Question

Explain Steward’s starch glucose interconversion theory.

Answer 1

(1) Starch hydrolysis theory:

(Starch – glucose inter conversion theory). This theory was proposed by steward (1964). It is modified theory of starch – sugar interconversion theory.

Steward's scheme for opeaning and closing of stomata

• According to this theory following changes bring the opening of stomata.
• During day time, guard cells consume (use) CO₂ for photosynthesis. So CO₂ concentration decreases in the guard cells. pH of guard cells rises to almost 7.
• Under low CO₂ concentration and at pH7, starch is hydrolysed into glucose -1 – P ( glucose -1 -phosphate) in the presence of phosphorylase enzyme.
• Glucose -1 – P is convented into glucose -6 – phosphate in the presence of phospho glucomutase.
• Glucose -6 – P is finally converted into glucose is the presence of phosphatase.
• When starch is hydrolysed, glucose formed in the guard cells. Since glucose is osmotically active, Osmotic Concentration & Osmotic Pressure of guard cells increases &yw decreases.
• Water is drawn into the guard cells from the surrounding subsidiary cells.
• Guard cells become turgid due to endosmosis. So turgor pressure is developed in the guard cells. The TP exert a pressure on the outer thin elastic wall of guard cell, resulting in stretching of outer wall. The inner thick wall also stretched outward. It results in the opening of stomata.

closing of stomata

Following changes bring closing of stomata. 

• During night, in the absence of photosynthesis, CO₂ accumulates in guard cells. Its concentration increases in guard cells. pH of guard cells decreases and it becomes 5(acidic). 
• Under high CO₂ concentration and at pH5, glucose is convented into glucose -1 – phosphate in the presence of hexokinase. 
• Glucose -1 – phosphate is further converted into starch in the presence of phosphorylase.
• Starch formed in guard cells, which is insoluble and osmotically enactive. So OC and OP of guard cells decreases, ψ ω of cell sap of guard cells is increased.
• Water from the guard cells diffuse out into the subsidiary cells due to exosmosis.
• Guard cells become flaccid, outward stretched walls regain their original position and stomata become closed.