Which structure opens and closes the stoma?

• A. Stomata
• B. Anther
• C. Pistil
• D. Guard cell

Answer: (D)

Guard cells control when the stoma opens and closes. They are the two specialized epidermal cells that surround the stomatal pore on the leaf surface. When these cells gain turgor—typically by actively moving potassium and other solutes into them—water follows by osmosis, the cells swell, especially at the outer walls, and the pore opens to allow gas exchange. When ions exit and water leaves, the guard cells lose turgor, become less swollen, and the pore closes. Factors like light, CO2 levels, and humidity influence this ion transport and turgor change, directing when the leaf can take in CO2 or conserve water. The stomata themselves are the openings, while the anther and pistil are reproductive parts of a flower, not involved in controlling stomatal movement.