Passive absorption of mineral salts occurs through simple diffusion down a concentration gradient without expending metabolic energy. Several theories have been proposed to explain the mechanism of passive absorption, including diffusion, ion exchange, Donnan equilibrium, and mass flow. Diffusion involves the net movement of minerals from high to low concentration until equilibrium is reached. Ion exchange involves replacing ions on the surface of cell walls. Donnan equilibrium describes the balance of ions across a selectively permeable membrane. Mass flow theory states that minerals are absorbed with water flow into roots during transpiration. However, some objections to purely passive absorption have been raised, as metabolic activities and accumulation against gradients have been observed.

1. Mechanism of passive
Absorption
Rahee

2. Mechanism of Mineral salt
absorption ( Passive
Absorption)
• When the concentration of mineral salts is higher in the outer solution than in the cell sap of the root cells,
the mineral salts are absorbed according to the concentration gradient by simple process of diffusion.
• This is called as passive absorption because it does not require expenditure of metabolic energy.
• Passive absorption is a physical phenomenon and it is also called physical absorption
• It is not affected by temperature or metabolic inhibitors.

3.  Passive absorption takes place in many ways
 Scientist have explained various theories about passive absorption
1. Theory of Diffusion
2. Theory of ion exchange
3. Theory of Donnon equilibrium
4. Theory of mass flow

4. Theory of Diffusion
 Diffusion is the net movement of material from an area of high concentration to an
area with lower concentration.
 The difference of concentration between the two areas is often termed as
the between the two areas is often termed as the concentration gradient.
 diffusion will continue until this gradient has been eliminated.

5.  As long as the concentration of solutes inside the cell is lower than its
concentration outside, this process remains continue
 When the concentration of solutes outside and inside become equal, the rate of
diffusion become zero in both direction
 Therefore, further absorption of solutes stops.

6. Theory of ion exchange
 Ion exchange mechanism is involved in the exchange of anions and cations from
inside the cell in the surrounding of the cells.
 Absorption of minerals also takws place by ionic exchange
 This mechanism may occur between the ions present on the surface of cell wall and
external solution,in which tissue is immersed.

7.  Ion exchange mechanism has been shown in barley roots with the use of
radioactive k*.
 When the roots of barley are placed in a solution of non radioactive k*, these ions
are replaced by non-radioactive k*.
 When the roots are placed in distilled water, the radioactive k* ions do not come
out.
 The ionic exchange is not affected by rate of respiration.

8. Theory of Donnan equilibrium
 When there is equilibrium of ions inside and outside the cell, it is called donnan
equilibrium
 Donnan equilibrium describes the equilibrium that exists between two solutions
that are separated by a membrane.
 The membrane is constructed such that it allows the passage of certain charged
components (ions) of the solutions.
 The membrane, however, does not allow the passage of all the ions present in the
solutions and is thus a selectively permeable membrane.

9.  In this mechanism non-diffusible ions (also called fixed ion) play an important role.
 E.g, Suppose on the inner side of the membrane non-diffusible anions are in
concentration i.e membrane is impermeable to these ions
 Now, if this solution is put in a solution containing cations and anions to which the
membrane is permeable, an equal number of cations and anions will diffuse across
the membrane until equilibrium is established
 However, to balance the non diffusible anions on the inner side of membrane,
more cations move inside.
 This is known as Donnan equilibrium concept.

10. Theory of mass flow
 This theory was proposed by Kramer( 1965) and Russel and Balber( 1960).
 According to this theory the ions are taken up by the roots with the mass flow of
water which is absorbed by roots due to transpiration.
 It is noticed that increase in transpiration increase the absorption of ions from the
soil.

11. Objections to passive absorption concept
 Some objects against the passive absorption have been put forth. These are as
follows:
1. It has been experimentally proved that in salt uptake metabolic activities are
involved
2. No theory of passive absorption explains properly the absorption and
accumulation of salts or ions against the osmotic gradient.