Question

According to proton transport theory of stomatal mechanism, protons are formed in the guard cells when ________ .

• A. starch is converted into cellulose during daytime.
• B. sugar is converted into starch during night time.
• C. starch is converted into malic acid during day time which dissociates into malate.
• D. potassium malate is formed during night time.

Hint:

Stomatal opening: Starch → Malic acid → Malate + H$^+$ → K$^+$ entry → Guard cells swell.

Answer 1

The Correct Option is C

Concept:
According to the proton (H$^+$) transport theory, stomatal opening involves active ion transport and metabolic changes in guard cells.

Step 1: Understand daytime process. During daytime:
• Starch in guard cells is converted into phosphoenolpyruvate (PEP)
• PEP is converted into malic acid

Step 2: Formation of protons. Malic acid dissociates as: \[ \text{Malic acid} \rightarrow \text{Malate}^{-} + H^{+} \] Thus, protons ($H^+$ ions) are released in guard cells.

Step 3: Analyze options.
• (A) Cellulose formation → Not related $\times$
• (B) Sugar to starch → Night process $\times$
• (C) Starch to malic acid → Proton formation \checkmark
• (D) Potassium malate at night → Incorrect $\times$

Step 4: Conclusion. \[ \text{Protons are formed when malic acid dissociates in guard cells} \]