Mineral salts in soil comprises the most important ingredient of mutation for plants. In addition to diffusion or passive movement of neutral molecules and ions across membranes, during transport, there is another active mode of transport which requires metabolic energy from the cell in form of ATP.
It has been observed that there is accumulation of large amount of mineral salts against the concentration gradient. This never is explained by simple diffusion. Transport of different molecules across the plasma membrane shows a high degree of specificity, that is the permeability of a molecule is related to its chemical structure.
It has been accepted that active absorption of mineral salts involves the operation of a specific carrier compound present in the plasm membrane of the cells of plant body across which salt transport takes place. These carrier compounds are the transport proteins or called as permeases. These carrier molecules behave same way with the minerals as the enzymes with the substrate molecules.
Carrier molecules present in the membrane forms bond with the minerals present outside in form of ions, resulting 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 pickup fresh ions.
The carrier concept of active transport of minerals is evidenced by following observed facts.
i. Isotopic exchange:
Use of radioactive ions like S35 O4 in the outer solution of a cell if was detected that this radioisotopes are ondce absorbed actively through the membrane can not diffused back, indicating that plasma membrane is not permeable to fee ions.
ii. Saturation effect:
An increased concentration of salt ions in outer solution does not bring about increase in the salt absorption. It is due to the fact that active sites of the permases become saturated in side the membrane.
Plasma membranes of the cells are not permeable to all type of ions, due to the fact that carriers are specific in nature having specific active sites to form the compels. Therefore absorption of salts is specific and selective.
Theories of Explain Carrier Mechanism:
There are two hypothesis has been popularly accepted on the explanation for carrier mechanism of salt transport.
Cytochromes Pump Theory:
This theory was proposed by lundegardh and burstrom (1933) believed that there is a definite correlation between reparation and anion absorption. Thus when a plant is transferred from water to a salt solution the rate of respiration increases.
This theory is based on the assumptions that:
i. Mechanism of anion and cation absorption is different.
ii. Anions are absorbed through cytochrome chain by an active process.
iii. Dehydrogenase reactions on inner side of membrane give rise to H* ions and (e) ions.
iv. The election travels over the cytochrome chain towards outside the membrane so that the “Fe” of the cytochrome become reduced to Fe++ on the outer surface and: Fe” is oxdised to (Fe++) on the inner surface.
v. On the outer surface, the reduced cytochrome is oxidised by oxygen releasing the electron and taking an anion.
vi. The electron thus released unites with H+ and oxygen to form water.
vii. The anion (A-) travels over the cytochrome chain towards inside.
viii. At the inner surface the oxdised cytochrome become reduced by taking an electron produced through the dehydrogenase reactions and the onion (A-) released.
ix. As a result, a cation (m+) moves passively from outside to inside to balance the onion.
Demerits of Lundegardh’s theory:
i. It does not explain selective absorption of ions.
ii. It principally concerned with active absorption of only anions, but cations passively absorbed and transported.
Protein – Lecithin Theory:
This theory was suggested by bennet Clarke (1956). According to his theory –
i. Carrier molecule composed of protein with the phospholipids called as lecithin.
ii. There are different phospholipid groups present in membrane correspond with the number of competitive group of cations and anions.
iii. Phosphate group in the phosphatide is regarded as active centers of carrier.
iv. The ions are liberated on the inner surface of the membrane by decomposition of the lecithin by the enzyme lecithinase.
v. The regeneration of the carrier lecithin from phosphatide acid and choline takes place in presence of enzymes viz. choline acetylase and choline esterase and ATP.
vi. ATP acts as the energy source for active transport.
Donnan’s Equlibrium Theory:
According to this theory there are some pre-existing ions in side the cell which can not diffuse outside through membrane such ions are called as in diffusible or fixed ions. However, the membrane is permeable to both anions and cations of the outer solution.
Normally equal number of anions and cations would have diffused in to the cell through an electrical potential to balance each other but to balance the fixed onions mere cations will diffuse into the cell.
If there are fixed numbers of cations inside the cell, the equilibrium will result in an accumulation of anions inside the cell.
Donnan’s equilibrium theory explains that there is no need of matabolic energy for transport of ions across the membrane.