Guard cells are specialised cells found in the epidermis of plant leaves, stems and other aerial organs. They are responsible for regulating the opening and closing of small openings called stomata. Guard cells have a distinctive kidney or dumbbell shape and are found on both sides of the stomatal pore. These cells have unique structural and functional properties that enable them to respond to environmental stimuli and control gas exchange, transpiration and water balance in plants. Through their ability to swell and shrink, guard cells modulate the size of the stomatal pore, regulating the entry of carbon dioxide for photosynthesis and the exit of oxygen and water vapour. The intricate regulation of guard cells plays a crucial role in maintaining the plant’s water status, optimising photosynthesis and responding to environmental stimuli.
Table of Contents
Guard Cell Definition
Guard cells are specialised plant cells found in the epidermis of leaves, stems and other plant organs. They surround and regulate the openings known as Stomata, which control gas exchange and water loss in plants.
Guard Cell Diagram
Structure and Location of Guard Cells
Guard cells are mainly located in the epidermis of plant leaves, especially in the lower epidermis. They are located around the stomata, which are also found on the leaf surface. Guard cells can also be found in the upper epidermis, but they are less abundant compared to the lower epidermis. The arrangement and density of guard cells can vary depending on the plant species.
The structure of guard cells is crucial to their function in regulating stomatal openings. The most important components of guard cell structure include –
Cell wall
The cell wall of guard cells is made of cellulose, pectin and hemicellulose. This rigid structure gives the Guard cells support and shape and enables them to maintain the stomatal pore.
Cytoplasm
The cytoplasm of guard cells contains all the organelles found in other plant cells, including the nucleus, mitochondria, chloroplasts and vacuoles. The cytoplasm is where the various metabolic activities of the guard cells take place.
Vacuole
The Guard cells have a large central vacuole filled with water and solutes. This vacuole plays an important role in maintaining turgor pressure within the guard cells. When the vacuole is filled with water, it exerts pressure on the cell walls, causing the guard cells to swell and the stomatal pore to open. If, on the other hand, the vacuole loses water, the turgor pressure drops, causing the stomata pore to close.
Chloroplasts
The guard cells contain chloroplasts, organelles responsible for photosynthesis. Chloroplasts contain chlorophyll, a pigment that captures light energy for photosynthesis. The presence of chloroplasts enables guard cells to produce energy-rich molecules such as ATP so that they can actively control the opening of the stomata.
Nucleus
The nucleus is the control centre of the cell and contains the genetic material (DNA) that controls cellular activities. It regulates gene expression and protein synthesis and plays a crucial role in coordinating the reactions of the guard cells to various environmental signals.
The structure of guard cells is critical to their function in controlling stomatal openings. The cell wall provides support and shape to the guard cells and enables them to resist changes in turgor pressure. The cytoplasm houses all the necessary organelles for metabolism, including the synthesis of signalling molecules and the regulation of ion transport. The central vacuole influences the opening and closing of stomata by maintaining turgor pressure. The chloroplasts enable the guard cells to carry out photosynthesis and supply them with the energy required for their physiological functions. The nucleus serves as a command centre and controls the activities and responses of the guard cells to environmental stimuli and hormonal signals.
Functions of The Guard cells
Guard cells play a crucial role in regulating the opening and closing of stomata, which are tiny pores on the surface of plants.
Here are the most important functions of the guard cells-
Regulation of Stomata
The main function of guard cells is to regulate the opening and closing of stomata. The stomata are responsible for gas exchange and allow plants to take in carbon dioxide for photosynthesis and release oxygen and water vapour. The Guard cells control the size of the stomata and thus influence the speed of gas exchange and water loss.
Also Check – How do the Guard cells Regulate Opening and Closing of Stomatal Pores?
Water balance
The guard cells help to maintain the water balance in the plant by controlling transpiration. When the plant suffers from water shortage, the guard cells can close the stomata to reduce water loss through evaporation. Conversely, they can open the stomata when water is abundant to facilitate gas exchange for photosynthesis.
