Stomata with Diagram, Structure, Types and Mechanism of Stomata

Stomata are tiny openings or pores found on the surfaces of plant leaves, stems, and other green parts. They play an important role in gas exchange, allowing plants to take in carbon dioxide for photosynthesis and release oxygen and water vapor. Stomata are surrounded by specialized guard cells that can open and close to regulate the rate of gas exchange and control water loss through transpiration.

Stomata Definition

Stomata are small pores or openings found on the surface of plant leaves and stems, facilitating gas exchange and regulating transpiration.

What is Stomata?

Stomata are pores or openings found in the epidermis of leaves, stems, and other organs of plants that control the rate of gas exchange. The pores are surrounded by a pair of specialized parenchymal cells called guard cells, which regulate the size of the stomata opening. Air, which contains oxygen and is used for respiration, and carbon dioxide, which is used for photosynthesis, flows through the stomata by gas diffusion. Water vapor diffuses through the stomata into the atmosphere, a process called transpiration.

Stomata are present in the sporophytic generation of all terrestrial plant groups except liverworts. In vascular plants, the number, size, and arrangement of stomata vary widely. Dicotyledonous plants generally have more stomata on the underside of the leaf than on the top of the leaf. Monocots such as onions, oats, and corn can have about the same number of stomata on both leaf surfaces.

Diagram of Stomata

The well-labeled diagram of the stomata is shown below:

Characteristics of Stomata

Stomata are small openings or pores surrounded by specialized guard cells. They open and close to regulate the rate of gas exchange and control water loss through transpiration. The characteristics of stomata are as follows:

• Stomata can vary in shape, such as oval or kidney-shaped, and may be arranged in a regular pattern on the leaf surface. Size varies by species, with a peak-to-peak length of between 10 and 80 µm and a width of between a few and 50 µm.
• Stomata are surrounded by two specialized guard cells that can swell or shrink, opening and closing the pore. These cells control the rate of gas exchange and regulate water loss through transpiration.
• Stomatal opening is influenced by environmental factors, with light being a key trigger. During the day, when photosynthesis occurs, stomata open to take in CO2.
• Stomata also help regulate water loss by closing in response to conditions like high temperatures or drought, reducing the rate of transpiration.
• They are more numerous on the lower surface of leaves, which helps minimize water loss due to exposure to direct sunlight.
• Leaves with stomata on the upper and lower side of the leaf are called Amphistomatous leaves. Leaves with stomata only on the underside are Hypostomatic and leaves with stomata only on the upper side are Epistomatic.
• In addition to photosynthesis, stomata play a role in respiration, allowing oxygen to enter plant cells and carbon dioxide to exit during nighttime.
• Stomatal behavior is sensitive to environmental factors like light intensity, humidity, and carbon dioxide concentration, allowing plants to adapt to changing conditions.

Structure of Stomata

The structure of the stomata consists of a kidney-shaped epidermal cell with an opening in the center called a pore. Stomata is surrounded by a pair of specialized parenchyma cells that are guard cells. They are responsible for regulating the size of the opening and thus protecting the plant from water loss. Each pore of the stoma has a similar structure. The shape of the cells may differ in part, but the mechanism and composition remain the same. The four essential parts of the stoma are:

Epidermal Cells of Stomata

• These are the cells of the plant that provide physical and mechanical support to the plants.
• Epidermal cells are irregularly shaped and form the outermost layer of the plant.
• These cells are present as the building blocks of a plant.
• Since they are stiff, they provide scope for the stomatal pores to close again.

Subsidiary Cells of Stomata

• These are present in parts throughout the plant, especially around the stomata.
• Unlike epidermal cells, subsidiary cells are soft, allowing guard cells to expand and open pores.
• They essentially form a working space for the stoma.
• Without them, the pores won’t be efficient to work for a long period of time.

Stoma Pore of Stomata

• The pore is the main opening through which all of the gaseous exchanges, vapor exchanges, and absorption from the atmosphere take place.
• Without the pores present, all of the functioning of Stomata would be completely useless.

Guard Cells of Stomata

• These are the most important part of a Stomata.
• These are kidney-shaped cells, with a thick inner cell wall.
• Since the entire function of the stomata depends on their opening and closing, and only the guard cells do this, they are very important for the general maintenance of the plant.

Types of Stomata

Based on the size, shape, and arrangement of the subsidiary cells that surround the two guard cells, different classifications of stomata types exist.

