movement is based on entry and exit of water molecules in guard cells. Many theories are there to explain how water enters and exits from guard cells. The theory of potassium transport enumerates two different reactions separately run for opening and closing of stomata. Contrary to ascent of sap by xylem in an upward direction, the path of solute which consists of the photosynthetic products is always in phloem and translocate multidirectional.
The point of origin of translocation is photosynthetic leaves which oxidase and are transferred to other members of chain as they transfer the electron to the next component (Figure . ). Figure . : Cytochrome Pump theory H - H ½ H O ¼ O Fe + Fe + Fe + Fe + Fe + Fe + e - e - A - A - A - A - A - A - A - A - A - A - A - Dehydrogenase Reactions c + c + OUTSIDE INSIDE The theory assumes that cations (C + ) move passively along the electrical gradient created by the accumulation of anions (A – ) at the inner surface of the membrane.
Main defects of the above theory are: (i) Cations also induce respiration. (ii) Fails to explain the selective uptake of ions. (iii) It explains absorption of anions only. .
Bennet-Clark’s Protein-Lecithin Theory: In , Bennet-Clark proposed that the carrier could be a protein associated with phosphatide called as lecithin . The carrier is amphoteric (the ability to act either as an acid or a base) and hence both cations and anions combine with it to form Lecithin- ion complex in the membrane. Inside the membrane, Lecithin-ion complex is broken down into phosphatidic acid and choline along with the liberation of ions. Lecithin again gets regenerated from phosphatidic acid and choline in the presence of the enzyme choline acetylase and choline esterase (Figure .
). ATP is required for regeneration of lecithin. Lecithin Lecithinase Phosphatidic Acid Choline Choline Esterase Acetyl Choline ATP Choline Acetylase OUTSIDE INSIDE C + C + C + A _ A _ A _ Figure