Diffusion Theory Mass Flow Theory Ion Exchange Theory Donnan's Equilibrium

2. Passive Theories- Mineral Absorption
Passive – without spending energy- spontaneous
Theories
1. Diffusion Theory
2. Mass Flow Theory
3. Ion Exchange Theory
a) Contact Ion Exchange Theory
b) Carbonic acid Ion Exchange Theory
4. Donnan’s Equilibrium

3. 1. Diffusion Theory
• Diffusion- movement of particles (ions and molecules) along the
concentration gradient, i.e. from higher concentration to lower
concentration
• Example : Sugar in Water, Perfume in air
• Mulder (1851)- the importance of diffusion in mineral absorption
• Supported by Sachs (1875) and Pfeffer (1900)
• Hoagland – studied extensively in Nitella – reported absorption is active
process that requires metabolic energy
• Kostytschew (1926), Olsen (1950), Knaus and Porter (1954) – experiments
did not support diffusion theory

4. 2. Mass Flow Theory
• Solutes move along with the flow (movement) of
water molecules
• Water tends to have opposite charge to pore
walls – hence move to cell wall
• From cell wall move to membrane portion
consisting similar charge as of cell wall.
• Hylmo (1953) proposed that mass flow of solutes
along with water to the root tissue and from
there to the other parts of plant is facilitated by
the transpiration

5. • Devaux (1916) was the first person - drew attention
to the cation exchange properties of roots
• Jenny and Overstreet (1938) elaborated the idea of
"contact exchange”
• Exchange of ions between roots and soil without
intervention of the soil solution.
• Ions are not tightly held together in root cells and
clay micelle
• When Root and Clay particles contact each other,
they may exchange the ions (especially cations) due
to overlapping oscillation volume
3. a) Contact Ion Exchange Theory

6. 3. a) Contact Ion Exchange Theory

7. • Overstreet et al. (1942) observed organic acids
produced by plants help in ion exchange
• The Carbon-di-Oxide released in the respiration
process by root cells reacts with water and form
Carbonic acid
• The Carbonic acid dissociates into H+ and HCO3
- in
the soil solution
• Now the H+ exchanged with cations (Eg. K+)present
in the clay particles
3. b) Carbonic Acid Exchange Theory

9. • Cells contain Macromolecules
• Fixed ions - Certain Ions inside the cells can not
diffuse out
• In general, cells contain more fixed anions (-ve ions)
compared to cations (+ve ions)
• Cations are mostly mobile and can diffuse through
membrane
• To balance this (-ve charge), cations will diffuse into
the cell from external solution
• This equilibrium is called Donnan’s Equilibrium
4. Donnan’s Equilibrium