OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What physiological state of guard cells causes stomata to open?

1. Flaccid

2. Turgid

3. Dry

4. Wrinkled

The correct answer is: Turgid

Guard cells are specialized cells located on the surface of leaves and play a crucial role in regulating gas exchange and water loss through the stomata. When these cells are turgid, they are filled with water, which causes them to swell and become firm. This swelling leads to the opening of the stomata. The mechanism behind this involves the osmotic movement of water into the guard cells. When water enters, the guard cells become turgid and bow outward, resulting in the opening of the stomatal pore. This allows gases like carbon dioxide to enter the leaf for photosynthesis while also enabling water vapor to exit. In contrast, when guard cells are flaccid (not filled with water), the stomata close, preventing gas exchange. If the surrounding environment is dry, it can lead to loss of water from the guard cells, further contributing to their flaccidity and closure of the stomata. Similarly, if guard cells appear wrinkled, it indicates a lack of sufficient water, correlating with closed stomata. Therefore, it's the turgidity of the guard cells that directly leads to the opening of stomata, facilitating essential physiological processes for the plant.