Guard cells function mainly in the control of gas exchange in the epidermis of leaves, stems, and other organs. Guard cells function, definition, structure, and location would be discussed. These bean-shaped specialized cells are formed in pairs, with a gap between them forming a stomatal pore.
Table of Contents
What are guard cells?
Guard cells are essentially two bean-shaped cells that surround a stoma. They play an important role in gaseous exchange in and out of plant leaves as epidermal cells by regulating the opening and closing of pores known as stomata. Guard cells are also the channels through which water is released from the leaves into the environment.
Guard cells, as a result, play an important role in photosynthesis by regulating the entry of materials required for the process. Aside from regulating gaseous exchange (as well as water release from leaves), they have been shown to contain chloroplasts, making them photosynthesis sites.
Definition of guard cells
Guard cells are defined in biology as a pair of crescent-shaped cells that surround a pore (stoma) in the epidermis. The stoma opens and closes due to changes in the turgidity of the cells.
Guard cell functions
The main function of guard cells in a leaf is in their ability to become turgid and flaccid. This ability is the primary mechanism in the opening and closing of the stomata whose function is to allow for gaseous exchange, transpiration, and photosynthesis. This function of guard cells also answers the question of how guard cells open and close and what is the function of guard cells.
How Guard cells function by the opening and closing mechanism
One of the really essential roles of guard cells in a leaf is to help regulate the stoma/pore closure and opening. While these pores allow water to escape into the environment, they also allow CO2 to enter the cell for photosynthesis (as well as the release of oxygen into the environment).
Factors affecting the function of guard cells
- Humidity
- Temperature
- Light
- Carbon dioxide sensing and signaling
- Potassium ions
- Hormones (Abscisic acid) sensing and signaling
Humidity
The guard cells lose more water than the surrounding epidermal cells. As humidity decreases, the water potential of the guard cells decreases in proportion to the humidity of the air, causing stomata to close. since the guard cells lose water when the humidity of the surrounding environment is low, it also means that it gains water when the humidity is high.
Temperature
Guard cell movement induced by high temperature necessitates components involved in blue light-mediated stomatal opening. This means on hot dry days guard cell expands due to high temperature, which means that stomatal pores open when guard cells become turgid allowing for the cooling of the leaves. Whereas low temperature promotes guard cell contraction, which closes stomatal pores.
Light
Light intensity for example influences the swelling or shrinkage of guard cells, and thus the opening and closing of pores. In terms of pore opening, this factor influences water uptake into the cell, causing guard cells to inflate. This inflation/swelling causes the pores to open, allowing for gaseous exchange (as well as water release/transpiration).
Carbon dioxide sensing and signaling
Carbon dioxide concentration is one of the factors that influence the swelling and shrinkage of guard cells. Anion channels are activated in cases of high carbon dioxide concentrations in the atmosphere, causing potassium ions to move out of the cells. Simultaneously, chloride is released from the cells, eventually reused in membrane depolarization.
As solutes move out of the cell, their concentration increases in comparison to the concentration inside the cell. This results in the loss of water from the plant cell through osmosis, and when the cell loses water, it shrinks thereby closing the pore or stoma.
Potassium ion
During the day, potassium ions are transported into the guard cells via a series of events, increasing solute concentration and drawing water into the cell. The conversion of starch to phosphoenolpyruvate, and thus malic acid, also results in an increase in potassium ions resulting in more water intake.
Abscisic acid sensing and signaling
ABA (a plant hormone) has a variety of functions in plants, ranging from controlling seed germination to influencing guard cells.
Roots have been shown to produce more of this hormone under environmental conditions such as drought or increased salinity in the soil. The detection of this hormone by guard cells causes changes in the intake or removal of ions from the cells, causing the stoma to open or close.
The efflux of anions and potassium through the channels occurs when there is a high concentration of ABA. Simultaneously, potassium ion importation is inhibited, preventing the ions from entering the cell (this would otherwise cause a high concentration of solutes in the cell).
When there is a high concentration of solutes outside the cell, water is forced out through osmosis, lowering the turgor pressure of the guard cells. As a result, the aperture closes, preventing the cells from losing any more water.
What do Guard Cells Do?
This section contains a summary of the function of guard cells in a leaf of a plant. To answer this question, one needs to look at how guard cells open and close stomata.
How do guard cells open and close stomata?
One main function of guard cells in the leaves of plants is to regulate the rate of transpiration in a plant. The closing and opening of stomatal guard cells involve two situations or conditions, namely, the presence of light and the absence of light.
In the presence of light
Water begins to enter the guard cell during this phase, causing it to swell and become turgid. It is followed by two series of events that cause the stomata to open:
- The guard cell absorbs potassium ions (K+) from its surroundings. This K+ influx raises the solute potential but lowers the water potential within the guard cell.
- Chloride ion uptake (Cl–)
Water begins to move inside the guard cell as the ion concentration rises, causing its thin side to bulge outwards, similar to an inflated balloon. The thick side moves in the same direction as the thin side, giving the guard cells the shape of the letter ‘O.’ The pressure inside the guard cell is controlled by regulating the entry and exit of ions and sugar molecules.
