by diffusion -- from a region of higher ionic concentration to one of lower concentration -- without any expenditure of energy. It can occur by direct ion exchange, indirect ion exchange, mass flow and Donnan equilibrium.

b. Donnan equilibrium : It is based on the assumption that certain negatively charged ions (anions) become fixed on the inner face of the cell membrane once they enter the cell and cannot diffuse back out; to electrically balance these trapped anions, additional mobile cations are drawn in from outside, even against their own concentration gradient, so that cations end up accumulating in the cell purely to neutralise the fixed anions.

Active Absorption : Active absorption is the uptake of mineral ions against their concentration gradient, and this always requires an expenditure of energy by the absorbing cell, drawn from respiration and supplied through ATP; if root cells are deprived of oxygen, active mineral absorption drops sharply. Once inside, mineral ions accumulated in the root hairs pass inward through the cortex and finally reach the xylem, from where they travel with the transpiration stream to the rest of the plant and are eventually assimilated into organic molecules and redistributed through the phloem.