Mechanism of Mineral uptake by Plants
Previously, it was thought that the absorption of mineral salts from the soil took place along with
the absorption of water, but it is now well established that the mineral salt absorption and water
absorption are two independent processes.
Mineral salts are absorbed from the soil solution in the form of ions. They are chiefly absorbed
through the meristematic regions of the roots near tips. The plasma membrane of the root cells is not
permeable to all the ions. It is selectively permeable. All the ions of the same salt are not absorbed in
an equal rate but there is unequal absorption of ions.
The first step in the absorption of mineral salts is by Ion Exchange. Once the nutrients come and
adsorb on the surface of the walls of the membranes of root cells, then the further process of the
absorption of mineral salts may be of two types. They are :
1. Passive Absorption
2. Active Absorption
Various theories have been proposed to explain the mechanism of mineral salt absorption, which
can be of two categories :
1. Passive Absorption of Mineral Salts
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 the simple
process of diffusion. This is called passive absorption because it does not require the expenditure of
metabolic energy.
This can also be called Physical Absorption. This process is not affected by temperature and
metabolic inhibitors. This theory is based on the movements of ions from the region of its
higher concentration to the lower concentration. Therefore, The direction of the initial uptake gets
reversed if the tissues are transferred back to a low concentration.
Important theories are Mass Flow, ion exchange, and Donnan equilibrium.
i. Mass Flow Theory (Bulk Flow)
According to this theory, the ions are taken up by the roots along with the mass flow of water under
the influence of transpiration. Therefore, the transpiration effect on salt absorption is direct.
ii. Ion Exchange Theory
According to this theory, ions from the external solution in which the tissue is immersed
may exchange with the ions absorbed on the surface of the cell wall or membranes of the tissue.
iii. Donnan Equilibrium
This theory explains the accumulation of ions inside the cells without involving the expenditure of
metabolic energy. According to this theory, there are certain pre-existing ions inside the cell, which
cannot diffuse outside through the membrane. Such ions are called indiffusible or fixed ions. However,
the membrane is permeable to both anions and cations of the outer solution (Fig.11)
Suppose, there are certain fixed anions in the cell which is in contact with the outer solution
containing anions and cations. Normally, an equal number of anions and cations would have diffused into
the cell through an electrical potential to balance each other, but to balance the fixed anions already
present in the cell (pre-existing), more cations will diffuse into the cell, This equilibrium is known as
Donnan’s Equilibrium. In this particular case, there would be more accumulation of cations inside the
cell.
However, if there are fixed cations (pre-existing)inside the cell, then Donnan’s equilibrium will
result in more accumulation of anions inside the cell to maintain the equilibrium.
2.Active Absorption of Mineral Salts
This process involved the metabolic energy for the transport of ions from soil solution to the
plants. Based on the nature of participation of metabolic energy, various theories have been proposed. It
includes the theories related to carrier concept such as the Cytochrome Pump hypothesis, ATP theories,
Protein-Lecithin as carrier theories etc.
i. Carrier Concept Theory (Honert,1973)
(for movement of both cation & anion)
According to this theory, the ion transport process is carried out by means of carriers, which may
be organic molecules or vesicles. This theory explains that the plasma membrane is impermeable to free
ions. The carrier combines with ions to form a 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 fresh ions (Fig.12).
Here, the metabolic energy is required in the process of formation of carrier-ion complex, its
transport, breakdown of complex, regeneration of carrier, and movement of carrier molecules back.
ii. Protein-lecithin as Carrier (Bennet-Clark,1956)
(for the uptake of both cation & anion)
It is suggested that because the cell membranes chiefly consist of phospholipids and proteins and
also certain enzymes seem to be located on them, the carrier could be a protein associated with the phosphatide
called lecithin. This theory believes in the participation of some amphoteric compounds as carriers
with which both cations and anions can combine.
According to this theory,
1. The acidic phosphate group in the phosphatide is regarded as the active center
binding the cation, and the basic choline group (N+) as the anion binding center.
2. The ions are liberated on the inner surface of the membrane by decomposition of the lecithin
by the enzyme lecithinase.
3. The regeneration of the carrier lecithin from phosphatidic acid and choline takes place in the
presence of the enzymes choline acetylase and choline esterase and ATP. The ATP acts as a
source of energy.
iii. Cytochrome-pump Theory
(For the movement of anions only)
Lundegardh and Burstrom (1933) claimed that a quantitative relationship exists between anion
absorption and respiration. When a plant is transferred from water to salt solution, the rate of respiration
increases. They called this increase in respiration Salt Respiration. The actual transport of
anions occurs through a cytochrome system (Fig.14)
1. Dehydrogenase reactions on the inner side of the membrane give rise to protons (H+) and
electrons(e_).
2. The electron travels over the cytochrome chain towards outside the membrane, so that the Fe
of the cytochrome becomes reduced (Fe++). on the outer surface and oxidized (Fe+++) on the
inner surface.
3. On the outer surface, the reduced cytochrome is oxidized by oxygen releasing the electron (e-)
and taking an anion (A-)
4. The electron thus released unites with H+ and oxygen to form water.
5. The anion (A+) travels over the cytochrome chain towards the inside.
6. On the inner surface, the oxidized cytochrome becomes reduced by taking an electron
produced through the dehydrogenase reactions and the anion (A+) is released.
7. As a result of anion absorption, a cation (M+) moves passively from outside to inside to balance
the anion.
iv. ATP Theories
According to this theory, ion uptake into the cell is energized by ATP molecules. The energy
from hydrolysis of ATP molecules can be made available to energize ion pumps through the action of
enzymes.
Case I
Here, the organic compound is first phosphorylated which on dephosphorylation makes the organic
compound capable to combine with cation. The cation is released when phosphorylation occurs
again.
Case II
In this case, the phosphorylated organic compound combines with cation and the cations are
released on hydrolysis of the complex (dephosphorylation). Thus, the role of ATP in this theory is of two
kinds. i.e., by removal or addition of phosphate group.
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