In the absence of light
During this phase, the loss of water from the guard cell via osmosis causes it to become flaccid and resemble the letter ‘I.’ This water influx occurs as a result of:
- K+ ion release from guard cells to neighboring cells
- Cl– ion and malate release from guard cells
Structure of guard cells in plants
Guard cells, as previously stated, are bean/kidney-shaped cells found on plant epidermis. As such, they are epidermal cells, just like trichomes and pavement cells.
A stoma (a pore) connects each pair of guard cells, allowing water and gases to exchange. The thickening and shrinking of guard cells on the epidermis allows these pores (collectively known as stomata) to open and close.
Ultrastructure of guard cells
Guard cells in leaves of various plant species have been shown to contain considerable amounts of typical cell organelles (among other structures) with some distinctive traits. For example, the cuticle of guard cells is more permeable to water vapor than the rest of the leaf, which impacts their activities/functions.
Guard cells have a large number of ectodesmata which is a cuticle. This cuticle is more permeable to various polar substances. This is especially significant because the concentration of these substances influences the thickening and shrinkage of guard cells.
Various components, in varying amounts and orientation, can also be found in various types of guard cells such as fibrillar filaments which are found radially in the outer wall of dumbbell-shaped guard cells. This orientation, however, may change as the cells thicken and shrink. Aside from fibrils and microfibrils, a variety of other substances have been discovered in guard cells.
Lignin, in addition to cellulose, has been discovered in Zea mays and pectin has been found in the guard cells of many plants. Pectin and cellulose are gradually deposited into the plasmodesmata of young and developing guard cells (a thin layer of cytoplasm). However, it vanishes as guard cells mature, and the few that remain serve no purpose. Perforations in their walls allow relatively large organelles to pass through. Plasmids and mitochondria, for example, can pass through these perforations.
Organelles found in guard cells
- Microtubules
- Endoplasmic reticulum
- Lysosomes
- Lipid droplets
- Nuclei
- Plastids
- Mitochondria
- Ribosomes
- Chloroplast
Microtubules
Microtubules allow for movement and flexibility in guard cells. They also play a role in the formation and development of guard cells. In addition, microtubules also aid in the orientation of cellulose microfibrils.
Endoplasmic reticulum (ER)
There are two kinds of endoplasmic reticulum namely, the rough endoplasmic reticulum (RER) when it is attached to ribosomes, and the smooth endoplasmic reticulum (SER) when it is not attached to the ribosomes. The rough endoplasmic reticulum, in conjunction with ribosomes, aids in protein synthesis. ER also aids in the formation of vesicles and vacuoles within the cell.
Lysosomes
Lysosomes consist of a number of molecules that play a role in the cell’s proper functioning. These molecules include digestive enzymes such as lipases, endopeptidases, phosphatases, and nucleases that aid in the breakdown of large complex molecules and in the metabolism of guard cells.
Lipid droplets
Lipid droplets in guard cells serve as intermediates in the synthesis of wax and cutin.
Nuclei
They are located in the center of the guard cells and contain the genetic material. The opening and closing of stomata cause nuclei to change shape.
Plastids
The number of plastids in guard cells, such as chloroplasts, varies from plant to plant. While some of these plastids are not fully developed, others are and can perform functions such as photosynthesis. During the night, guard cells with functional chloroplasts consume a large amount of starch.
Mitochondria
Mitochondria is the organelle that generates energy. The presence of many mitochondria in guard cells in a leaf of a plant indicates that they have high metabolic activity.
Ribosomes
Ribosomes aid in the synthesis of proteins. They are either bound to the endoplasmic reticulum or free in the cytosol.
Chloroplast
They are double-membrane-bound organelles that vary in number between plants. Chloroplast assists the guard cells in photosynthesis and the production of a large amount of starch at night.
Structural adaptations of guard cells
- They have perforations through which solutes and water enter or leave the cells
- Guard cells contain chloroplasts
- Guard cells in leaves of plants contain hormone receptors
- Bean/kidney-shape
- Guard cells are surrounded by a thin, elastic outer cell wall
- Location
Guard cells have perforations through which solutes and water enter or leave the cells
Because the movement of solutes and water in and out of guard cells causes them to shrink or swell, this is one of the most important adaptations of guard cells. As a result, the stoma/pore, through which water and gases are exchanged, closes or opens.
They contain chloroplasts
Guard cells have been shown to be the only epidermal cells with chloroplasts, despite having fewer chloroplasts than mesophyll cells. As such, soma plant guard cells are photosynthetic sites where sugars and energy are produced. It’s worth noting that in some guard cells, the chloroplast is either absent or inactive.
Guard cells in plants contain hormone receptors
containing hormone receptors enable guard cells to react appropriately to changes in their surroundings Water scarcity in the soil, for example, causes the release of a hormone (abscisic acid (ABA)). This hormone is transferred from root cells to guard cell receptors, causing the guard cells to close the stoma to prevent excessive water loss.
Bean/kidney-shape
The bean or kidney shape of guard cells in a leaf of a plant makes it easy to close and open the stoma to regulate gaseous exchange and water release.
