What is Active Mineral Salt Absorption
Direct analysis of vacuolar sap of plant shows that both cations and anions accumulate in plant cell in a large quantity against the concentration gradient. The extent of accumulation is such that it cannot be explained by ion exchange or Donnan equilibrium theory.
Hober (1945) observed that freshwater algae Nitella accumulates K+ ion to a concentration 1000 times greater than the concentration of K+ ions in surrounding medium. In addition to this, it is also observed that:
- In xylem, the accumulation of ions is much higher even when the rate of transpiration is low.
- Concentration of ions in the xylem sap falls when a rapidly transpiring plant is allowed to grow in a concentrated salt solution.
The above observation indicates the presence of an ion barrier in the cell which is less permeable to ion than water. The ions are not allowed to cross the barrier as rapidly as they are carried to the shoot by the transpiration strain. However, in order to cross the barrier, metabolic energy is required.
Evidence of Metabolic Activities
Following are the evidences in support of the involvement of metabolic activities in absorption of salt.
- Steward (1932) and Hopkins (1956) observed that increased rate of transpiration causes an increase in salt absorption. By decreasing oxygen content in the medium, the salt absorption also decreases.
- Robertson (1945) and Lundegardh (1952-1954) observed that respiratory inhibitor strongly inhibit salt absorption.
- Lundegardh observed that absorption of salt result in an increase in the rate of respiration.
The above-mentioned evidences are sufficient to believe that metabolic energy is required for the uptake of salt. The absorption of ion at the expense of metabolic energy is termed as active absorption.
Mechanism of Active Salt Absorption
According to this theory, the plasma membrane is impermeable to free ions but some compound 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. The concept got support from the following three observations:
Isotopic exchange: It has been found that actively absorbed radioactive ions cannot diffuse back or be exchanged with other ions in the outer solution indicating impermeability of plasma membrane to free ions.
Saturation effect: It has been found that continuous increase in the concentration of salt in the external medium does not increase the rate of absorption beyond a certain limit. This is because the active sites on the carrier compound become saturated with ions.
Specificity: Carriers are specific in nature and they absorb specific ions. They resemble enzymes in this property. This explains selective ion absorption by roots. The whole volume of tissue can be divided into three parts:
- Outer free space: It includes the space which allows free diffusion of ions.
- Inner space: The space in which ions penetrate with the aid of metabolic energy.
- Intermediate space between outer free space and inner space: This is not clearly defined, however, it is thought that it is present somewhere in the middle of cytoplasm. This space is impermeable to free ions.
The carrier molecules combine with ions in outer free space and form carrier-ion-complex. This moves into inner space where it releases ions. These ions once taken into inner space cannot return back to outer space. The carrier compound returns back to outer space to pick up free ions.
Two possible mechanism of salt absorption based on carrier concept are as follows:
Cytochrome Pump Theory
This theory was given by Lundegardh (1954). According to this theory, dehydrogenase reactions on inner side of the membrane gives rise to protons and electrons. The electron travels over the cytochrome chain towards outside the membrane. Cytochrome becomes reduced on the outer surface and oxidised on the inner surface.
Then on the outer surface, the reduced cytochrome is oxidised by oxygen releasing the electron (e-) and taking an anion (A-). The electron thus released unites with H+ and oxygen to form water. The anion (A-) travels over the cytochrome chain towards inside.
On the inner surface, the oxidised cytochrome becomes reduced by taking an electron produced through the dehydrogenase reaction and the anion (A-) is released. As a result of anion absorption, a cation (M+) moves passively from outside to inside to balance the anion.
Criticism of Cytochrome Pump Theory
- It does not explain the involvement of metabolic energy in salt absorption.
- It does not explain selective uptake of ions.
- It has been found that cations also stimulates respiration.
Protein – Lecithin Theory
This theory was proposed by Bennet Clark (1956). According to this theory, a phospholipid compound called lecithin is involved in the transport of ion, as carrier.
The compound lecithin is composed of phosphatidic acid and choline. The phosphate group in phosphatide acts an active center for binding the cation and choline.
The complete lecithin molecules when combined with anion and cation move inside and split into phosphatidic acid and choline in the presence of enzyme, lecithinase on the inner space. Cations and anions are now released to inner space from phosphatidic acid and choline respectively.
After releasing anions and cations, phosphatidic acid and choline react to form acetyl choline in the presence of enzyme choline acetylase and ATP. Acetyl choline in the presence of enzyme choline esterase is converted into lecithin. The lecithin then moves towards the outer space.