Stomata are tiny openings or pores in plant tissue that allow for gas exchange. Stomata are typically found in plant leaves but can also be found in some stems. Specialized cells known as guard cells surround stomata and function to open and close stomatal pores. Stomata allow a plant to take in carbon dioxide, which is needed for photosynthesis. They also help to reduce water loss by closing when conditions are hot or dry. Stomata look like tiny mouths which open and close as they assist in transpiration.
Plants that reside on land typically have thousands of stomata on the surfaces of their leaves. The majority of stomata are located on the underside of plant leaves reducing their exposure to heat and air current. In aquatic plants, stomata are located on the upper surface of the leaves. A stoma (singular for stomata) is surrounded by two types of specialized plant cells that differ from other plant epidermal cells. These cells are called guard cells and subsidiary cells.
Guard cells are large crescent-shaped cells, two of which surround a stoma and are connected to at both ends. These cells enlarge and contract to open and close stomatal pores. Guard cells also contain chloroplasts, the light-capturing organelles in plants.
Subsidiary cells, also called accessory cells, surround and support guard cells. They act as a buffer between guard cells and epidermal cells, protecting epidermal cells against guard cell expansion. Subsidiary cells of different plant types exist in various shapes and sizes. They are also arranged differently with respect to their positioning around guard cells.
Types of Stomata
Stomata can be grouped into different types base on the number and characteristics of the surrounding subsidiary cells. Examples of different types of stomata include:
- Anomocytic Stomata: Possess irregularly shaped cells, similar to epidermal cells, that surround each stoma.
- Anisocytic Stomata: Features include an unequal number of subsidiary cells (three) surrounding each stoma. Two of these cells are significantly larger than the third.
- Diacytic Stomata: Stomata are surrounded by two subsidiary cells that are perpendicular to each stoma.
- Paracytic Stomata: Two subsidiary cells are arranged parallel to the guard cells and stomatal pore.
- Gramineous Stomata: The guard cells are narrow in the middle and wider at the ends. The subsidiary cells are parallel to the guard cells.
Two Main Functions of Stomata
The two main functions of stomata are to allow for the uptake of carbon dioxide and to limit the loss of water due to evaporation. In many plants, stomata remain open during the day and closed at night. Stomata are open during the day because this is when photosynthesis typically occurs. In photosynthesis, plants use carbon dioxide, water, and sunlight to produce glucose, water, and oxygen. Glucose is used as a food source, while oxygen and water vapor escape through open stomata into the surrounding environment. Carbon dioxide needed for photosynthesis is obtained through open plant stomata. At night, when sunlight is no longer available and photosynthesis is not occurring, stomata close. This closure prevents water from escaping through open pores.
How Do They Open and Close?
The opening and closing of stomata are regulated by factors such as light, plant carbon dioxide levels, and changes in environmental conditions. Humidity is an example of an environmental condition that regulates the opening or closing of stomata. When humidity conditions are optimal, stomata are open. Should humidity levels in the air around plant leaves decrease due to increased temperatures or windy conditions, more water vapor would diffuse from the plant into the air. Under such conditions, plants must close their stomata to prevent excess water loss.
Stomata open and close as a result of diffusion. Under hot and dry conditions, when water loss due to evaporation is high, stomata must close to prevent dehydration. Guard cells actively pump potassium ions (K +) out of the guard cells and into surrounding cells. This causes water in the enlarged guard cells to move osmotically from an area of low solute concentration (guard cells) to an area of high solute concentration (surrounding cells). The loss of water in the guard cells causes them to shrink. This shrinkage closes the stomatal pore.
When conditions change such that stomata need to open, potassium ions are actively pumped back into the guard cells from the surrounding cells. Water moves osmotically into guard cells causing them to swell and curve. This enlarging of the guard cells open the pores. The plant takes in carbon dioxide to be used in photosynthesis through open stomata. Oxygen and water vapor are also released back into the air through open stomata.
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- Ferry, R J. "Stomata, Subsidiary Cells, and Implications." MIOS Journal, vol. 9 iss. 3, Mar. 2008, pp. 9-16.