Question
What are stomata? Describe their structure and explain the mechanism by which they open and close.
Solution — Step by Step
Stomata (singular: stoma) are microscopic pores present mainly on the epidermis of leaves, and sometimes on stems. The word comes from Greek — “stoma” means mouth. Stomata allow gas exchange: CO₂ enters for photosynthesis, and O₂ and water vapour exit. They are particularly concentrated on the lower epidermis of leaves (to reduce direct sunlight exposure and minimise water loss).
A typical leaf has 100–300 stomata per mm² — a sunflower leaf has about 156 on the upper surface and 175 on the lower surface per mm².
Each stoma consists of two specialised epidermal cells called guard cells, which surround the pore.
Guard cell features:
- Bean-shaped (kidney-shaped) in dicots; dumbbell-shaped in monocots (grasses)
- Contain chloroplasts — unlike other epidermal cells, so they can photosynthesise
- Have unevenly thickened cell walls: the wall facing the pore (inner wall) is thick and inelastic; the outer wall is thin and flexible
- Contain vacuoles and are turgid/flaccid depending on water content
Subsidiary cells: Epidermal cells immediately surrounding the guard cells, supporting them and facilitating ion movement.
Opening (daytime, in presence of light):
- Light (especially blue light) is detected by the guard cells.
- This activates proton pumps (H⁺-ATPase) in the guard cell membrane, which pump H⁺ out of the guard cells.
- The resulting electrical gradient drives K⁺ (potassium ions) into the guard cells from surrounding epidermal cells via K⁺ channels.
- As K⁺ accumulates, the water potential inside the guard cell decreases (becomes more negative).
- Water enters the guard cell by osmosis, making it turgid (swollen).
- Because the inner wall (facing the pore) is thicker and inelastic, the turgid guard cells bow outward — the pore opens.
Malate²⁻ ions are also produced by PEP carboxylase activity and help balance the K⁺ charge buildup.
Closing (night, water stress, high CO₂):
- In darkness, K⁺ channels operate in reverse — K⁺ flows OUT of guard cells.
- Alternatively, under water stress, the plant hormone abscisic acid (ABA) is released from roots/mesophyll.
- ABA triggers Ca²⁺ release inside guard cells and activates anion channels, leading to efflux of Cl⁻ and malate.
- K⁺ follows the negative ions out.
- Guard cell water potential increases, water leaves by osmosis → guard cells become flaccid (limp).
- The elastic outer wall pulls the guard cells back together → pore closes, reducing water loss.
Why This Works
The mechanism exploits a beautiful principle: guard cells can regulate their turgor pressure rapidly and reversibly by controlling K⁺ flux. Since water follows solutes by osmosis, controlling ion concentration effectively controls cell shape and pore size.
The asymmetric cell wall thickness is the key structural feature — thick inner walls mean that when the cell swells, it can only bend outward (away from the pore), thereby opening it. When it loses water, it straightens back and the pore closes.
Alternative Method
The starch-sugar hypothesis (older, less accepted) proposed that starch is converted to glucose in light (lowering water potential), causing water entry and pore opening. While partially correct (sugars do contribute to osmotic potential), the K⁺ transport theory is the modern, accepted explanation and what NEET/CBSE expect.
NEET Biology (Plant Physiology chapter) and CBSE Class 11 regularly ask: “Explain the mechanism of stomatal opening and closing” as a 3-5 mark question. The K⁺ transport model is the answer to write. Key words to include: K⁺ influx, osmosis, turgor pressure, guard cell shape change, ABA for closing.
Common Mistake
A very common error: students say stomata open at night and close during the day (getting it backwards). The general rule is: stomata open in light (for photosynthesis, which needs CO₂ to enter) and close in darkness. CAM plants (cacti, succulents) are the major exception — they open stomata at night and close during day to reduce water loss in arid environments. Unless the question specifies CAM plants, assume the standard light-opens pattern.