Write a detailed account of the mechanism of mineral salt absorption.

Question

Filo · Accepted Answer

Plants absorb mineral salt from the soil in the form of cations and anions through different methods. The mechanism of mineral absorption can be understood under the following heads.
(I) Passive absorption.

Mass flow hypothesis.
Ion exchange theory.
Donnan equilibrium theory.

(II) Active absorption.

Carrier concept.
Ion pump or cytochrome pump concept.
Electrochemical gradient hypothesis.

(I) Passive absorption:
According to this theory, mineral elements enter the cell from the soil solution along their electrochemical potential gradient i.e. from their higher concentration to the place of their lower concentration without any expenditure of energy of the cell. Three theories have been put forwarded in support of passive absorption of minerals.

1. Massive flow hypothesis:
According to this theory, under the influence of transpiration pull, mineral ions are also absorbed along with the stream of water. Mass flow, also called as the bulk flow is the unidirectional movement of molecules or ions through the root along with a stream of water due to the suction force created by transpiration pull. An increase in the rate of transpiration results in to increase in the rate of passive absorption.

2. Ion Exchange theory:
Exchange of cations and anions in between the external solution (soil solution) and the surface of the root is called ion exchange. This hypothesis assumes that exchange of cations and anions takes place by ions of similar charge i.e.

K^{+}

of the external solution can exchange with

H^{+}

on the surface of the membrane and similarly the anion can also exchange with free hydroxyl ion.

3. Donnan equilibrium theory:

This theory was proposed by Donnan (1927).
According to this theory, there are some fixed ions in the cell which cannot pass out of the cell membrane. These are called stable ions, whereas both anions and cations can enter the membrane from outside.
Normally there is an equilibrium of ions between external and the internal solution. This theory assumes that a cell has a concentration of fixed ions to which its membrane is impermeable. An equal number of anions and cations from the external solution will diffuse across the membrane until it reaches the state of equilibrium.
This theory explains the accumulation of anions against a concentration gradient without the participation of metabolic energy.

Note: It is assumed that at least a part of the total salt uptake may result from passive absorption and it may account for salt accumulation within the plant tissue. The rate of mineral salt absorption is too rapid and cannot be explained by passive absorption which requires metabolic energy.

(2) Active absorption:
According to this theory, mineral elements move against the concentration gradient or the electrochemical potential and metabolic energy of the cell is used in the process. This process requires ATP molecules. This absorption is called active absorption. Following theories have been put forwarded in support of this theory.
1. Carrier Concept:

This theory was proposed by Van Den Honert (1937).
According to this concept, the part of the cell or tissue in which ions are absorbed by the use of metabolic energy is called inner space and the area outside this is called outer space.
The boundary or barrier (cell membrane or plasma membrane and tonoplast) between inner and outer space is impermeable to the movement of free ions.
According to Honert (1937), protein molecule present at the surface of membrane acts as a carrier and combines on the outer space with a specific ion in the soil solution, transports it across the membrane and releases it in the inner space.
The process can be explained with the help of following steps.

Carrier + ATP (Activation of Carrier).
Activated carrier + Ion - Carrier ion complex.
Carrier ion complex - Carrier + Ion.

2. Ion Pump or Cytochrome Pump Theory:

This theory was proposed by Lundegard and Burstrom (1933).
According to this theory, there is a direct relationship between the rate of respiration and the absorption of anions.
Anions are transported from the outer surface of membranes to the inner surface by cytochromes.
To balance the electrochemical potential, the cations move from the outer surface to the inner surface of the membrane.
This theory assumes that anions are taken up by expenditure of energy whereas movement of cations is passive.

Outer Surface	Membrane	Inner Surface
Anion (-)	Cytochrome pump	Anion (-)
Cation	Passive flow	Cation (+)

3. Electrochemical gradient hypothesis:

This hypothesis has been proposed by Peter Mitchell (1968).
According to this hypothesis, anions are transported across the membrane due to electrochemical potential developed on the outer and inner face of the membrane.
In this process, the ATPase enzyme plays and important role.

Question Text	Write a detailed account of the mechanism of mineral salt absorption.
Answer Type	Text solution:1
Upvotes	150

Write a detailed account of the mechanism of mineral salt absorption.

Text solutionVerified

Practice questions from similar books