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Stomata

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Stomata

Let’s do a breathing exercise- take a deep breath in and a deep breath out. Then, do it a few more times. Well done. You’ve breathed out some carbon dioxide and in some oxygen. A plant’s stomata do a similar job, except they take in carbon dioxide for the plant and expel oxygen. Stomata are pores on the leaf surface that allow for gas exchange and help to control water loss.

What are stomata in plants?

Stomata are openings or pores that allow for the exchange of gas between the plant tissues and the atmosphere. In particular, a plant takes in carbon dioxide through its stomata and expels oxygen, a byproduct of photosynthesis. Stomatal openings are found in the epidermis of the plant or, in other words, the plant dermal tissue.

Stomata are often found on the surfaces of leaves and some stems. Leaves, being the main site of photosynthesis, must have access to carbon dioxide. Stomata allow for this intake, thus making them an important addition to the leaf surface.

The singular of stomata is "stoma" or sometimes "stomate".

The definition of stomata in biology

So how exactly would you describe the term “stomata” to your AP biology buddy? Well, stomata are most notably pores, that can be opened or closed, resting on plant leaves (sometimes on stems) that allow for gas exchange between the plant and the surrounding atmosphere.

How did stomata evolve?

Stomata are an important stage in the evolution of plants. Scientists believe that stomata predate even the vascular system, which is a feature of many of the plants that make up our ecosystems! Early land plants evolving from aquatic species had to face the biggest challenge: how to not dry out in a terrestrial environment. As a result, plants evolved waxy cuticles that helped reduce the amount of water that could be lost as water vapor through the plant. However, these cuticles also prevented gases from diffusing across the plants' membranes for photosynthesis. What was the solution? Stomata, of course!

Stomata allowed plants to control gas exchange between their membranes and the air, despite having cuticles to prevent drying out. Because water vapor can also pass through stomata, they are not always open. Stomata open and close based on environmental cues, which helps to prevent excess water loss. All plants beside the liverworts have stomata! That includes mosses, hornworts, the vascular plants (Fig. 1).

Function and structure of stomata

As we have discussed throughout this plant biology module, the structure of plants and the function they serve are studied together, as they are inextricably linked. The basic function of stomata is to allow for gas exchange between the air and the plant, allowing in carbon dioxide and releasing oxygen.

Stomata and transpiration

As a result of the direct opening of stomata, a process called transpiration occurs. Transpiration is the evaporation of water through the stomata. Transpiration creates a water pressure difference in plants, helping to drive water up the xylem tissue of vascular plants.

Transpiration is the evaporation of water through the plant body, particularly through the stomatal openings.

Transpiration also means that a plant is losing water. Approximately 90% of all water lost in plants is lost through the stomata, which are only 1% of a leaf's surface area!1 This means that controlling the number of stomata, when a plant opens and closes its stomata, and the density of stomata on leaves can help a plant prevent water loss.

Stomatal structure

Stomata are found in the epidermis of leaves and sometimes stems. Surrounding the stomatal pores are modified epidermal cells known as guard cells. Guard cells tend to be classified as either appearing “kidney” shaped or “dumbbell” shaped.

Guard cells have cell walls that are not uniform but can expand when water enters them. They have cellulose (the component fortifying plant cell walls) microfibrils that help the cells expand and contract depending on their turgidity. Guard cells also contain chlorophyll and chloroplasts, which make them capable of photosynthesis. The presence of chloroplasts also helps guard cells detect changes in light, which can influence whether they are open or closed.

Surrounding the guard cells are subsidiary cells, which vary in function but may offer mechanical or storage support to the guard cells2. The number of subsidiary cells surrounding guard cells, their sizes, and their shapes vary from plant to plant.

Stomata: where to find them?

Most stomata are found on the dermal tissue of a leaf. This means they exist in the outer layers of a plant and its tissues. Stomata occur both on the undersides of leaves and the top of them as well.

In biology, the underside of the leaf is known as the abaxial surface and the top is known as the adaxial surface.

Depending on the species or type of plant, you may observe stomata on both the abaxial and adaxial surfaces, or on one or the other. For example, in most tree species, stomata are found on the underside, or abaxial surface, of the leaves.

How do stomata open and close?

