In the active absorption minerals ions moves against the concentration gradient with the expenditure of energy is known as active absorption. The main theories put forward to explain the process of mineral salt absorption are the carrier concept, cytochrome pump hypothesis etc.
i. Carrier concept:
This theory was proposed by the Honert in 1937. According to this theory there are certain organic molecules which act as carriers or transporters. These carrier molecules are highly specific for each ions, it carries ions from the outer space to inner space by forming the ion carrier complexes. Theory also supports that when the concentration of the ions is increased in the outer side it will result in the increase in the rate of ion uptake. The rate of uptake of ions remains constant even if the concentration of the mineral ions is increased, because all the carrier sites will be employed.
ii. Cytochrome pump hypothesis:
Cytochrome pump theory was proposed by H. Lundegardh in 1954. Theory supports that there is direct co relation between the anion absorption and respiration. The transport of anions takes place through cytochrome system, so the cytochrome acts as carrier. Oxygen gradient oxidation in the outer surface and reduction in the inner surface.
iii. Protein-Lecithin theory:
Bennet Clark 1956 proposed the theory in which the carrier is a phosphatide protein. The Lecithin carries both the anions and cations by forming the lecithin-ion complex. This is mediated by the enzyme lecithinase.