Stomata
Stomata are small openings in plant leaves and stems that control gas exchange and water loss. Each stoma is surrounded by guard cells that regulate its opening and closing. Stomata play a crucial role in photosynthesis, transpiration, and maintaining plant water balance.
This Story also Contains
- What are Stomata?
- Structure of Stomata
- Types of Stomata
- Functions of Stomata
- Mechanism of Stomatal Movement
- Factors Influencing Stomatal Movement
- Distribution and Density of Stomata
- Adaptations of Stomata
- Evolutionary Significance of Stomata
- Agricultural Importance of Stomata
- NEET MCQs (With Answers & Explanations)
- Recommended Video On 'Stomata'
What are Stomata?
Stomata refers to small openings or pores mainly in leaves and stems that provide gas exchange. Each stomata is bordered by two specialised cells, called guard cells, which control the opening and closing of stomata.
Stomata are essential in photosynthesis since they are responsible for taking in carbon dioxide gas and giving out oxygen gas. They also promote cooling in plants by facilitating transpiration and allowing the flow of specific nutrients within the plant. The structure, types, functions, mechanisms and adaptations of stomata all help explain the importance they play in agriculture and evolution.
Structure of Stomata
The structure of stomata is complex and of prime importance for its function in plant physiology. Knowledge of this structure helps in understanding how they work.
Guard Cells
Guard cells are bean-shaped cells surrounding the stomata and in charge of its opening and closing by taking different shapes.
Stoma (Pore)
Stoma is the pore or the actual opening through which the gas exchange occurs.
Accessory Cells
Accessory cells are the cells alongside the guard cells that support stomatal function in some plant species.
Types of Stomata
The various types of stomata display diversity in plant species to the number and arrangement. The classification of stomata is based on the number and arrangement.
Anisocytic
Stomata are flanked by three subsidiary cells of unequal sizes.
Examples include members of the family Brassicaceae, like mustard.
Anomocytic
Stomata are surrounded by cells which are similar in size and shape.
Commonly occurs in most dicots, for example, Sunflower.
Diacytic
Stomata are accompanied by two subsidiary cells oriented at right angles to the guard cells.
Commonly found in plants like members of the family Caryophyllaceae.
Paracytic
Stomata are flanked by one or more subsidiary cells parallel to the guard cells.
This is typical in the Rubiaceae family.
Gramineous
Specialised form occurring in grasses, guard cells are dumbbell-shaped.
Occurs in monocots like maize and wheat.
Functions of Stomata
The stomata carry out several vital functions that are essential for the survival and growth of the plants.
Gas Exchange
Stomata facilitate the entry of CO₂ into the leaf, which is used in the process of photosynthesis. It also permits issuance of oxygen, which is a by-product of photosynthesis.
Transpiration
The stomata control the loss of water vapour, which in turn cools down the plant and has a role in the water balance.
Water Regulation
Stomata regulates the rate of transpiration and thus helps to maintain the water balance in plants.
Photosynthetic Efficiency
Stomata enables the exchange of gases by allowing CO₂ to enter for photosynthesis and O₂ to exit as a byproduct.
Mechanism of Stomatal Movement
The movement of stomata, i.e., opening and closing of stomata is a complicated process controlled by several factors.
Role of Guard Cells
The guard cells become turgid to open the stoma and flaccid to close it.
Role of Turgor Pressure
The changes in turgor pressure in guard cells are responsible for the opening and closing of stomata.
Light and CO₂ Influence
Light increases the uptake of potassium ions and hence the stomata opens. High levels of internal CO₂ causes the stomata to close.
Factors Influencing Stomatal Movement
The factors influencing the movement of stomata are:
Light Intensity
Stomata open in presence of light to allow CO₂ enter for photosynthesis and close in dark to reduce water loss
CO₂ Concentration
Low CO₂ stimulates the opening of the stomata while high levels cause the stomata to close.
Humidity and Water Availability
High humidity causes the stomatal opening, while low humidity or water scarcity leads to closure to prevent excessive transpiration.
Temperature
Moderate temperatures promote stomatal opening, but high temperatures increase the rate of transpiration and close the stomata to prevent further water loss.
Distribution and Density of Stomata
The stomata density and its distribution vary in different plants and are dependent on environmental and genetic factors.
Dorsiventral Leaves (Dicots): Stomata more on the lower surface.
Isobilateral Leaves (Monocots): Stomata equally distributed on both surfaces.
Adaptations of Stomata
Plants developed several stomata adaptations to adapt to different environments.