Stomata allow the exchange of gases for photosynthesis and control the loss of water, as we have discussed. What factors, then, might influence whether stomates remain open or closed? If you guessed concentrations of CO2, changes in light, or humidity (water content) in the air, then you'd be right. All of these may be internal or external signals that a stoma should open to continue the exchange of gases or close to limit water loss.

A stoma may open due to:

  • Increase in the amount of light

  • Increase in humidity in the atmosphere

  • Low levels of CO2 in the mesophyll tissue surrounding the stomatal pore

A stoma may close due to:

  • Decrease in the amount of light

  • Decrease in humidity in the atmosphere

  • High levels of CO2 in the mesophyll tissue

Turgor pressure, guard cells, and stomata

When environmental cues are present, the guard cells of stomata undergo a change in turgor pressure to either open or close. When the stomata are closed, the guard cells are flaccid. However, the stomatal opening is caused by the movement of water into guard cells, causing them to become turgid, and curve outwards, allowing a direct path to the mesophyll tissue below (Fig. 3).

What causes a change in turgor pressure? The environmental signal that is detected by the stomata will cause the guard cells to pump out protons or H+ ions. This action will then cause potassium ions (K+) from the surrounding cells and chloride ions (Cl-) from the surrounding cells to enter the guard cells. As a result, these ions create a negative gradient that causes water to flow into the guard cells, increasing the turgor pressure and making them turgid.

Stomatal adaptations for preventing water loss

As we have discussed, the presence of stomata is important for gas exchange. However, we also learned that stomata offer an easy passage for water out of a plant through transpiration. Plants control the amount of water they lose through stomata through different mechanisms or adaptations. Controlling the amount of water lost through transpiration means controlling the stomata. One way a plant manages its stomata is by opening and closing them at strategic times.

Plants also control the number of stomata. They may do this by shedding extra leaves, or if a plant faces long drought periods it may even decrease the stomatal numbers on new leaves. Some plants have their stomata in crevices called stomatal crypts, which are indentations on the surface of leaves. The stomata are at the bottom of these crypts (Fig. 4).

Opening and closing stomata

Most plants open their stomata during the day when the sunlight is present so that CO2 gas that enters the plant can be used for photosynthesis. The plant, however, must respond to extreme dryness or heat in the atmosphere that can cause water stress.

Abscisic acid

Plants respond to sudden water stress brought on by high temperatures or increased drought by closing their stomata. One plant hormone, in particular, abscisic acid, helps aid in a plant’s rapid response.

If the water potential is low (negative) in the mesophyll tissues of leaves, the plant will activate an abscisic acid response. This means abscisic acid will signal the plant to close the guard cells, preventing further water loss through transpiration.

Crassulacean acid metabolism (CAM) plants

Most plants open their stomata during the day when the sunlight is ample for photosynthesis to take place. However, if a plant lives in an arid climate like the desert, then opening stomata during the day is a recipe for excess water loss. As a result, some plants that live in hot, dry environments have developed a Crassulacean Acid Metabolism (CAM) which lets them open stomata during the cool night and keep them closed during the heat of the day.

At night, the stomata open, and CAM plants concentrate carbon dioxide in the mesophyll tissue, converting it into a preliminary carbon compound used in the Calvin cycle of photosynthesis. Then, during the day, the plant has carbon to carry out photosynthesis without opening stomata.

Stomata - Key takeaways

  • Stomata are openings on the surfaces of leaves and some stems that allow for gas exchange between the plant tissues and the surrounding air.
  • Providing a passage for water to evaporate through, stomata serve as the main source of water loss by transpiration in a plant.
  • The stomata are made up of modified epidermal cells that become the guard cells, or the doors that open and close the stomata and supporting subsidiary cells.
  • Stomata are open when guard cells are turgid and closed when guard cells are flaccid. Stomata react to environmental signals to determine whether they need to open or close.
  • Plants control excess water loss by opening and closing stomata and by changing the number or density of stomata on the leaf surface.

References

  1. AP Biology by Deborah T. Goldberg
  2. Gray, Antonia, Liu, Le, and Facette, Michelle. Flanking Support: How Subsidiary Cells Contribute to Stomatal Form and Function. Frontiers in Plant Science (11), 2020, edited.

