The structure of a stoma consists of a pore surrounded by two specialized guard cells, primarily responsible for regulating the opening and closing of the pore.
Here's a breakdown of the stomatal structure:
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Stomatal Pore: The central opening or pore through which gases (carbon dioxide, oxygen, and water vapor) are exchanged between the plant and the atmosphere.
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Guard Cells: These are specialized parenchyma cells that flank the stomatal pore. They are typically bean-shaped (kidney-shaped) and control the size of the stomatal opening.
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Cell Walls: The guard cells have unevenly thickened cell walls. The wall adjacent to the pore is thicker than the wall on the opposite side. This differential thickening plays a crucial role in the mechanism of stomatal opening and closing.
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Chloroplasts: Guard cells contain chloroplasts, enabling them to perform photosynthesis. While their photosynthetic activity contributes to their function, the primary role of chloroplasts in guard cells is to generate ATP (energy) required for ion transport processes.
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Turgor Pressure: Changes in turgor pressure (water content) within the guard cells cause them to swell or shrink. When turgid (swollen with water), the guard cells bend outwards, opening the stomatal pore. When flaccid (lacking water), they become less curved, and the pore closes.
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Subsidiary Cells (Optional): In some plant species, specialized epidermal cells called subsidiary cells (or accessory cells) surround the guard cells. These cells may assist in the stomatal movement by acting as a reservoir of water and ions, aiding in the turgor pressure changes within the guard cells. Their presence and morphology vary greatly among different plant species.
Summary:
In essence, a stoma is composed of a pore formed by the separation of two guard cells. These guard cells regulate the pore size based on environmental factors and the plant's physiological needs. The uneven thickness of the guard cells' walls and the changes in turgor pressure are the driving forces behind the opening and closing mechanism.