Plant Types | Adaptation | Example |
Xerophytes | Reduced number and sunken stomata to reduce loss of water | Nerium, Pine |
Hydrophytes | Stomata only on upper surfaces for exchange of gases in water. | Lotus |
Mesophytes | Stomata are evenly distributed for optimum gas exchange and water conservation | Sunflower |
Evolutionary Significance of Stomata
Stomata have had a significant role in plant evolution, thus helping classify plant groups. It enabled plants to handle loss of water while letting gasses cross through. The evolution from simple pore in bryophytes to complex guard cells in angiosperms
Bryophytes have insignificant, rather non-specialised stomata.
Pteridophytes have more developed structures of stomata.
Gymnosperms possess well-developed stomata to adapt to various environments.
Angiosperms have highly specialised stomata mirroring a wide range of habitats.
Agricultural Importance of Stomata
Stomata play a very critical role in agriculture since proper functioning has direct links to crop productivity and resilience. Plant breeding with optimal stomatal functioning therefore holds great potential in improving water-use efficiency in crops. Selecting for stomatal traits that confer greater drought tolerance may lead to better performance of crops.
NEET MCQs (With Answers & Explanations)
The key concepts to be covered under this topic for different exams are:
Structure of Stomata
Types of Stomata
Mechanism of stomatal movement
Practice Questions for NEET
Q1. Grass leaves curl inwards during very dry weather. Select the most appropriate reason for the following:
Closure of stomata
Flaccidity of bulliform cells
Shrinkage of air spaces in spongy mesophyll
Tyloses in vessels
Correct answer: 2) Flaccidity of bulliform cells
Explanation:
Opening and closing of stomata are governed by the change in osmotic pressure or turgidity of guard cells.
When guard cells are turgid, stomata open, and when guard cells are flaccid, stomata close.
The rolling or curling of grass leaves is mainly done due to bulliform cells. These are large bubble-shaped cells distributed between the epidermal cells of the leaf. During stressful conditions (such as dry weather), they become flaccid, causing the leaves to curl or roll. They unroll the leaves by gaining turgidity once stress conditions are over.
Hence, the correct option is 2) Flaccidity of bulliform cells.
Q2. Which of the following has an active role in stomatal opening?
Cl-
K+
H+
All of these
Correct answer: 2) K+
Explanation:
Stomata, located on leaf surfaces, are minute openings encircled by guard cells. Their opening and closing are governed by guard cell turgor pressure fluctuations, which depend on ion movement, notably potassium (K+).
Potassium's Role:
During the stomatal opening, primarily for CO₂ intake in photosynthesis, K+ ions are transferred from nearby epidermal cells into the guard cells. This influx raises osmotic pressure within guard cells, inducing water entry through osmosis and thus guard cell turgidity. Consequently, the stomata dilate.
Stomatal Closure Mechanism:
For water conservation, especially under drought or at night, K+ ions are actively removed from guard cells. This osmotic pressure decrease leads to water exit and guard cell flaccidity. The outcome is stomatal closure.
Hence, the correct answer is option 2) K+.
Q3. Basis of stomatal opening is
Exoosmosis
Endosmosis
Decreases in cell sap concentration
Plasmolysis of guard cells
Correct answer: 2) Endosmosis
Explanation:
Stomatal opening is primarily controlled by the turgor pressure of guard cells. When light stimulates the cells, potassium ions are actively pumped into the guard cells, lowering their osmotic potential. This causes water to enter the cells by osmosis, increasing their turgor pressure. As the guard cells swell, they curve, leading to the opening of the stomatal pore. Factors like light, CO₂ concentration, and hormones like abscisic acid also regulate stomatal movement, balancing gas exchange and water loss.
Movement of water in the guard cells increases their turgidity. This causes outward bending of the outer wall of the guard cells and the opening of the stomata.
Hence, the correct answer is option 2) Endosmosis.
Also Read:
Recommended Video On 'Stomata'
Frequently Asked Questions (FAQs)
The small openings on plants do the gas exchange and transpiration for photosynthesis to occur and to maintain the balance of water.
Stomatal density affects photosynthetic potential in a plant and controls water loss of the plant hence influencing its overall growth and stress tolerance.
Light, CO2 concentration, relative humidity, and temperature alter the turgor pressure in guard cells and hence control the opening and closing of stomata.
Stomata are classified into types like anisocytic, anomocytic, diacytic, paracytic, and gramineous, each differing in structural characteristics and found in different plant families.
It is through turgor pressure changes, as a result of environmental factors that alter the shape of guard cells in modulating the opening and closure of the stomata.
