Mechanism of Mineral uptake by Plants

Mechanism of Mineral uptake

    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.
    • Plasma membrane of the root cells is not permeable to all the ions as it is selectively permeable.
    • All the ions of the same salt are not absorbed at equal rate but there is unequal absorption of ions.
    • First step in the absorption of mineral salts is by Ion Exchange.

    Once the nutrients come and adsorb on the surface of the walls or the membranes of root cells, then the further process of the absorption of mineral salts may be of two types. They are:

    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 simple process of diffusion. This is called as passive absorption because it does not require expenditure of metabolic energy. This can also be called as 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 mass flow
    • of water under the influence of transpiration. Therefore, 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.

    Ion Exchange Theory
    Negatively charged Cl- and Br- are exchanged without disturbing the electrical neutrality
    III. Donnan Equilibrium
    • This theory explains the accumulation of ions inside the cells without involving the expenditure of the metabolic energy.
    • According to this theory, there are certain preexisting ions inside the cell, which cannot diffuse outside through membrane.
    • However, the membrane is permeable to both anions and cations of the outer solution Normally, 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, and 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 (preexisting) inside the cell, then the Donnan’s equilibrium will result in more accumulation of anions inside the cell to maintain the equilibrium.

    Explanation of Donnan’s Equilibrium
    2. Active Absorption of Mineral Salts
    This processes 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 with carrier concept such as 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 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. Here, the metabolic energy is required in the process of formation of carrier-ion complex, its transport, and breakdown of complex, regeneration of carrier and movement of carrier molecules back.

    Carrier Concept
    II. Protein-lecithin as Carrier (Bennet-Clark, 1956)
    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 phosphotide called as 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 centre binding the cation, and the basic choline group (N+) as the anion binding centre.
    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 relaionship exists between anion absorption and respiration. When a plant is transferred from water to salt solution, the rate of respiration will increases. They called this increase in respiration as Salt Respiration. The actual transport of anions occurs through a cytochrome system.
    1. Dehydrogenase reactions on 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 oxidised 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 inside.
    6. On the inner surface, the oxidised 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+) move 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 energies 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.



Last modified: Tuesday, 26 June 2012, 4:04 AM