This theory assumes that the membrane is impermeable to ions alone. But there are certain compounds called ‘Cytochromes’ which act as carriers. The ions combine with the carriers to form an “ion carrier complex”. This ion carrier complex can pass through the membrane. Within the cell, the ion carrier complex breaks up releasing the free ions.

The carrier now is free to receive one more ion Metabolic energy is supposed to be involved in the activation of the carrier There are two hypotheses based on the carrier con­cept. These are Lundegardh’s cytochrome pump theory and Bennet Clark’s protein lecithin theory Cytochrome pump hypothesis viauslises the exist­ence of a direct relationship between anion absorption and salt respiration. (The increase in the rate of respiration of a plant organ when transferred to a salt solution is called salt respiration). Lundegardh postulated that anions are actively absorbed via a cytochrome pump (series of cytochromes); here cytochromes act as carriers.

Bennet Clark’s protein lecithin theory postulates that the carrier could be a Protein combined with a phospholipid called lecithin instead of the cytochrome as visualised by Lundegardh. Their view is based on the preva­lence of proteins and lipids in the membrane.