Question

According to ${ H }^{ + }$ −${ K }^{ + }$ exchange pump mechanism of stomatal movement
I. ${ K }^{ + }$ ions accumulate in guard cells during the day.
II. ${ H }^{ + }$ ions are pumped out from guard cells and ${ K }^{ + }$ ions are pumped into guard cells.
III. Water potential of guard cells decreases with the accumulation of ${ K }^{ + }$ ions together with ${ Cl }^{ - }$ and organic acid ions.
IV. Water enters into guard cells from epidermal and mesophyll cells due to decreased water potential of guard cells.
(A) I and II are correct
(B) I, II, and IV are correct
(C) II, III, and IV are correct
(D) All are correct

Answer 1

They are the tiny pores i.e stomata present in the chlorenchymatous plant tissues that are found on the surface of leaves for gaseous exchange in the plants and function in the presence of air and water and opening and closing of stomata depends on the turgidity of the guard cells.

Complete answer:
${ H }^{ + }$ −${ K }^{ + }$ exchange pump mechanism was known to be proposed by Levitt. According to this mechanism, during the day, starch is converted to phosphoenol acid which mixes with CO and forms oxaloacetic acid (OAA). OAA is then converted to malate. ${ H }^{ + }$ ions are pumped out from the respectively guard cells and ${ K }^{ + }$
ions are pumped in and accumulate inside guard cells along with the side malate. This is often called a natural process. Water potential of guard cells decreases with the buildup of ${ K }^{ + }$
ions which are known to be balanced by taking in some ${ Cl }^{ - }$ ions. This natural process occurs using ATP as energy. Increased ${ K }^{ + }$ concentration leads to the decreased water potential of guard cells which ends up in absorption of water from epidermal and mesophyll cells. This successively increases the turgor pressure of guard cells which results in the opening of stomatal pores.

Additional Information:
-The stomach is surrounded by two parenchymatous guard cells.
-The guard’s cells are kidney-shaped structures consisting of Chloroplasts.
-Stoma will help in the process of respiration and photosynthesis.
-The turgor pressure created in the guard cells is responsible for the occurrence of the mechanism of
opening and closing of stomata.
-When the sugar or solute which is formed during photosynthesis due to the presence of light, enters
the guard cells then the osmotic potential and water potential of the guard cell is lowered.
-The water potential between the guard cells and the nearby cells will increase the turgor pressure in the
guard cells.
-The guard cells will now become turgid and swell causing stomatal opening.
-When the light is absent, at that time no starch is produced as no photosynthesis occurs.
-The concentration of the solute in the guard cells gets lowered leading to the movement of water from the cell to the outside through osmosis.
-The guard cells will eventually become flaccid and the stomata get closed.

So, the correct answer is ‘All are correct’.

Note: The guard cells become turgid and swollen due to the accumulation of potassium ions resulting in lowering of water potential and opening of stomata while the closing of stomata occurs due to the
lowering the amount of water, light, and carbon dioxide decreasing photosynthesis.