Biology · Anatomy of Flowering Plants · NEET
Water moves into the two guard cells, so they become turgid (full and swollen). Because the inner wall (facing the pore) is thick and stiff and the outer wall is thin and stretchy, the outer wall bulges outward. This bending pulls the two guard cells apart in the middle, and the stomatal pore opens. When water leaves, the cells become flaccid and the pore closes.
It is because of uneven wall thickness plus radial microfibrils. The thin outer wall stretches easily, but the thick inner wall does not. When the cell swells, only the outer wall expands, so the whole cell curves away from the pore. Both guard cells curve outward together, so the gap between them (the pore) widens. Turgid guard cells always open the pore; flaccid guard cells close it.
The cellulose microfibrils in the guard cell wall are arranged radially (like spokes of a wheel) instead of lengthwise. Radial arrangement stops the cell from getting longer and forces the swelling to bend the cell outward, opening the pore. This exact point was asked in NEET 2017 - radial orientation facilitates opening, not longitudinal.
In light, guard cells actively pump in K+ ions (potassium ions). This lowers the water potential inside the guard cells, so water enters by osmosis and the cells become turgid, opening the pore. At night or under water stress, K+ moves out, water leaves, the cells go flaccid, and the pore closes. So K+ movement is the trigger that controls water entry.
The inner wall (towards the stomatal pore) is highly thickened, and the outer wall (away from the pore) is thin. NEET has directly tested this: the guard cell has thin outer walls and thick inner walls. This uneven thickening is the reason turgid guard cells bend outward and open the pore.
Light, temperature and CO2 concentration control stomatal movement. Light usually opens stomata; high CO2 tends to close them. NEET 2018 tested this - O2 concentration does NOT affect stomatal movement, while light, temperature and CO2 do.
Which of the following facilitates opening of stomatal aperture?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Guard cells are the only epidermal cells that possess chloroplasts. This helps them respond to light and control the opening and closing of stomata.
When water leaves the guard cells, they become flaccid (soft and shrunken). The bending straightens out, the two guard cells come together, and the stomatal pore closes. This reduces water loss by transpiration.
No. In most dicots the guard cells are bean-shaped (kidney-shaped). In grasses (monocots) they are dumb-bell shaped - narrow in the middle and swollen at the ends. Both types open and close by changing turgor.
Because NEET repeatedly tests it. You must know: outer wall thin, inner wall thick, microfibrils radial, and turgidity opens the pore. These four facts answer almost every guard cell question.
