Stomata (singular: stoma) are microscopic pores found mainly on the lower surface of leaves, though they can occur on both surfaces and on stems. Each stoma is surrounded by two specialised guard cells that control its opening and closing. Stomata serve two main functions: gas exchange (CO₂ in, O₂ out during photosynthesis) and transpiration (water vapour loss). The opening and closing of stomata is regulated by changes in turgor pressure of the guard cells.
Stomata are microscopic pores in leaf epidermis surrounded by two guard cells.
Guard cells in dicots are kidney/bean-shaped; in monocots (grasses) they are dumbbell-shaped.
Only guard cells in the epidermis contain chloroplasts.
Stomata open when guard cells are turgid (K⁺ enter → water enters by osmosis).
Stomata close when guard cells are flaccid (K⁺ leave → water leaves by osmosis).
ABA (abscisic acid) causes stomata to close during drought stress.
Functions: gas exchange (CO₂ in, O₂ out) and transpiration (water vapour out).
Leaves of aquatic plants (floating leaves) have stomata on the upper surface only.
Structure: • Stoma = a small pore (opening) in the leaf epidermis • Surrounded by 2 guard cells (bean/kidney-shaped in dicots; dumbbell-shaped in monocots) • The epidermis cells surrounding the guard cells are called subsidiary cells / accessory cells
Guard cells: • Only epidermal cells that contain chloroplasts • Have unevenly thickened cell walls: – Inner wall (facing the pore): thicker and inelastic – Outer wall: thinner and elastic • When guard cells gain water (turgor), they bulge outward → pore opens • When guard cells lose water (flaccid), they straighten → pore closes
Distribution: • Dicots: mostly on lower leaf surface (hypostomatous) • Monocots: both surfaces equally (amphistomatous) — e.g., grasses • Water plants (hydrophytes): stomata on upper surface only • Some aquatic plants: no stomata (absorb gases directly)
Numbers: A leaf may have 1,000–100,000 stomata per cm² depending on species.
Mechanism of stomatal opening (turgor mechanism):
Opening:
Closing:
Factors that cause stomata to open: • Light (daytime) • High CO₂ concentration inside leaf • High temperature • Low water vapour pressure
Factors that cause stomata to close: • Darkness (night) • Drought / low water supply • ABA (abscisic acid) • Very high temperature
Two primary functions:
Gas exchange: • During photosynthesis (day): CO₂ enters → O₂ exits • During respiration (day and night): O₂ enters → CO₂ exits • Stomata account for >90% of gas exchange in land plants
Transpiration: • Water vapour escapes through open stomata • Creates a water potential gradient — pulls water up from roots • Helps in mineral transport (dissolved minerals move with water) • Cools the leaf (evaporative cooling)
Transpiration pull: Water lost through stomata → creates tension in xylem → water pulled from roots upward → TACT mechanism (Transpiration, Adhesion, Cohesion, Tension)
Conflict: • Stomata must be open for photosynthesis (CO₂ needed) and nutrient uptake • But open stomata cause water loss (transpiration) • Plants must balance: open in light (photosynthesis) vs close in drought (conserve water) • CAM plants open stomata only at night to minimise water loss
Stomata are microscopic pores (openings) found mainly in the lower epidermis of leaves. Each stoma is flanked by two guard cells. They are also found on stems and some fruits. Stomata allow gas exchange (CO₂ and O₂) and water vapour (transpiration) in and out of the plant.
Guard cells control the opening and closing of stomata. When guard cells absorb water and become turgid, they curve apart, opening the stoma. When they lose water and become flaccid, they straighten and close the stoma. Guard cells are the only epidermal cells that contain chloroplasts.
In light, K⁺ ions enter guard cells → water follows by osmosis → guard cells become turgid → curved shape opens the pore. In darkness or drought, K⁺ ions leave → water exits → guard cells become flaccid → pore closes. Abscisic acid (ABA) triggers closure under drought stress.
Two main functions: (1) Gas exchange — CO₂ enters for photosynthesis and O₂ exits; (2) Transpiration — water vapour leaves the leaf, which drives water uptake from roots and helps cool the plant.
In dicots (e.g., beans, roses), guard cells are kidney/bean-shaped and stomata are mainly on the lower leaf surface. In monocots (e.g., grasses, maize), guard cells are dumbbell-shaped and stomata are present on both leaf surfaces in equal numbers.
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