Top 5 Theories of Ion Uptake by Plants

This article throws light upon the top five theories of ion uptake by plants. The theories are: 1. Lundegardh Theory 2. Contact Exchange Theory 3. Carbonic Acid Exchange Theory 4. Bennet-Clark’s Protein Lecithin Theory 5. Donnans’ Equilibrium Theory.

1. Lundegardh Theory (Electro-Chemical Theory) of Ion Uptake:

Lundegardh (1945) proposed that an electrical potential gradient existing between the cell membrane of plants and outside solution is responsible for the passage of electrons from inside solution (cytoplasm) to the outside membrane (apoplasm). Such flow or passage of electrons causes movement of anions from outside to inside.

Oxygen deficit in cytoplasm of plant cell induced by respiration is responsible for a drop in redox potential of the cytoplasm (Fig. 20.11). It is believed that cytochromes (Fe-containing substances) present on the outer membrane, where Fe is present in Fe³⁺ form because of higher redox potential when these cytochromes move through thermal motion across the membrane.

Fe³⁺ is reduced to Fe²⁺ by acquisition of electrons from the respiratory chain.

Fe³⁺ + e → Fe²⁺

In the outside membrane, Fe of cytochrome is oxidised by losing an electron, which combines with H⁺ to form water. Outward flow of the electron is balanced by the counter movement of anions into the cell.

Lundegardh deduced that since cytoplasm of the cell is negatively charged compared to the outside, energy must be required in pumping anions into cytoplasm and cations move along with anions for maintaining balance between anions and cations. However, the hypothetical scheme of the Lundagardh theory is shown in the Fig. 20.11.

Limitations of :

(i) Cytochromes are not located in cell wall of plants, and

(ii) Ideally, four anion molecules should move in response to consumption of one molecule on O2, but actually, this number is much higher.

Carrier Hypothesis of Ion Uptake:

In 1953, Epstein proposed the carrier hypothesis to describe primarily the active accumulations of ions across the cell membrane. According to this hypothesis, there is a carrier present on the membrane which combined with the ion at the outer surface of the plasma lemma, the carrier-ion complex traversed the membrane which is relatively impermeable to the ion alone.

On the inner side of the membrane due to some chemical changes, the membrane-ion complex breaks, releasing the ion which cannot leak because the membrane is impermeable to free ion. On releasing the ion, the carrier moves back to the outer surface of the membrane where it can again combine with an ion.

Usually, the carrier can take up ions only in the phosphorylated form, and on the inside of the membrane, both phosphate and ions are released. Metabolic energy is required for the accumulation of ions in the carrier hypothesis.

The carrier hypothesis can explain:

(a) An accumulation of salts against the concentration gradient,

(b) The specificity in ion uptake usually found in all plants, and

(c) The transport of ions through membranes which by themselves are impermeable.

Chemically, this cycle has been proposed by Mengel and Kirkby (1982) as follows:

However, schematically the carrier ion transport is depicted as follows:

2. Contact Exchange Theory of Ion Uptake:

According to this theory the ions adsorbed or the surface of roots cells and clay particles are not held tightly but oscillate within small volume of space.

If the roots and clay particles are in close contact with each other, the oscillation volume of ions absorbed on root surface may over by the oscillation volume of ions adsorbed on clay particles, and the ions adsorbed on clay particle may be exchanged with the ions adsorbed on root surface directly without first being dissolved in soil solution.

3. Carbonic Acid Exchange Theory of Ion Uptake:

According to this theory, the CO₂ released during respiration of root cells combines with water to form carbonic acid (H₂CO₃). Carbonic acid dissociates into H⁺ and an anion HCO₃ in soil solution. These H⁺ ions may be exchanged for cations adsorbed on the clay particles.

The cations thus released into the soil solution from the clay particles, may be adsorbed on root cells in exchange for H⁺ ions or as in ion pairs with bicarbonate. Thus, the soil solution plays an important role in carbonic acid exchange theory.

4. Bennet-Clark’s Protein Lecithin Theory of Ion Uptake:

In 1856, Bennet-Clark suggested that because the cell membranes of plants 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. He also assumed the presence of different phosphatides to correspond with the number of known competitive groups of cations and anions.

According to this theory:

1. Phosphate group in the phosphatide is regarded as the active centre binding the cations and the basic choline group as the anion binding centre.

2. The ions are liberated on the inner surface of the membrane by decomposition of lecithin by the enzyme lecithinase.

3. The regeneration of the carrier lecithin form phosphatidic acid and choline takes place in the presence of the enzyme choline acetylase and choline esterase and ATP. The latter acts as a source of energy.

5. Donnans’ Equilibrium Theory of Ion Uptake:

The accumulation of ions inside the cells without involving expenditure of the metabolic energy can be explained to some extent by Donnan’s equilibrium theory.

According to this theory there are certain pre-existing ions inside the cell which cannot 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 solutions.

Suppose there are certain fixed anions in the cell which is in contact with outer solution containing anions and cations. 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 more cations will diffuse into the plant cell.

This equilibrium is known as Donnan’s equilibrium. In this particular case, there would be an accumulation of cations inside the cell. If however, there are fixed cations inside the plant cell, the Donnan’s equilibrium will result in the accumulation of anions inside the plant cell.