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.
The definition of stomata in biology
In particular, a plant takes in carbon dioxide (CO2) through its stomata and expels oxygen (O2), a byproduct of photosynthesis. Stomatal openings are found in the epidermis of the plant or, in other words, the plant's dermal tissue.
Stomata are openings or pores that allow for the exchange of gas between the plant tissues and the atmosphere.
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".
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.
Stomata and transpiration
As a result of the direct opening of the 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.
Structure of stomata
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.
Stomata function: how do stomata open and close?
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 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.
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.
Stomata in plants: the 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.
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.
- Plantscontrol excess water loss by opening and closing stomata and by changing the number or density of stomata on the leaf surface.
References
- Deborah T. Goldberg, AP Biology, 2008
- Gray, Antonia, Liu, Le, and Facette, Michelle. Flanking Support: How Subsidiary Cells Contribute to Stomatal Form and Function. Frontiers in Plant Science (11), 2020.
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