The Definition, Function, Structure Of Stomata On Plants (Guard Cells)

What Is A Guard Cell?

Guard cells consist of the two cells that surround the stoma. These cells are shaped like beans and play a crucial role in the gaseous exchange between plant leaves and their surrounding environment.

They are otherwise known as epidermal cells and assist in the regulation of the opening of pores on a plant’s leaves (the stoma). Additionally, the guard cells are also the cells that channel water from the leaves into the surrounding environment. 

In this sense, guard cells play a pivotal role in the photosynthesis process as they regulate the channeled materials that are necessary for a plant to photosynthesise.

Asides from regulating this exchange of gasses, as well as releasing water from the leaves of the plant, they also contain chloroplasts which make them the focal site where photosynthesis can occur.

The factors the influence the activities of these guard cells include the following:

• Temperature
• Light
• Humidity
• Carbon Dioxide
• Hormones
• Potassium ions

The word stoma derives from the greek word for mouth, and although stomata are usually found within the leaves of the plant, they are also found on the stem. 

The Structure Of Guard Cells

Guard cells are located on the epidermis of the plant. In this sense, they are similar to pavement cells and trichomes, which are also considered to be epidermal cells.

A stoma is located in between each pair of guard cells and this is where gasses and water are channeled/exchanged. The pores are collectively known as the stomata and they open and close by proxy of the guard cells shrinking on the epidermis. 

The amount of stomata contained in the leaves of a plant depends on the variety of plant as well as its surrounding environment. 

The Ultrastructure Of Guard Cells

In different specimens of plant, the guard cells will contain a differing amount of cell organelles. These organelles will also have their own characteristics that are specific to that particular section.

For example, the cuticle of a guard cell is far more easily able to absorb water vapor which inherently influences its activity/overall functionality. 

Guard cells have been proven to contain a variety of ectodesmata. This cuticle has also been proven to be far more permeable to a variety of polar substances.

This is crucial in regard to the concentration of specific substances that will influence the shrinking and expansion of the guard cells. 

The cuticle on guard cells tends to be far thicker on the outer parts and the permeability of the cuticle is also greatly dependent on the overall chemical composition of these cells. 

In younger, developing guard cells, the pectin and cellulose are deposited gradually into the plasmodesmata which consists of a thin layer of cytoplasm.

However, these will dissipate as the guard cells begin to mature and any that remain will not serve a functional purpose.

Perforations on the walls also allows large organelles to pass through and this allows for mitochondria and plastids to easily pass through without any issues arising. 

There are a variety of components that can be found in the vast ranging varieties of guard cells. Within dumbbell-shaped cells, the fibrils will be contained within the outer wall.

This specific orientation may actually change as the walls thicken or shrink. 

Asides from the microfibrils and fibrils contained within the guard cells, there are also other substances that are contained within them.

In certain plants like Zea mays, lignin has also been identified as well as cellulose and pectin has also been identified within the guard cells of a number of plants. 

Examples Of Organelles Contained In Guard Cells

Endoplasmic Reticulum 

The amount of endoplasmic reticulum that is present within guard cells is high and this culminates in protein synthesis. Endoplasmic reticulum is also included in the formation of vesicles and vacuoles.

Microtubules

The microtubules are useful in orienting microfibrils and cellulose. They also assist in the building and development process of the guard cells.

Lysosomes

The lysosomes include a variety of molecules that assist in the overall functioning of the guard cells. These lysosomes include endopeptidases, phosphates, DNAse, and lipase.

Nuclei

The nuclei are located at the heart of the guard cells. They have been proven to adapt their overall shape during the opening and closing of the stomata. 

Lipid Droplets

Guard cells, the lipid droplets are the middleman in the synthesis of cutin and wax.

Plastids

The plastids and chloroplasts in guard cells vary in numbers depending on which plant you are studying.

Whilst many of the plastids may be underdeveloped, some are extremely well developed and are thus, more than capable of assisting in the process of photosynthesis. 

Mitochondria

There is a high amount of mitochondria that is contained in guard cells in comparison to mesophyll cells which indicates high metabolic activity within these cells. 

What Are Stomata?

Stomata refers to the pores on the epidermis and the surrounding guard cells. Subsidiary cells surrounding the guard cells are useful in classifying the various types of stomata.

Whilst the stoma is the channel where gasses are exchanged, the opening and closing of these pores is regulated by the surrounding guard cells situated on the epidermis. 

Different Classifications Of Stomata

Stomata are usually identified and classified on the basis of structure and overall distribution. There are a number of stomata located on a plant and these are classified via the following placements: 

Apple type (Mulberry)

These stomata are usually located on the lower surface of the leaf. In this sense, they can be located on walnut trees, apple trees and peach trees amongst other plants. 