Frequently Asked Questions about Stomata

The main function of stomata is to allow a plant to exchange gases with the surrounding atmosphere. In particular, stomatal openings allow the uptake of carbon dioxide, a key ingredient in photosynthesis. They also allow for the plant to release the oxygen gas that is a byproduct of photosynthesis. 


Stomata also play a role in controlling water loss. Because stomata provide a route for water to evaporate (transpiration), they are regulated by plants. Regulation of stomata includes opening and closing them at strategic times, controlling how many stomata exist on leaf surfaces, and adaptations that allow for less water loss (stomatal crypts). 

No, not all plants have stomata. Although, most plants do have stomata for gas exchange. The evolution of stomata precedes the development of the vascular system. This means a number of nonvascular plants have stomata (mosses and hornworts) on their sporophyte (diploid) structures. The liverworts do not have stomata. 


All species of known vascular plants have stomata. 

Stomatal openings are made from modified epidermal cells on the outer layer of dermal plant tissue.  Therefore, stomata are pores located on the surface of leaves and sometimes on stems as well. 


Stomata are found on both the underside (abaxial side) and on the top (adaxial side) of leaves. Some leaves have stomata only on one side, and some have stomata on both sides. 

Stomata are small pores or openings on the leaf surface (sometimes stems as well) that can be opened or closed to allow for gas exchange between a plant, and its atmosphere. In particular, plants need carbon dioxide for photosynthesis and must expel oxygen gas as a byproduct of photosynthesis.


 Stomata consist of two modified epidermal cells known as guard cells that can open and close to control gaseous exchange. Guard cells also have supporting cells that vary in shape and size, known as subsidiary cells. 

When environmental signals are present, the guard cells of stomata undergo a change in turgor pressure to either open or close. When the stomata are closed, the guard cells are flaccid. However, the stomatal opening is caused by the moving of water into guard cells, causing them to become turgid, and curve outwards, allowing a direct path to the mesophyll tissue below. 


More specifically, when stomata respond to an environmental signal, they pump out protons or Hions of guard cells. As a result, potassium and then chloride ions move into the guard cells. When these ions move in, they create a negative gradient with the surrounding cells, causing water molecules to also fill the guard cells and make them turgid. 

Final Stomata Quiz

Question

Stomata are ______ in the surface of plant leaves (and sometimes stems) that allow for gas exchange between the plant and the atmosphere. 

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Answer

pores/ openings

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Question

True or False: Stomata are found only on the surfaces of leaves. 

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Answer

False: Stomata can be found on the surfaces of stems as well. Although, they mostly exist on leaf surfaces.

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Question

_______ is the evaporation of water through the stomata. 


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Answer

Transpiration

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Question

True or False: All plants have stomata.

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Answer

False: Stomata are found on most terrestrial plants; however, liverworts do not have stomata. 

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Question

Which of the following is true about plant evolution?

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Answer

Plants evolved waxy cuticles to prevent water loss and stomata to provide a passage for gas exchange. 

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Question

Transpiration creates a water pressure difference in plants, helping to drive water up the ____ _____ of vascular plants.


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Answer

Xylem tissue

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Question

About ____ % of the water that a plant loses is lost by transpiration through the stomata. 

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Answer

2

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Question

When the guard cells are turgid (filled with water) they ____.

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Answer

Open

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Question

Which of the following would most likely cause a stoma to open? 

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Answer

Detection of increase in light levels. 

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Question

Which of the following would most likely cause a stoma to close? 

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Answer

Detection of increase in light levels. 

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Question

When a stoma receives an environmental signal to open it will first...

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Answer

Pump out protons into surrounding cells

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Question

True or False: After the protons (H+) are pumped out of guard cells in response to an environmental signal to open the stomata, the presence of potassium and chloride ions increases. 

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Answer

True

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Question

What are some of the ways plants can prevent water loss?

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Answer

Controlling the opening and closing stomata

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Question

CAM plants will _____ their stomata at night and _____ them during the day.

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Answer

Open; close 

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Question

_____ ____ is a plant hormone that will help a plant respond to water stress signals by triggering the closing of stomata. 

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Answer

Abscisic acid 

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