Stomata: Structure, Diagram, Functions, Types & Mechanism

Stomata Structure Overview

Stomata, the tiny openings on the surface of green plants, play a vital role in facilitating the exchange of water and carbon dioxide between the plant and its surrounding atmosphere. When observed under a microscope, these stomata are easily visible. Each individual opening is referred to as a stoma, and they are present in the epidermis of leaves, stems, and other plant organs. The surface of leaves is adorned with thousands of these stomata. The significance of stomata lies in their contribution to crucial plant processes such as transpiration and photosynthesis, which are essential for the plant's survival. These processes are carried out through well-defined structures and procedures. In this comprehensive article, we will delve into the intricacies of stomata, providing detailed information on their structure, types, diagrams, functions, mechanisms, and more.

What are Stomata?

Stomata are small pores or openings found on the surface of leaves, stems, and other plant organs. They serve as gateways for gas exchange between the plant and its environment, allowing for the intake of carbon dioxide (CO2) and the release of oxygen (O2) and water vapor. Each stoma is surrounded by specialized cells called guard cells, which control the opening and closing of the stomatal pore. The presence of stomata is crucial for the processes of photosynthesis, transpiration, and respiration in plants. These minute structures play a vital role in maintaining the overall health and functioning of plants.

Structure of Stomata

Stomata consist of a kidney-shaped epidermal cell housing a central opening known as a pore. Surrounding the stomata are a pair of specialized parenchymal cells called guard cells. The guard cells play a crucial role in controlling the size of the stomatal opening, thus helping to prevent excessive water loss from the plant. Although the shape of the cells may vary slightly, the overall structure and composition of each stoma pore remain the same, ensuring the proper functioning of the stomata.

A stoma's four vital parts are: 

• Pore
• Guard cells
• Subsidiary cells 
• Epidermal cell

Structure of Stomata: Features

Epidermal Cells

1. These are the plant cells that give the plants their mechanical and physical support.
2. The outermost layer of plants is made up of irregularly shaped epidermal cells.
3. These cells serve as the building blocks of a plant.
4. Since they are stiff, they give the stomatal pores room to close again.

Subsidiary Cells

1. These can be found at various points throughout the plant, but are most common around the stomata.
2. Subsidiary cells, in contrast to epidermal cells, are soft and provide room for the guard cells to expand and the pore to open.
3. They essentially serve as a working space for the stoma.
4. The pores won't be able to work efficiently for a long time without them.

Stoma Pore

1. The pore serves as the primary opening for all gaseous exchanges, gaseous exchanges, and atmospheric absorption.
2. The ability of the stomata to operate at all would be entirely meaningless without the presence of pores.

Guard Cells

1. These are a Stomata's essential part.
2. These cells are kidney-shaped and have a thick inner cell wall.
3. These are extremely important to the general maintenance of the plant since the Guard cells are the only ones that can open and close the Stomata, which is necessary for all of its functions.

Structure of Stomata: Types

Stomata exhibit different types based on the characteristics of the guard cells and the arrangement of subsidiary cells. From an evolutionary perspective, stomata can be classified into the following four types:

1. Moss-type stomata are predominantly found in the capsules of specific moss species such as Physcomitrium patens.
2. Gymnospermous type stomata are present in naked seeded plants. These stomata are sunken to minimize water loss through transpiration.
3. Coniferous type stomata are also sunken. In their central regions, the guard cells have an elliptical cross-section with narrow lumina.
4. Gramineous type stomata are characteristic of the grass family. They consist of two guard cells surrounded by two lens-shaped subsidiary cells.

The Dicotyledonous type is another significant type of stomata, which holds diagnostic importance among these classifications.

These various types of stomata exemplify the diversity and adaptations found in different plant groups, highlighting their distinct structural features and functions.

Structure of Stomata: Functions

Stomata serve several important functions in plants, including:

1. Gas Exchange:Stomata facilitate the exchange of gases between the plant and its surroundings. They allow for the uptake of carbon dioxide (CO2) needed for photosynthesis, while also enabling the release of oxygen (O2) produced as a byproduct of photosynthesis. This gas exchange is vital for the plant's energy production and respiration.
2. Transpiration: Stomata play a crucial role in the process of transpiration, which is the loss of water vapor from the plant. By regulating the opening and closing of stomatal pores, plants can control the rate of transpiration, thereby maintaining water balance and preventing excessive water loss.
3. Regulation of Water Loss:Through the opening and closing of stomata, plants can regulate the amount of water vapor that escapes from their leaves. By closing the stomata during times of water scarcity or high temperatures, plants can reduce water loss and conserve moisture. This helps prevent dehydration and maintains proper hydration levels in plant tissues.
4. Photosynthesis:Stomata are essential for photosynthesis, the process by which plants convert sunlight, water, and carbon dioxide into glucose and oxygen. The uptake of carbon dioxide through the stomata enables plants to carry out this crucial process, which is fundamental for their growth, development, and production of carbohydrates.
5. Control of Leaf Temperature:Stomata also contribute to the regulation of leaf temperature. When the stomata open, water vapor is released through transpiration, leading to evaporative cooling of the leaves. This process helps to prevent overheating and maintain optimal temperatures for photosynthesis and other metabolic activities.