Water Lily

The stomata on the water lily are located on the upper epidermis of its leaves. These can usually be located in the same place on a variety of aquatic plants. 

Potato Type

This variety of stomata are located on the lower surface of the leaves whilst some may be found on the upper surface of the leaves. In this sense, they are located in both anisostomatic and amphistomatic leaves.

Oat Type

This variety of stomata are often located on the leaves and are often evenly distributed on the lower and upper surfaces of these leaves.  

Potamogeton Type

These stomata are usually non-functional and located on aquatic plants that are submerged. 

Overall Structure

The following are examples of the differing structures that surround and accompany the guard cells and stoma:

Anomocytic

The anomocytic are a limited number of subsidiary cells that are situated around the stomata. In this majority of instances, these subsidiary cells will look exactly like epidermal cells.

Cruciferous 

These subsidiary cells surround the stoma. There are three types of these subsidiary cells that vary in size depending on the plant itself. 

Paracytic

The stoma is also surrounded by two paracytic, subsidiary cells that are usually arranged in parallel to the guard cell’s axis. 

Graminaceous 

The guard cells appear dumbbell-shaped when in graminaceous form, and have subsidiary cells arranged parallel to them. 

Cyclocytic

This structure consists of four subsidiary cells that are surrounding the guard cells. 

Diacytic

The stoma in this instance includes two guard cells and the surrounding subsidiary cells, including the stoma itself, are positioned at a right angle in conjunction with the guard cells. 

It is important to note that up to 90% of transpiration will occur via the stomata. It is also important to note that a lot of water is lost during the process of cuticular and lenticular transpiration. 

Whilst it is only a small amount of water that is absorbed (approximately 2%), this is all that is required for photosynthesis in most plants. 

Adaptations Of Guard Cells

Guard cells will be able to implement a variety of adaptations that contribute to their overall functionality. These adaptations include the following processes:

• Perforations through which water can enter or leave the cells. This process is one of the most crucial adaptations that can occur in guard cells as the movement of water in and out causes them to shrink or expand accordingly. This results in the stoma opening or closing and the relevant amount of water and gasses being exchanged. 
• Guard cells contain chloroplasts, and whilst they may not contain as many of these chloroplasts as the mesophyll cells, the guard cells have been proven to be the only epidermal cell that does contain chloroplasts. In this sense, the guard cells are hotspots of photosynthesis where energy and sugar can be appropriately produced and distributed throughout the plant. It is also worth considering that chloroplast can be inactive in some guard cells. 
• Guard cells contain hormone receptors which allow them to respond accordingly to any changes in their surrounding environment. In this instance, any lack of water within the soil causes hormones like abscisic acid to be released and this hormone is then transported to the receptors on the guard cells which causes the stomata to close, preventing any additional water loss. 
• The kidney-bean shape of guard cells is notably useful for the closing and opening of the stoma as this enables them to properly regulate the exchange of gasses and water.
• Guard cells are also encompassed by a thin, elasticated layer on the outer wall. This layer offers a key contribution to the movement process and ensures that water is able to enter and exit the cell easily. 
• The location of the guard cells depends on the surrounding habitat of the plant, however, they may be located on either the lower or upper surface of the leaves. This enables the plant to appropriately regulate any water that exits into the surrounding environment. In the majority of aquatic plants, the guard cells will be located on the upper surface as this enables more water to be released. In contrast, plants located in hotter climates will have guard cells located on the lower surface to reduce water loss. 

The Closing And Opening Mechanism Of Guard Cells

One of the key functions of the guard cells is their ability to control the opening and closing of the stoma. This is one of the most crucial functions of these cells as it is imperative to appropriately control the closing and opening of the pores.

Whilst opening the stoma will allow water to be released into the atmosphere, it also allows for carbon dioxide to be transmitted into the plant in order for photosynthesis to occur. In this sense, guard cells play a key role in the photosynthesis process. 

Factors that can influence the mechanics of these guard cells include light intensity and hormones and these factors can influence the shrinkage and swelling process of the guard cells which inherently impacts their ability to open or close.

With regard to pore opening, these factors also influence the amount of water that the cell intakes, inherently causing the guard cells to inflate.

The swelling of these cells results in the opening of the stomata which allows for gasses to be exchanged and water to be transported accordingly. 

Whilst this process may appear simplistic, the path of signals that influence the activity of these guard cells is not easy to fully understand.

Because of this, a variety of theories have been derived in order to describe this overall process. Irrespective of which theory you believe, there are several key aspects that are wholly understood. 

What Are These Theories?

