The mechanism of opening and closing of stomata in plants is based on

1. Analyze the question The question asks what the opening and closing of stomata depends on. Stomata are tiny pores on leaves, each surrounded by two bean-shaped guard cells.

2. Explain the concept

• Guard cells control the size of the stomatal pore.
• In the day, guard cells photosynthesize and accumulate solutes (sugars and K⁺ ions). Water enters them by osmosis, so they become turgid. Their inner walls are thicker than the outer walls, so the cells bow outward and the pore opens.
• At night, or when the plant is short of water, the guard cells lose water and become flaccid. They straighten and the pore closes.

3. Step-by-step solution

• The opening mechanism is the guard cells becoming turgid.
• The closing mechanism is the guard cells becoming flaccid.
• Osmosis is how water moves in or out, but the state of the guard cells (turgid or flaccid) is what directly opens or closes the pore.
• Option A (turgidity and diffusion): diffusion does not explain closing.
• Option C (osmosis and diffusion): neither describes the two states, opening and closing.
• Option D (diffusion and flaccidity): this misses turgidity, which is needed for opening.
• Option B (turgidity and flaccidity) covers both opening (turgid) and closing (flaccid).

4. Answer The correct option is B. turgidity and flaccidity.

5. Exam tip Remember: Turgid = Tube open, Flaccid = Fully closed. Whenever a question asks about opening and closing, look for the option that pairs two opposite states, here turgid and flaccid.