Question

Identify the theory of passive absorption of mineral salts.

• A. Carrier concept
• B. Cytochrome pump theory
• C. Carbonic acid exchange theory
• D. None of the above

Answer 1

Answer: (C)

Carbonic acid exchange theory

The roots respire continuously giving out carbon dioxide into the rhizosphere. Carbon dioxide combines with water and forms carbonic acid. Carbonic acid dissociates into H${}^{+}$ and HCO${}_3$${}^{-}$. Because of high respiratory activity, a zone of carbonic acid is developed in root tips. The H${}^{+}$ ions are exchanged with cations adsorbed on the clay micelle. The cations then come into the soil and diffuse onto the root surface. This is the carbonic acid exchange theory. Thus the correct answer is option C.