What is function of the stomata? Stomata are tiny pores found mainly on the lower surface of leaves. Their primary functions are: (1) gas exchange — allowing carbon dioxide (CO₂) to enter for photosynthesis and oxygen (O₂) to exit, (2) transpiration — regulating water loss from the plant, and (3) respiration — allowing oxygen in and carbon dioxide out during cellular respiration. Each stoma is surrounded by two kidney-shaped guard cells that control its opening and closing. Stomata open during the day for photosynthesis and close at night or during water stress to prevent excessive water loss. This guide covers all functions of stomata, their structure, opening and closing mechanism, types, factors affecting them, and exam-ready FAQs.
Stomata are tiny pores on leaf surfaces, mainly the lower epidermis
Main functions: gas exchange (CO₂ in, O₂ out), transpiration, and regulation
Each stoma is surrounded by two kidney-shaped guard cells
Guard cells control opening (turgid) and closing (flaccid) of stomata
90–95% of transpiration occurs through stomata (stomatal transpiration)
Stomata open in light and close in darkness
K⁺ ions enter guard cells → water follows by osmosis → cells become turgid → stoma opens
Abscisic acid (ABA) hormone triggers rapid stomatal closure during drought
A single leaf can have 10,000 to 100,000+ stomata
CAM plants (e.g., cactus) open stomata at night to conserve water
The three main functions of stomata are:
Gas Exchange • CO₂ enters the leaf through stomata → used in photosynthesis • O₂ produced during photosynthesis exits through stomata • During respiration, O₂ enters and CO₂ exits through stomata
Transpiration • Water vapour escapes from the leaf through open stomata • This water loss (transpiration) creates a pull that draws water up from roots through the xylem • Transpiration also cools the plant
Regulation • Guard cells open and close stomata to balance gas exchange and water loss • Stomata open when CO₂ is needed (daytime) and close to conserve water (night, drought)
In short: stomata are the 'breathing pores' of plants — they let gases in and out while controlling how much water the plant loses.
Each stoma (singular of stomata) consists of:
Stomatal Pore • The tiny opening (pore) between two guard cells • Diameter: approximately 10–80 μm when open • Found mainly on the lower epidermis of leaves (to reduce direct sun exposure and water loss)
Guard Cells • Two bean-shaped (kidney-shaped) cells surrounding the pore • In grasses: guard cells are dumbbell-shaped • Contain chloroplasts (unlike other epidermal cells) — they can photosynthesise • Inner wall (facing the pore) is thick; outer wall is thin • When turgid (full of water): guard cells curve apart → stoma opens • When flaccid (lost water): guard cells straighten → stoma closes
Subsidiary Cells (Accessory Cells) • Epidermal cells surrounding the guard cells • Help in the opening and closing movement of guard cells • Differ in shape and number depending on the plant species
Location of stomata: • Most abundant on the lower surface of dorsiventral leaves (dicots) • Equal on both surfaces in monocot leaves (e.g., grass) • Floating leaves (e.g., lotus): stomata only on the upper surface • Submerged aquatic plants: no stomata at all • A single leaf can have 10,000 to 100,000+ stomata
The most important function of stomata is gas exchange, which is essential for two processes:
A. Photosynthesis (during the day): • CO₂ from the atmosphere diffuses into the leaf through open stomata • CO₂ reaches the mesophyll cells where photosynthesis occurs • Equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of light and chlorophyll) • O₂ produced as a by-product diffuses out through stomata • Without stomata, CO₂ cannot enter the leaf and photosynthesis stops
B. Cellular Respiration (day and night): • O₂ enters through stomata for cellular respiration • CO₂ produced during respiration exits through stomata • Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP) • Respiration occurs 24 hours, but during the day, the rate of photosynthesis exceeds respiration, so there is a net intake of CO₂ and net release of O₂
Stomata act as the interface between the plant's internal air spaces and the external atmosphere. The gas exchange occurs by diffusion — gases move from regions of higher concentration to lower concentration through the stomatal pore.
Transpiration is the loss of water vapour from the aerial parts of the plant, mainly through stomata. About 90–95% of transpiration occurs through stomata (called stomatal transpiration).