Guard cells are surrounded by a thin, elastic outer cell wall
This actually adds to the flow of water and solutes into and out of the cell.
Location
Guard cells can be found on either the upper or lower surface of the leaf, depending on the environment. This limits the amount of water that is lost to the environment.
Most aquatic plants have guard cells, and thus stomata, on the upper surface of the leaf, allowing more water to be released into the environment. However, in hotter/dryer climates, these cells are located on the lower surface of the leaf and are fewer in number.
Guard cells facts
- Guard cells regulate the opening and closing of a leaf pore (stoma).
- Each is a bean or kidney-shaped cell with varying degrees of rigidity in its wall.
- The inside wall is thickened and rigid, meanwhile the outside wall is thin and extensible.
- The paired cells swell as they absorb water, and the thin-walled region curves outwards, pulling the nonextensible thicker wall with it and opening the stomatal pore of a leaf.
- Water loss has the opposite effect, causing the guard cells to shrink and the pore to close.
- The transport of potassium ions (K+) into and out of the cell controls the movement of water in and out of the cell. K+ ions are pumped into the guard cells at sunrise, increasing their internal ionic concentration and causing water to follow by osmosis. As a result, the guard cells swell and the pore opens. K+ ions are pumped out of the guard cells to close the pore, causing them to lose water osmotically and shrink.
FAQ on guard cells in plants
When do guard cells swell?
Guard cells swell when they take in more solutes (K+ and Cl-) which creates a concentration gradient and allows the cells to take in water through osmosis and thereby becoming swollen.
What is the function of guard cells in a leaf?
Guard cells function in photosynthesis when they open the stoma to create room for the intake of carbon dioxide necessary for photosynthesis and the release of oxygen which is a byproduct of photosynthesis.
What do guard cells do in a leaf?
The function of guard cells in leaves is to help in transpiration, gaseous exchange, and photosynthesis through the mechanism of opening and closing of the stomata.
What are guard cells are responsible for?
Guard cells are responsible for the opening and closing of the stoma of a plant.
What type of tissue is guard cells?
Guard cells are specialized cells that are found in the epidermal tissue of a plant.
Do guard cells have a nucleus?
From the above diagram of the structure of guard cells, it can be stated that guard cells have a nucleus, which is located at the center of the cell and contains all the genetic material.
What mechanism causes stomata to open when the guard cells are in good conditions?
The closing and opening of stomatal guard cells involve the following mechanism first, is the intake of water in the presence of light. This process involves the intake of K+ and Cl– to create an environment for water to enter the cells thereby making it turgid and opening the stomata.
Secondly is the release of water in the absence of light and the closure of the stomatal opening to prevent further loss of water through transpiration. This process is achieved when the K+ and Cl– are released from the cell into the surrounding environment to create a loss of water through osmosis from the cells to the environment.
Where are guard cells located?
Two guard cells with stoma are located in the epidermal tissue of a plant
In the interaction between guard cells and a leaf opening which is the stomata, one can see the relationship between guard cells and the stomata when the guard cells swell via the intake of solutes (ions) in its environment, it opens the stomata. Also, when the solutes (ions) are released from the cell back into the environment, the guard cells become flaccid through the loss of water, and this results in the closure of the stomatal pore.
Are guard cells dermal tissue?
No, guard cells are not dermal tissue, but guard cells are found in dermal tissues. This is because biology has taught us that cells are not tissues but different cells make up a tissue.
What are guard cells in biology?
Guard cells in biology or guard cells in botany are specialized cells that are located in the epidermis of the leaf of a plant. The shape of guard cells is sausage-like or kidney-shaped and they become turgid when they take water and flaccid when they release water.
Are guard cells vascular tissue?
Guard cells are not vascular tissue even though they are found in a plant. This is because guard cells are responsible for the stomatal opening and closing while vascular tissue which comprises of xylem and phloem and these vascular bundles are responsible for the transport of water and nutrient and not for the intake of the water.
What evidence suggests that ABA from roots can signal guard cells to close?
Roots have been shown to produce more of this hormone under environmental conditions such as drought or increased salinity in the soil. The detection of this hormone by guard cells causes changes in the intake or removal of ions from the cells, causing the stoma to open or close.
When there is a high concentration of solutes outside the cell, water is forced out through osmosis, lowering the turgor pressure of the guard cells. As a result, the aperture closes, preventing the cells from losing any more water.
Why are guard cells important?
Guard cells are important because they help in the whole life process of a plant, through the intake of water and CO2 that is really needed for the production of the plant’s food through photosynthesis. Not only in the aspect of photosynthesis are guard cells important, but also in the transpiration of water in plants and in the exchange of gaseous substances between the plant and its environment.
What do guard cells control?
Guard cells control the opening and closing of the stomata.
How many guard cells per stoma?
There is a pair of guard cells per stoma, which means that there are 2 guard cells that surround a stoma.
Do guard cells have chloroplasts?
Guard cells have chloroplast that assists in photosynthesis and the production of a large amount of starch at night. Because of the presence of chloroplasts, guard cells are known as photosynthetic sites.