Plants absorb minerals as ions. The rate of mineral absorption is independent of its concentration in the soil. It has two phases, the initial phase and the metabolic phase.
In metabolic phase, the ions pass into inner space (cytoplasm and vacuole) by two methods :
(a) Passive absorption
(b) Active absorption.
(i) Passive mineral absorption :
(a) It occur by the physical processes like diffusion. Diffusion do not involve any direct expenditure of metabolic energy.
(b) There are ion channels in the membrane.
(c) There are transmembrane proteins, which function as selective pores for the diffusion of ions.
(d) Similarly there are aquaporins for the diffusion of water. Oxygen and carbon dioxide are also transported across the membrane passively.
(ii) Active mineral absorption :
(a) Active transport occurs through the agency of specific carriers present in the plasma membranes of the absorbing cells. It requires energy, which is supplied by ATP.
(b) The enzymes at pores present on the plasma membrane of root epidermal cells use the energy of ATP to pump protons across the membrane. This establishes an electrochemical proton gradient that supplies energy for moving ions into the cells.
(c) The cells of endodermis have many transport proteins embedded in their plasma membrane.
(d) The active uptake of ions is partly responsible for the water potential gradient in roots, and therefore for the uptake of water by osmosis.
(e) Ions diffuse down their concentration gradient from the epidermis to the xylem.
