The Carrier Concept
Active Absorption (Active Uptake):-
Ø It has often been observed that the cell sap in plants accumulates large quantities of mineral salts ions
against the concentration gradient. It is an active process which involves the expenditure of metabolic energy through respiration.
Ø Following evidences favor this view:-
i. The factors like low temp., deficiency of O2, metabolic inhibitors etc. which inhibit metabolic activities like respiration in plants also inhibit accumulation of ions.
ii. Rate of respiration is increased when a plant is transferred from water to salt solution. It is called salt respiration.
The Carrier Concept:- According to this theory the plasma membrane is impermeable to free ions. But some compounds present in it acts as carrier and combines with ions to form carrier-ion-complex which can move across the membrane. On the inner surface of the membrane this complex breaks releasing ions into the cell while the carrier goes back to the outer surface to pick up fresh ions.
