Question

Stomatal opening and closing involves the role of various ions. In the given figure, arrows depict the movement of certain ions during stomatal opening in light. Identify the ions (P, Q, R and S) and select the correct option.

	P	Q	R	S
(a)	Malate$^{2-}$	$K^+$	$Cl^-$	$H^+$
(b)	$K^+\,\,$	$H^+\,\,$	$Cl^-$	Malate$^{2-}$
(c)	$H^+\,\,$	$K^+\,\,$	$Cl^-\,\,$	Malate$^{2-}$
(d)	$K^+\,\,$	Malate$^{2-}$	$H^+\,\,$	$Cl^-$

• A. a
• B. b
• C. c
• D. d

Answer 1

Answer: (C)

In light, starch is converted into phosphoenol pyruvic acid $(PEP)$ in the presence of $PEP$case enzyme. $PEP$ combines with $CO_2$ to form $OAA$ and then malic acid. Malic acid dissociates into malate ions (malate$^{2-}$) and $H^+$ ions in the guard cells. $H^+$ ions are transported to epidermal cells and $K^+$ ions are taken into the guard cells in exchange of $H^+$ (ion exchange). Some $Cl^-$ ions are also taken in to neutralize some of the $K^+$ ions. $H^+ - K^+$ exchange is an active process which involves involvement of $ATP$. Increased $K^+$ and malate ion concentration in the vacuole of guard cells causes sufficient osmotic pressure to absorb water from surrounding cells, resulting in increased turgor of guard cells and thus opening of stomata.