Active Absorption

Active Absorption

Movement of ions from the outer space of the cell to the inner space is generally against the concentration gradient and hence requires energy. This energy is obtained through metabolism either directly or indirectly. Various evidences indicate the active uptake of ions by carrier mechanism.

Sub Topics

• Carrier mechanism

• Ion traffic into the root

• Translocation of solutes

• Goldacre's Theory

• Cytochrome Pump Salt Respiration or Electron Transport Theory

Carrier mechanism

In carrier mechanism, activated ions combine with carrier proteins and from ion carrier complex. This complex moves across the membrane and reaches the inner space by the expenditure of energy.



Within the cytoplasm, the complex breaks to release the ions. The carrier moves out of the cytoplasm and is again ready to attach another ion to from a complex.

Ion traffic into the root

Mineral nutrients absorbed from the root has to be carried to the xylem. This transport follows two pathways namely apoplastic pathway and symplastic pathway.

In apoplastic pathway, mineral nutrients along with water moves from cell to cell through spaces between cell wall by diffusion. The ions, which enter the cell wall of the epidermis move across cell wall of cortex, cytoplasm of endodermis, cell walls of pericycle and finally reach the xylem.

In symplastic pathway, mineral nutrients entering the cytoplasm of the epidermis move across the cytoplasm of the cortex, endodermis of pericycle through plasmodesmata and finally reach the xylem.



Translocation of solutes

P.R. Stout and Dr. Hoagland have proved that mineral nutrients absorbed by the roots are translocated through the xylem vessel. Mineral salts dissolved in water moves up along the xylem vessel to be transported to all the parts of the plant body. Translocation is aided, by transpiration. As water is continuously lost by transpiration on the upper surfaces of the plant, it creates a transpirational pull, by which water along with mineral salts is pulled up along the xylem vessel.

Active absorption of energy can be achieved only by an input of energy. Following evidences show the involvement of metabolic energy in the absorption of mineral salts.

• Higher rate of respiration increases the salt accumulation inside the cell.

• Respiratory inhibitors check the process of salt uptake.

• By decreasing oxygen content in the medium, the salt absorption is also decreased.

These evidences indicate that salt absorption is directly connected with respiratory rate and energy level in the plant body, as active absorption requires utilization of energy.

Goldacre's Theory

Contractile proteins of membrane show their existence in folded or unfolded condition. Proteins in unfolded conditions are able to bind ions by free valencies exposed at membrane surface. Proteins in folded (contracted) condition release ions as free valencies of proteins get satisfied in folded condition. In this theory role of carrier has been emphasised with utilisation of ATP energy. This theory however has not been proved.



Diagrammatic Representation of the Goldacre Concept

Cytochrome Pump Salt Respiration or Electron Transport Theory

This theory was proposed by H. Lundegardh, who suggested that anions could be transported across the membrane by cytochrome system. Energy is supplied by direct oxidation of respiratory intermediates.



Diagrammatic representation of cytochrome pump hypothesis On salt absorption, anions (A^-) are actively absorbed via a cytochrome pump and cations (M⁺) are passively absorbed.

The rate of respiration, which is solely due to anion absorption, is called as anion respiration or salt respiration. The original rate of respiration (without anion respiration) can be observed in distilled water and is called ground respiration.

Total respiration (R₁) = Ground respiration (R_g) + Salt or anion respiration (R_a).