Q.Stomata of a plant open due to

Phosphorylase opens the stoma by converting starch to osmotically active sugar in guard cells during the day, which draws in water by osmosis and makes the guard cells turgid enough to bow apart.

The starch–sugar interconversion theory (an early explanation of stomatal movement, proposed by Sayre) links the opening and closing of stomata to a reversible enzymatic interconversion between starch and sugar in the guard cells, driven by pH changes linked to photosynthesis and respiration.

Mechanism during the day (stoma opens):

1. In the light, the guard cells (which contain chloroplasts) photosynthesise, using up CO2. This lowers the CO2 concentration inside the guard cells.
2. As CO2 falls, less carbonic acid (H2CO3) is formed, so the pH of the guard cell sap rises and becomes more alkaline (closer to or above pH 7).
3. This alkaline shift activates the enzyme phosphorylase, which — under alkaline conditions — favours the breakdown (hydrolysis) reaction, converting the guard cells' stored, osmotically inactive starch into osmotically active, soluble sugar (glucose-1-phosphate/glucose).
4. This sugar production increases the solute concentration inside the guard cells, lowering (making more negative) their osmotic/water potential relative to the neighbouring epidermal cells.
5. Water therefore moves into the guard cells by endosmosis, and the guard cells become turgid.
6. Because the guard cells' inner (ventral) wall, facing the pore, is thicker and less elastic than their outer (dorsal) wall, the turgid guard cells expand unevenly — the thinner outer wall stretches more, forcing the whole guard cell to bow outward in a curve — and this bowing apart of the pair of guard cells opens the stomatal pore. …

Stomatal opening and closing is driven by K+-ion-mediated osmotic changes in guard cells, controlling their turgor.

Mechanism of K+-mediated stomatal movement:

1. K+ influx and stomatal opening: In light, the guard cells actively pump protons (H+) out of the cell using a plasma-membrane H+-ATPase (proton pump). This creates an electrochemical gradient that drives K+ ions into the guard cells through voltage-gated K+ channels. Malate ions (and sometimes Cl-) also accumulate to balance charge.
2. Osmotic water uptake: The increased concentration of K+ (and accompanying anions) inside the guard cells lowers their water potential, causing water to enter by osmosis.
3. Turgor and cell shape: Guard cells have unevenly thickened walls — the wall bordering the stomatal pore (inner wall) is thicker and less elastic than the outer wall. As the guard cells become turgid, the thin outer walls stretch/bulge more than the thick inner walls, causing the guard cells to bow outward and the pore (stoma) to open. …