Role in photosynthesis
Facilitating the uptake of carbon dioxide
One of the main functions of sentinel cells is to regulate stomatal apertures and thus control the influx of carbon dioxide (CO2) for photosynthesis. During stomatal opening, CO2 diffuses through the stomatal pores into the leaf and reaches the chloroplasts in the guard cells and mesophyll cells. The CO2 is then used in the process of photosynthesis to produce carbohydrates, which serve as a source of energy and building blocks for plant growth and development.
Preventing water loss during gas exchange
In addition to facilitating CO2 uptake, guard cells also play a crucial role in preventing excessive water loss through transpiration. By regulating the stomatal openings, they can optimise the balance between CO2 uptake and water loss. In times of high temperatures or water scarcity, the guard cells close the stomata to reduce transpiration and conserve water. This closure helps to maintain the plant’s water balance, especially in environments with limited water availability.
Regulation of gas exchange
The Stomata allow carbon dioxide (CO2) to enter the plant, which is used in photosynthesis. They also allow oxygen (O2) to escape from the plant, which is a by-product of photosynthesis. In addition, the stomata help regulate the plant’s water loss. When the stomata are open, water vapour can escape from the plant, which can lead to water loss. However, when the stomata are closed, water loss is minimised.
Respiration
Oxygen (O2) can escape from the plant through the stomata, which is a by-product of Respiration. Respiration is the process by which plants use oxygen to convert carbohydrates into carbon dioxide and water. This process releases energy that the plant uses for growth and development.
Regulation of Transpiration
Guard cells influence the rate of transpiration, i.e. the process by which water evaporates from the plant surface. By opening and closing the stomata, they can control the amount of water vapour released into the atmosphere.
Also Check – 15 Important Difference between Transpiration and Guttation
Evapotranspiration
Evapotranspiration is the process by which water is lost from the plant through the stomata. This water is evaporated from the leaves and released into the air. Evapotranspiration is important for cooling the plants and for the water cycle.
Also Check – 14 Important Differences between Transpiration and Evaporation
Defence Mechanisms
Guard cells can respond to various environmental stimuli, such as light intensity, temperature, humidity and carbon dioxide concentration. In response to these stimuli, they can rapidly change their shape and control the opening of stomata. In this way, plants can adapt to different conditions and protect themselves from stress.
Ion Regulation
Guard cells play a role in maintaining the correct ion balance in the plant. They actively transport ions such as potassium (K+) and chloride (Cl-) in and out of the cells, which affects osmotic pressure and water movement and ultimately influences the opening and closing of the stomata.
Pathogen Defence
Guard cells are involved in the plant defence against pathogens. They can recognise certain pathogenic signals and initiate defense responses, such as the production of reactive oxygen species or the activation of defence genes to prevent the invasion of the pathogen.
Environmental Sensing
Guard cells act as environmental sensors, sensing changes in light, temperature, humidity and atmospheric gases. These stimuli trigger signaling pathways that lead to the opening or closing of stomata, allowing plants to respond and adapt to their environment.
Also Check – Mechanism of opening and Closing of the Stomata
Factors Affecting Guard Cell Activities
Humidity
Humidity plays an important role in regulating the opening and closing of the stomata. When humidity is high, the guard cells lose water and become flaccid, causing the pores of the stomata to close. This helps to minimise water loss through transpiration. Conversely, when humidity is low, guard cells absorb water and become turgid, opening the stomata and facilitating gas exchange.
Temperature
Temperature affects the behaviour of guard cells and the opening and closing of stomata. In general, stomata open at higher temperatures and promote gas exchange and transpiration. In contrast, stomata close at lower temperatures, reducing water loss and conserving the plant’s resources.
Light
Light is a major trigger for stomata opening. When light hits the guard cells, a protein called phototropin is activated. This activation sets in motion a series of events that lead to the influx of potassium ions (K+) into the guard cells. The influx of K+ ions causes the guard cells to swell, leading to the opening of the stomata and allowing gas exchange and transpiration.
