Active Mineral Absorption
Active Absorption (Active Uptake):-
Ø It has often been observed that the cell sap in plants accumulates large quantities of mineral salts ions
against the concentration gradient. It is an active process which involves the expenditure of metabolic energy through respiration.
Ø Following evidences favor this view:-
i. The factors like low temp., deficiency of O2, metabolic inhibitors etc. which inhibit metabolic activities like respiration in plants also inhibit accumulation of ions.
ii. Rate of respiration is increased when a plant is transferred from water to salt solution. It is called salt respiration.
a. The Carrier Concept:- According to this theory the plasma membrane is impermeable to free ions. But some compounds present in it acts as carrier and 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 up fresh ions.
b. Cytochrome Pump Theory:-
Ø Lundegardh and Burstrom (1933) believed that there was a definite correlation between respiration and anion absorption. Thus when a plant is transferred from water to a salt solution the rate of respiration increases. This increase in rate of respiration over the normal respiration has been called as anion respiration or salt respiration.
Ø The inhibition of salt respiration and the accompanying absorption of anions by CO and cyanides (which are known inhibitors of cytochrome oxidase of electron transport chain in mitochondria), later on led Lundegardh (1950, 54) to propose cytochrome pump theory. This is based on the following assumptions:-
i. The mechanism of anion and cation absorption is different.
ii. Anions are absorbed through cytochrome chain by an active process.
iii. Cations are absorbed passively.
Ø According to this theory:-
i. ehydrogenase reactions on inner side of the membrane give rise to protons (H+) and electrons (e–).
ii. 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.
iii. On the outer surface, the reduced cytochrome is oxidized by oxygen releasing the electron (e–) and taking an anion (A–).
iv. The electron thus released unites with H+ and oxygen to form water.
v. The anion (A–) travels over the cytochrome chain towards inside.
vi. On the inner surface the oxidized cytochrome becomes reduced by taking an electron produced through the dehydrogenase reactions, and the anion (A–) is released.
vii. As a result of anion absorption, a cation (M+) moves passively from outside to inside to balance the anion.
Ø Main defects of the above theory are:-
i. It envisages active absorption of only anions.
ii. It does not explain selective uptake of ions.
iii. It has been found that cations also stimulate respiration.
c. Protein-Lecithin Theory:-
Ø In 1956, Bennet-Clark suggested that because the cell membranes chiefly consist of phospholipids and proteins and certain enzymes seem to be located on them, the carrier could be a protein associated with the phosphatide called as lecithin.
Ø He also assumed the presence of different phosphatides to correspond with the number of known competitive groups of cations and anions (which will be taken inside the cell).
Ø According to this theory:-
i. The phosphate group in the phosphatide is regarded as the active centre binding the cations, and the basic choline group as the anion binding center.
ii. The ions are liberated on the inner surface of the membrane by decomposition of the lecithin by the enzyme lecithinase.
iii. 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.
Pathways of Mineral Ions Movement:-
Once mineral salts reaches inside the epidermal cells of the root, their ionic form move from one cell to another by:-
i) Apoplastic pathway (i.e., through cell walls and intercellular spaces)
ii) Trans membrane pathway (i.e., by crossing the membranes)
iii) Symplastic pathway (i.e., through plasmodesmata)
Ultimately mineral salts reach to xylem vessels and tracheids, from where they are carried to different parts of the shoot along with ascent of sap.