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.
(a) Passive mineral absorption : (i) It occurs by the physical processes like diffusion. Diffusion do not
involve any direct expenditure of metabolic energy, For this, either the matrix of the membrane is
soluble to the diffusing particles or the membrane has pores, (ii) There are ion channels in the membrane, (iii) There are transmembrane proteins,
which function or selective pores for the diffusion of ions, (iv) Similarly there are aquaporins for the
diffusion of water. Oxyen and carbon dioxide are j also transported across the membrane passively, Diffusion is of two types:
(1) Passive Diffusion,
(2) Facilitated Diffusion.
(b) Active mineral absorption : (i) 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.
(ii) 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, (iii) The cells of endodermis have many transport proteins embedded in their plasma membrane, (iv)The active uptake of ions is partly responsible for the water potential gradient in roots, and therefore for the uptake of water by osmosis, (v) Ions diffuse down their concentration gradient from the epidermis to the xylem.