Also Check – 10 Important Factors that Affect the Rate of Transpiration
Carbon dioxide
The concentration of Carbon dioxide (CO2) in the environment also influences the regulation of stomatal opening and closing. Higher CO2 levels favour stomatal closure, reducing water loss through transpiration and optimising the plant’s ability to take up carbon dioxide for photosynthesis.
Potassium Ions
Potassium ions (K+) play a critical role in regulating guard cell function. The movement of potassium ions in and out of guard cells affects their osmotic pressure, which in turn influences stomatal behaviour. The influx of potassium ions leads to the opening of the stomata, while their outflow leads to the closure of the stomata.
Hormones
Plant hormones, such as abscisic acid (ABA), also contribute to the regulation of guard cell activities. ABA is produced in response to various stress conditions, including drought and high temperatures. When ABA binds to receptors on guard cells, it triggers a signalling cascade that causes the efflux of potassium ions. As a result, the guard cells shrink, causing the stomata to close and reducing water loss under harsh environmental conditions.
Also Check – 8 Important Functions of Stomata
Frequently asked questions on Guard cells
What are guard cells responsible for?
Answer- The guard cells are responsible for regulating the opening and closing of the stomata.
Where are guard cells found in plants?
Answer- Guard cells are found in the epidermis of plant leaves, stems and other aerial organs.
Also Check – Stomatal Transpiration
What is the shape of guard cells?
Answer- Guard cells have a characteristic kidney or dumbbell shape.
Are guard cells present on both sides of the stomatal pore?
Answer- Yes, guard cells are found on both sides of the stomatal pore.
How do guard cells regulate the size of the stomatal pore?
Answer- The guard cells can swell and shrink, changing the size of the stomatal pore.
What is the function of the guard cells in gas exchange?
Answer- The guard cells regulate the entry of carbon dioxide for photosynthesis and the exit of oxygen and water vapour.
What role do guard cells play in water balance?
Answer- Guard cells help maintain the plant’s water balance by regulating transpiration and controlling the opening and closing of stomata.
Where are the guard cells mainly located in the plant leaves?
Answer- Guard cells are mainly located in the epidermis of plant leaves, especially in the lower epidermis.
What components does the guard cell wall consist of?
Answer- The guard cell wall consists of cellulose, pectin and hemicellulose.
What is the function of the vacuole in the guard cells?
Answer- The vacuole in the guard cells maintains the turgor pressure and influences the opening and closing of the stomata.
What is the function of the chloroplasts in the guard cells?
Answer- The chloroplasts in the guard cells are responsible for photosynthesis and provide energy for physiological functions.
Which organelle regulates gene expression in the guard cells?
Answer- The cell nucleus, which contains the genetic material (DNA), regulates gene expression in the guard cells.
Why is the structure of the guard cells important?
Answer- The structure of the guard cells is important because it allows them to resist changes in turgor pressure and perform their functions in regulating stomatal opening.
What metabolic activities take place in the cytoplasm of the guard cells?
Answer- Various metabolic activities take place in the cytoplasm of guard cells, including the synthesis of signalling molecules and the regulation of ion transport.
Also Check – What are the Components of Transport System in Highly Organised Plants
What is the role of the nucleus in guard cells?
Answer- The nucleus serves as a command centre and controls the activities and reactions of the guard cells to environmental stimuli and hormonal signals.
What is the main function of guard cells?
Answer- The main function of the guard cells is to regulate the opening and closing of the stomata.
How do guard cells control gas exchange and water loss in plants?
Answer- Guard cells control gas exchange and water loss in plants by regulating the size of stomata.
What role do guard cells play in maintaining water balance in plants?
Answer- Guard cells help maintain water balance in plants by controlling transpiration. They can close stomata to reduce water loss during water shortage and open stomata when water is abundant.