Structure of Stomata: Mechanism

The mechanism of stomata involves the coordinated actions of specialized cells, primarily the guard cells, in response to various internal and external factors. Here is a breakdown of the mechanism:

1. Stomatal Opening:
   • Light Stimulation: When light intensity increases, it triggers the activation of chloroplasts in the guard cells. This leads to the synthesis of ATP (adenosine triphosphate) through photosynthesis.
   • ATP Production: The conversion of light energy into chemical energy results in the production of ATP, which provides the necessary energy for stomatal opening.
   • Ion Transport: ATP activates proton pumps in the plasma membrane of guard cells, leading to the movement of hydrogen ions (H+) out of the guard cells.
   • Potassium Uptake: The loss of positive charges due to hydrogen ion movement creates an electrochemical gradient that facilitates the uptake of potassium ions (K+) into the guard cells.
   • Water Uptake: As potassium ions enter the guard cells, water follows through osmosis, causing the guard cells to become turgid and swell.
   • Opening of Stomatal Pore: The increased turgor pressure within the guard cells causes them to bow outwards, resulting in the opening of the stomatal pore.
2. Stomatal Closure:
   • Environmental Factors: Various environmental factors such as high temperatures, low humidity, and water scarcity can trigger stomatal closure to reduce water loss through transpiration.
   • Abscisic Acid (ABA): A hormone called abscisic acid, produced in response to water stress, plays a crucial role in stomatal closure. ABA triggers several cellular responses that lead to the closure of stomata.
   • Ion Efflux: ABA promotes the efflux of potassium ions from the guard cells, decreasing their turgor pressure.
   • Loss of Water: As the turgor pressure decreases, water exits the guard cells through osmosis, causing them to become flaccid.
   • Closure of Stomatal Pore: The loss of turgidity in the guard cells causes them to collapse, resulting in the closure of the stomatal pore.

The mechanism of stomata is a dynamic process that responds to changes in environmental conditions and internal plant signals. This regulation ensures optimal gas exchange, water conservation, and overall plant health and survival.

Structure of Stomata: Stomatal Transpiration

Stomatal transpiration is the process of water vapor loss from plants through the stomata, which are small openings on the leaf surface. It plays a crucial role in plant water regulation, gas exchange, and cooling. Stomata are opened to allow gas exchange and transpiration, releasing water vapor into the atmosphere. Factors such as light, temperature, humidity, and plant water status influence the rate of stomatal transpiration. It is essential for plant functioning but can be regulated to prevent excessive water loss in unfavorable conditions.

Structure of Stomata: Opening and Closing of Stomata

The stomata open and close in response to various internal and external factors. Here's a brief explanation of the process:

Opening of Stomata:

1. Light stimulates the activation of chloroplasts in the guard cells.
2. ATP is produced through photosynthesis, providing energy for stomatal opening.
3. Proton pumps in the plasma membrane of guard cells transport hydrogen ions (H+) out of the cells.
4. Potassium ions (K+) are taken up into the guard cells, followed by water through osmosis.
5. The increase in turgor pressure causes the guard cells to bow outwards, resulting in the opening of the stomatal pore.

Closing of Stomata:

1. Environmental factors like high temperatures, low humidity, and water scarcity can trigger stomatal closure.
2. Abscisic acid (ABA), a hormone produced in response to water stress, promotes stomatal closure.
3. ABA induces the efflux of potassium ions from the guard cells, decreasing their turgor pressure.
4. Loss of water from the guard cells causes them to become flaccid and leads to the closure of the stomatal pore.

The opening and closing of stomata are dynamic processes that allow for gas exchange, regulate water loss, and respond to changing environmental conditions, ensuring the optimal functioning and survival of plants.

Structure of Stomata: Things to Remember

1. Stomata are minute pores found on the surface of plant leaves, stems, and other organs.
2. Each stoma consists of a pore surrounded by a pair of specialized cells called guard cells.
3. The guard cells regulate the opening and closing of the stomatal pore.
4. Subsidiary cells are present around the guard cells and assist in their movement.
5. Stomata are typically more abundant on the lower surface of leaves.
6. The shape of stomata can vary depending on the plant species.
7. Stomata allow for gas exchange, facilitating the uptake of carbon dioxide for photosynthesis and the release of oxygen.
8. The opening and closing of stomata are influenced by environmental factors, such as light intensity, humidity, and water availability.
9. Stomata play a vital role in transpiration, the process of water loss from plants through evaporation.
10. Understanding the structure of stomata helps in comprehending their functions and the overall physiology of plants.