In Dicots, several different types of stomata occur such as

• Actinocytic: Actinocytic stomata are those stomata that have guard cells surrounded by at least five radiating cells forming a star-shaped circle.
• Anisocytic: Anisocytic stomata have guard cells between two larger and one much smaller subsidiary cell.
• Anomocyte: Anomocyte stomata have guard cells surrounded by cells of the same size, shape, and arrangement as the rest of the epidermal cells.
• Diacytic: Two-celled stomata have guard cells flanked by two subsidiary cells, each surrounding one end of the opening and joining opposite the middle of the opening.
• Hemiparacytic: Hemiparacytic stomata are lined with a single subsidiary cell, which differs from the surrounding epidermal cells in that it is parallel in length to the stoma opening.
• Paracytic: Paracytic stomata have one or more subsidiary cells parallel to the opening between the guard cells. These subsidiary cells may or may not extend beyond the guard cells.

There are different types of stomata in Monocots, such as

• Graminoid: Graminoid stomata have two guard cells surrounded by two lens-shaped accessory cells. The guard cells are narrowest in the middle and bulbous at both ends.
• Hexazide: Hexazide stomata have six subsidiary cells surrounding the two guard cells, one at each end of the stoma opening, one adjacent to each guard cell, and one between the last subsidiary cell and the standard of the epidermal cells.
• Tetracytic: Tetracytic stomata have four subsidiary cells, one at each end of the opening and one adjacent to each stomata.

In ferns, four different types

• Hypocytic: Hypocytic stomata have two guard cells in one layer with only ordinary epidermis cells but with two subsidiary cells on the outer surface of the epidermis.
• Pericytic: Pericytic stomata have two guard cells that are entirely encircled by one continuous subsidiary cell.
• Desmocytic: Desmocytic stomata have two guard cells that are entirely encircled by one subsidiary cell that has not merged its ends.
• Polocytic: Polocytic stomata have two guard cells that are largely encircled by one subsidiary cell, but also contact ordinary epidermis cells.

Opening and Closing of Stomata

When conditions favor the opening of the stomata (e.g., high light intensity and high humidity), the proton pump pushes protons (H⁺) out of the guard cells. This means that the electrical potential of the cells becomes more and more negative. To maintain this internal negative voltage so that potassium ions are not trapped, negative ions balance the potassium influx. In some cases, chloride ions enter the cells. This increases cell volume and turgor pressure. Then, because of rings of cellulose microfibrils that prevent the width of the guard cells from swelling, and thus only allow the extra turgor pressure to elongate the guard cells, creating an open pore through which gas can diffuse.

When the roots begin to sense a water shortage in the soil, abscisic acid (ABA) is released. ABA binds to receptor proteins in the guard cells’ plasma membrane and cytosol, which first raises the pH of the cytosol of the cells and cause the concentration of free Ca²⁺ to increase in the cytosol. This causes the chloride (Cl⁻) and organic ions to exit the cells. Second, this stops the uptake of any further K⁺ into the cells and, subsequently, the loss of K⁺. The loss of these solutes causes an increase in water potential, which results in the diffusion of water back out of the cell by osmosis. This makes the cell plasmolyzed, which results in the closing of the stomatal pores.

Functions of Stomata

The main functions of stomata are as follows:

• Stomata facilitate the exchange of gases, allowing plants to take in carbon dioxide (CO2) for photosynthesis and release oxygen (O2) and water vapor (H2O) as byproducts.
• Stomata release water vapor through a process called transpiration, which helps regulate the plant’s temperature and maintain moisture levels in the plant.
• Stomata enable oxygen (O2) to enter plant cells, supporting cellular respiration, where energy is produced by breaking down glucose.
• Stomata can open and close to control the rate of transpiration, reducing water loss during dry or hot conditions and preventing dehydration.
• Stomatal closure can act as a defense mechanism against pathogens and pests by limiting their entry into plant tissues.
• Stomata can also facilitate the uptake of essential mineral nutrients, such as potassium, from the soil through ion exchange.

FAQs on Stomata

1. What is Stomata?

Stomata are small openings or pores found on the surfaces of plant leaves, stems, and other green tissues that facilitate gas exchange and water regulation.

2. Where is Stomata Located?

Stomata are mainly located on the surfaces of plant leaves, with a higher concentration on the lower leaf surface. They can also be present on stems and other green parts of the plant.

3. What is Function of Stomata?

The function of stomata is to regulate the exchange of gases, allowing plants to take in carbon dioxide for photosynthesis and release oxygen and water vapor.

4. How do Stomata Open and Close?

Stomata open and close due to changes in turgor pressure within specialized guard cells that surround them. When guard cells gain water, they swell and open the stomatal pore; when they lose water, they shrink, closing the pore.

5. What Factors Influence Stomatal Opening and Closure?

Stomatal behavior is influenced by environmental factors such as light intensity, humidity, temperature, and carbon dioxide concentration. Plant hormones like abscisic acid (ABA) help in regulating stomatal closure during drought conditions.