In plants, what is the primary function of guard cells?

• A. They transport water through xylem
• B. They store carbohydrates
• C. They change shape to open or close stomata
• D. They conduct photosynthesis

Answer: (C)

Guard cells regulate gas exchange and water loss by changing their shape to open or close the stomatal pore. When they take up ions like potassium and chloride, water follows by osmosis, the guard cells swell, and the pore opens, allowing carbon dioxide to diffuse in for photosynthesis while water vapor can exit. When signals indicate drought or darkness, ions leave and water exits, the cells become flaccid, and the pore closes to conserve water. This specialized turgor-driven mechanism is what enables plants to balance carbon intake with water conservation.