How it works:
Importance of transpiration: • Creates transpiration pull — the suction force that draws water up from roots to leaves (even in tall trees) • Cools the plant — evaporation of water has a cooling effect, preventing overheating • Helps in absorption and transport of minerals from the soil • Maintains cell turgidity in leaves
Transpiration and stomata regulation: • When water is abundant: stomata remain open → high transpiration • When water is scarce (drought): stomata close → transpiration is reduced to conserve water • This is a trade-off: closing stomata saves water but stops CO₂ entry, slowing photosynthesis
Types of transpiration: • Stomatal transpiration: through stomata (90–95%) • Cuticular transpiration: through the cuticle on leaf surface (5–10%) • Lenticular transpiration: through lenticels on stems (very small amount)
Stomata play a key regulatory role in maintaining the plant's internal balance:
Water Balance • By opening and closing, stomata control how much water the plant loses • In dry conditions, stomata close to prevent wilting and dehydration • In humid conditions, stomata can remain open longer
CO₂ Regulation • Stomata regulate the internal CO₂ concentration in the leaf • When CO₂ inside the leaf drops (due to active photosynthesis), stomata open to allow more CO₂ in • When CO₂ builds up (at night, when photosynthesis stops), stomata close
Temperature Regulation • Transpiration through open stomata cools the leaf surface • This is especially important in hot environments • Similar to sweating in humans — evaporation removes heat
Oxygen Release • During photosynthesis, plants release O₂ through stomata • This O₂ is vital for the respiration of all aerobic organisms on Earth • Plants are the primary source of atmospheric oxygen
Prevention of Pathogen Entry • Some plants close stomata in response to bacterial pathogens • This acts as a first line of defence — if the pore is closed, bacteria cannot enter • This is an active immune response discovered in recent research
The opening and closing of stomata is controlled by the guard cells:
Opening of Stomata:
Closing of Stomata:
Role of Abscisic Acid (ABA): • ABA is a plant hormone produced during drought stress • ABA signals guard cells to close stomata rapidly • This is the plant's emergency response to prevent excessive water loss • ABA causes K⁺ ions to leave guard cells, triggering closure
Summary: Turgid guard cells → stoma opens. Flaccid guard cells → stoma closes.
Several environmental and internal factors affect whether stomata are open or closed:
Light • Light → stomata open (for photosynthesis) • Darkness → stomata close • Blue light is most effective at triggering stomatal opening
CO₂ Concentration • Low CO₂ inside leaf → stomata open (to take in more CO₂) • High CO₂ inside leaf → stomata close
Water Availability • Sufficient water → stomata open • Water stress/drought → stomata close (to conserve water) • ABA hormone triggers rapid closure during drought
Temperature • Moderate temperature (25–30°C) → stomata open optimally • Very high temperature (>35°C) → stomata close (to reduce water loss) • Very low temperature → stomata close
Humidity • High humidity → stomata remain open (low transpiration rate, less water loss risk) • Low humidity (dry air) → stomata tend to close
Wind • Gentle wind → increases transpiration, may trigger partial closure • Strong wind → stomata close to prevent excessive water loss
Plant Hormones • Abscisic acid (ABA) → promotes stomatal closure • Cytokinins → promote stomatal opening
Stomata are classified based on the arrangement of surrounding subsidiary cells:
Anomocytic (Irregular-celled) Stomata • Subsidiary cells are not distinct from other epidermal cells • Found in: Ranunculus (buttercup), many dicots • Also called: Ranunculaceous type
Anisocytic (Unequal-celled) Stomata • Three subsidiary cells of unequal size surround each stoma • Found in: Solanum (potato), Nicotiana (tobacco), Brassica (mustard) • Also called: Cruciferous type
Paracytic (Parallel-celled) Stomata • Two subsidiary cells parallel to the guard cells • Found in: Rubiaceae family, many monocots • Also called: Rubiaceous type
Diacytic (Cross-celled) Stomata • Two subsidiary cells at right angles to the guard cells • Found in: Caryophyllaceae family, Acanthaceae • Also called: Caryophyllaceous type
Gramineous Stomata • Guard cells are dumbbell-shaped (not kidney-shaped) • Found in: grasses, cereals (wheat, rice, maize) • Subsidiary cells are parallel to the guard cells
Some plants have evolved special stomatal strategies to survive in hot, dry environments:
CAM Plants (Crassulacean Acid Metabolism): • Examples: Cactus, pineapple, jade plant, succulents • Stomata open at NIGHT and close during the DAY (opposite of most plants) • At night: CO₂ enters, is fixed into malic acid, and stored in vacuoles • During the day: stomata close (preventing water loss), malic acid is broken down to release CO₂ for photosynthesis • Adaptation: minimises water loss in extremely dry (arid) environments
C4 Plants: • Examples: Maize (corn), sugarcane, sorghum, many tropical grasses • Stomata open during the day but can partially close in heat stress • C4 pathway concentrates CO₂ around RuBisCO, so plants can maintain photosynthesis even with partially closed stomata • More water-efficient than C3 plants — they need less stomatal opening for the same amount of photosynthesis
C3 Plants (most plants): • Examples: Rice, wheat, most trees, most vegetables • Stomata open during the day for CO₂ intake • Less water-efficient — need stomata open wider and longer • In hot, dry conditions, they lose more water per unit of carbon fixed compared to C4 plants
Stomata perform three main functions: (1) Gas exchange — CO₂ enters the leaf for photosynthesis and O₂ exits as a by-product. During respiration, O₂ enters and CO₂ exits. (2) Transpiration — water vapour escapes through open stomata, creating a pull that draws water from roots upward. (3) Regulation — guard cells open and close stomata to balance CO₂ intake with water conservation, and to cool the plant through evaporation.
Guard cells are two specialised kidney-shaped (or dumbbell-shaped in grasses) cells that surround each stomatal pore. They control opening and closing. When K⁺ ions are pumped in, water follows by osmosis, making guard cells turgid — the thick inner wall and thin outer wall cause them to curve apart, opening the pore. When K⁺ ions leave, water exits, guard cells become flaccid, and the pore closes. Guard cells contain chloroplasts, unlike other epidermal cells.
Stomata open during the day because light triggers photosynthesis in guard cells, K⁺ ions are pumped in, water enters by osmosis, and guard cells become turgid. The plant needs CO₂ for photosynthesis, which only occurs in light. At night, photosynthesis stops, so CO₂ is not needed. Stomata close to prevent unnecessary water loss through transpiration. Exception: CAM plants (cactus, pineapple) open stomata at night to conserve water in arid environments.
Transpiration is the loss of water vapour from the aerial parts of a plant. About 90–95% of transpiration occurs through stomata (stomatal transpiration). Water evaporates from mesophyll cell surfaces into air spaces and exits through open stomata. Transpiration creates a suction pull that draws water up from the roots, cools the plant, and helps transport minerals. Stomata regulate transpiration by opening and closing based on water availability.
In most dicot plants, stomata are more abundant on the lower epidermis (underside) of leaves — this reduces direct sun exposure and minimises water loss. In monocots (grasses), stomata are roughly equal on both surfaces. In floating aquatic plants like lotus, stomata are only on the upper surface (since the lower surface touches water). Submerged aquatic plants have no stomata. A single leaf may have 10,000 to 100,000+ stomata.
Abscisic acid (ABA) is a stress hormone that causes rapid stomatal closure during drought. When the plant senses water stress, ABA is produced and transported to guard cells. ABA triggers the efflux of K⁺ ions from guard cells, causing water to leave by osmosis. Guard cells become flaccid and the stomata close. This is the plant's emergency water-conservation response, preventing wilting and dehydration during dry conditions.
CAM plants (Crassulacean Acid Metabolism — e.g., cactus, pineapple, succulents) open their stomata at night and close them during the day — the opposite of most plants. At night, CO₂ enters and is stored as malic acid. During the day, stomata close (preventing water loss in hot, dry conditions), and the stored malic acid releases CO₂ for photosynthesis internally. This adaptation allows CAM plants to survive in extremely arid environments.
Key factors: (1) Light — light opens stomata, darkness closes them. (2) CO₂ — low internal CO₂ opens stomata, high CO₂ closes them. (3) Water — sufficient water opens, drought closes (via ABA hormone). (4) Temperature — moderate temps open, extreme heat/cold closes. (5) Humidity — high humidity keeps stomata open, dry air closes them. (6) Wind — strong wind causes closure. (7) Hormones — ABA closes stomata, cytokinins open them.
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