How do guard cells facilitate the uptake of carbon dioxide for photosynthesis?
Answer- The guard cells regulate the stomata so that carbon dioxide can diffuse into the leaf and reach the chloroplasts in the guard cells and mesophyll cells where it is used for photosynthesis.
What role do the guard cells play in preventing excessive water loss during gas exchange?
Answer- The guard cells play a crucial role in preventing excessive water loss through transpiration by regulating the stomatal openings. They optimise the balance between carbon dioxide uptake and water loss.
How do stomata help regulate gas exchange and water loss in plants?
Answer- The stomata allow carbon dioxide to enter the plant for photosynthesis and allow oxygen to escape as a by-product. They also regulate water loss through transpiration.
What role do the guard cells play in the respiration process?
Answer- The guard cells allow oxygen to escape from the plant through the stomata, which is a by-product of respiration. Respiration is the process by which plants use oxygen to convert carbohydrates into carbon dioxide and water, releasing energy in the process.
How do guard cells affect the rate of transpiration?
Answer- Guard cells influence the rate of transpiration by controlling the opening and closing of the stomata, which affects the amount of water vapour released into the atmosphere.
What is evapotranspiration and why is it important?
Answer- Evapotranspiration is the process by which water is lost from the plant through the stomata, where it evaporates from the leaves and is released into the air. Evapotranspiration is important for cooling the plants and for the water cycle.
What other functions do guard cells perform besides regulating the stomata?
Answer- Guard cells serve as defence mechanisms against pathogens, maintain the ion balance in the plant, serve as environmental sensors and respond to various environmental stimuli such as light, temperature, humidity and atmospheric gases.
How do guard cells contribute to the defence against pathogens in plants?
Answer- Guard cells can recognize pathogenic signals and initiate defence responses, such as the production of reactive oxygen species or the activation of defence genes to prevent invasion by the pathogen.
What is the role of guard cells as environmental sensors?
Answer- Guard cells act as environmental sensors by sensing changes in light, temperature, humidity and atmospheric gases. These stimuli trigger signalling pathways that lead to the opening or closing of stomata, allowing the plants to respond and adapt to their environment.
How does the humidity of the air affect the opening and closing of the stomata?
Answer- When humidity is high, the guard cells lose water and become flaccid, causing the stomata to close and minimising water loss through transpiration. When humidity is low, on the other hand, the guard cells absorb water and become turgid, causing the stomata to open and facilitating gas exchange.
How does temperature influence the behaviour of the guard cells and the opening of the stomata?
Answer- In general, higher temperatures promote the opening of the stomata and facilitate gas exchange and transpiration. At lower temperatures, the stomata tend to close, reducing water loss and conserving the plant’s resources.
What role does light play in the opening of the stomata?
Answer- When light hits the guard cells, a protein called phototropin is activated. This activation triggers a series of events, including the influx of potassium ions (K+), which swells the guard cells and leads to the opening of the stomata for gas exchange and transpiration.
How does the concentration of carbon dioxide (CO2) affect the regulation of stomata?
Answer- A higher carbon dioxide content favours the closing of the stomata, reducing water loss through transpiration and optimising the plant’s ability to take up carbon dioxide for photosynthesis.
What role do potassium ions (K+) play in regulating the function of guard cells?
Answer- The movement of potassium ions in and out of guard cells influences their osmotic pressure, which in turn influences the behaviour of the stomata. The inflow of potassium ions leads to the opening of the stomata, while their outflow leads to the closing of the stomata.
How do plant hormones, especially abscisic acid (ABA), contribute to the regulation of guard cell activities?
Answer- Abscisic acid (ABA), produced in response to stress conditions such as drought and high temperatures, binds to receptors on guard cells. This binding triggers a signalling cascade that causes the efflux of potassium ions, leading to the shrinkage of guard cells, the closing of stomata and the reduction of water loss under harsh environmental conditions.
Also Check – What are the Methods used by Plants to get Rid of Excretory Products
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