acid exchange. i. Contact Exchange Theory: Figure . : Contact Exchange theory According to this theory, the ions adsorbed on the surface of root cells and clay particles (or clay micelles) are not held tightly but oscillate within a small volume of space called oscillation volume .
Due to small space, both ions overlap each other’s oscillation volume and exchange takes place (Figure . ). ii. Carbonic Acid Exchange Theory: According to this theory, soil solution plays an important role by acting as a medium for ion exchange.
The CO released during respiration of root cells combines with water to form carbonic acid (H CO ). Carbonic acid dissociates into H + and HCO – in the soil solution. These H + ions exchange with cations adsorbed on clay particles and the cations from micelles get released into soil solution and gets adsorbed on root cells (Figure . ).
Figure . : Carbonic Acid Exchange theory . . Active Absorption Absorption of ions against the concentration gradient with the expenditure of metabolic energy is called active absorption .
In plants, the vacuolar sap shows accumulation of anions and cations against the concentration gradient which cannot be explained by theories of passive absorption. Mechanism of active absorption of salts can be explained through carrier concept. Carrier Concept: This concept was proposed by Van den Honert in . The cell membrane is largely impermeable to free ions.
However, the presence of carrier molecules in the membrane acts as a vehicle to pick up or bind with ions to form carrier-ion-complex, which moves across the membrane. On the inner surface of the membrane, this complex breaks apart releasing ions into cell while carrier goes back to the outer surface to pick up fresh ions (Figure . ). Figure .
: Carrier Concept The concept can be explained using two theories: . Lundegardh’s Cytochrome Pump Theory: Lundegardh and Burstrom ( ) observed a correlation between respiration and anion absorption. When a plant is transferred from water to a salt solution the rate of respiration increases which is called as anion respiration or salt respiration . Based on this