The carrier concept theory is an accepted theory that the active absorption of mineral salts.
Carrier Concept Theory
According to this carrier concept theory, there are some carrier components present in the plasma membrane of a cell that are involved in the process. This type of movement of the ions by the carrier depends on metabolic energy and is called active transport.
This mechanism for ion transport employs a concept of carriers. The carriers are considered to be functional in membranes combining with ions at the external interphase, moving as a carrier ion complex to the inside surface, with subsequent internal release of ions.
The loss of these ions is prevented by the impermeable nature of the membrane. This theory assumes that with an increase in concentration, the material will become saturated and create a maximum uptake rate.
Beyond a certain limit, an increased concentration of salts in the outer solution does not bring about an increase in the rate of mineral salt absorption. This is because the active sites on the carrier compound had become saturated with the ions.
Active sites on carrier compounds may be specific which can bind only some specific ions. This explains the selective and unequal absorption of ions by plants.
There are two common hypotheses – Lundergardh’s Cytochrome pump theory and Bennet-Clarke’s Protein Pump Theory, based on the carrier concept to explain the mechanism of salt absorption although they are not universally accepted.
Lundegardh’s Cytochrome Pump Theory
Lundegårdh and Burstorm (1933) proposed that there is a connection between respiration and ion absorption. So when a plant is transferred from water to salt solution, the rate of respiration increases. This increase in the rate of respiration has been called anion respiration or salt respiration.
Later, Lundegardh proposed the theory based on these assumptions.
- The mechanism of anion and cation exchange is different.
- Anions were absorbed through the cytochrome chain by an active process.
- Oxygen concentration gradients exist from the outer surface to the inner surface of the membrane favoring the inner surface of the membrane.
- Actual transport of anions occurs through a cytochrome system by active process.
- Cations are absorbed passively.
According to Lundegårdh, protons H+ and electrons are produced on the inner surface as a result of the dehydrogenase reaction. Through a cytochrome chain, the electrons move outwards to be released to unite with protons and oxygen to form water and in the process, the reduced ion of cytochrome on the outer surface gets oxidized by losing an electron and picking up an anion.
The oxidized ion of cytochrome on the inner surface gets reduced by taking an electron released by the dehydrogenase reaction. The anions picked up by the oxidised iron of cytochrome on the outer surface are released on the inner side in the last step. To balance the potential difference due to the accumulation of anions on the inner surface, cations are absorbed passively.
The main defects of the above theory are,
- It envisages the active absorption of only anions.
- Does not explain selective uptake of ions.
- It has been found that cations also stimulate respiration.
Bennet- Clarke’s Protein-Lecithin Theory
According to this theory, some amphoteric compounds serve as carriers. It applies to both cation and anion uptake. Clarke proposed that a carrier can be a protein associated with phosphate, ie, lecithin.
In this process, ions from the outer side are picked up by lecithin to produce a lecithin-ion complex which moves inside and releases ions on hydrolysis of the complex in a cyclic manner.
Donnan’s Equilibrium
According to this theory, there are certain free existing ions inside the cell which cannot be diffused outside through the membrane. Such ions are called in-diffusible ions or fixed ions. But the membrane is permeable to both anions and cations present outside.
Suppose there are certain fixed anions in the cell which are in contact with the outer solution containing anions and cations, normally equal in number, will be diffused into the cell to balance each other.
But to balance the fixed anions, more cations will diffuse into the cell to have an accumulation of cations in the cell. This equilibrium is called Donnan’s Equilibrium. However, if there are fixed cations in the cell, Donnan’s Equilibrium will result in the accumulation of anions inside.
Donnan showed that the product of the concentration of any pair of oppositely charged diffusible anions and cations on one side of a membrane is equal to the product of the concentration of the same ion on the other side of the membrane.
Suppose there is on one side of a membrane, the unit volume of NaCl solution with both ions the membrane is permeable to and on the other side the same volume of solution of another sodium salt to the anion of which the membrane is impermeable.
It is clear that at equilibrium concentration of diffusing salt will be less in the solution on the side of the membrane containing diffusible ions than on the side containing diffusible salts only. Thus Donnan’s equilibrium is very important in plant physiology.
References
Bhatla, C. Satish and Manju A. Lal, Plant Physiology, Development and Metabolism, Springer Nature Singapore Pte Ltd. 2018. https://doi.org/10.1007/978-981-13-2023-1