The theories that aim to explain the overall movement of water in and out of guard cells include the following: 

pH Theory 

This theory stipulates that an increase in the concentration of hydrogen within the plant will cause a decrease in the pH levels which inherently results in glucose-1-phosphate being converted into starch. 

Starch-Sugar Theory

This theory states that the conversion of starch into sugar inherently causes the osmotic capabilities of the plant to increase, causing water to be drawn into the guard cells. 

Active K+ Transport Theory 

This theory states that any increase in potassium ions, resulting from the conversion of starch to phosphoenolpyruvate, causes malic acid to form. 

Proton-Potassium Pump Theory

A series of events leads to potassium ions forming and being transported into the relevant guard cells during the daytime, increasing the overall concentration of solute and drawing water into the cell inherently. 

Sensing And Signaling Carbon Dioxide

One of the key factors that influences the shrinkage and swelling of the guard cells is the amount of carbon dioxide that is concentrated within the environment.

In the instance that there is a vast amount of carbon dioxide concentrated within the atmosphere, anion channels become activated which inherently causes potassium ions to exit the cells.

During this process, chloride is also released which is then reused in the depolarisation of the plant’s membrane. 

As solutes exit the cell, the concentration outside of the cell will increase compared to the inside of the cell, this results in water being forcefully removed from the cell via a process of osmosis.

This inherently leads to the cell shrinking and closing the pores. When there is a low amount of carbon dioxide in the atmosphere, the reverse process will occur. 

Sensing And Signaling Abscisic Acid 

In a variety of plants, abscisic acid will have many purposes that range from assisting in controlling the germination process to impacting the functionality of guard cells.

When there is a drought in the surrounding environment, roots will produce ABA in higher quantities.

Whenever this hormone is detected by guard cells, changes can occur in regard to the intake or removal of ions from the guard cells which inherently causes the stomata to open or close.

When there are high amounts of ABA within a plant, an efflux of anions and potassium will occur through the relevant channels.

The importation of potassium will also become inhibited which inherently prevents ions from moving into the cell, causing a higher concentration of solutes to be contained outside the cell. 

This higher concentration of solute externally, causes water to be removed from the cell via osmosis which also reduces the turgor pressure within the guard cells.

In turn, causing the aperture to become closed and preventing the cells from losing any additional water.

Typically speaking, under normal environmental circumstances, the stomata will remain open during the day to ensure that carbon dioxide is properly absorbed and will close during the night to allow for photosynthesis to occur.

Water will also enter via the subsidiary cells at night, from the guard cells, which will cause them to reduce the turgor pressure and become flaccid, closing the stoma by proxy. 

Conclusion

To conclude, guard cells will use osmotic pressure and processes in order to open and close the stomata, which allows the plant to appropriately regulate the amount of water within it.

It is important to note that in order for plants to produce enough energy and maintain their overall cellular function, they will need to undergo the innate process of photosynthesis. 

The stoma is critical in this process and multiple stomata will be loaded on the upper or lower layers of the leaves and stems in order for this process to occur.

This is because an opened stoma will facilitate the photosynthesizing process by allowing light to enter into the cells and triggering the process by proxy.

These stomata also allow for carbon dioxide to enter the plant which is key to producing an appropriate amount of energy.

Stomata also allow for oxygen to be expelled into the surrounding environment by proxy of the photosynthesis process, ensuring that we are able to breathe and live our lives. 

The guard cells are therefore key in ensuring that the stoma is able to open and close accordingly and are imperative in facilitating the process of photosynthesis.

However, having an open stoma does come with a negative side effect in the form of excessive water loss. It has been stipulated that up to 95% of all water loss within plants will occur through the stomata.

And thus, it is important that the guard cells play their role effectively to maintain the appropriate balance to allow for light and gas to pass between the cells. Otherwise, the plant runs the risk of becoming dehydrated and ultimately dying. 

Luckily, this problem is easily averted by the guard cells. This is because these pairs of cells surround each opening of the stomata.

In order for the stoma to open, these cells must be triggered by chemical or environmental signals such as direct sunlight or a high level of carbon dioxide within the cell.

As a response to these signals, the guard cells will absorb potassium, sugar and essential chloride ions or solutes via their membranes.

This overall increase in solutes will induce the transmission of water across the membrane of the guard cells and as the overall volume of these guard cells expands, they will take their kidney-bean form. 

Whilst expanding, they will also open the stoma in the middle and once they have fully inflated, the stoma will remain open to engage in the exchange of gasses and water between the plant’s environmental surroundings and its cells.

The pores of the stoma will close when the opposite process occurs, and this is triggered by excessive loss of water via the stoma that will occur during a drought.

The chemical reactions that occur will signal ions and water to exit the guard cells and as these solutes exit, these cells will shrink which inherently closes the stomata. 

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