A Level Biology Practice Exam

Guard cells play a crucial role in regulating gas exchange and water loss in plants. Positioned on either side of the stomata, the small openings on the surface of leaves, guard cells control the size of these openings based on the plant's needs and environmental conditions.

When guard cells take up water, they swell and bend, causing the stomata to open. This opening allows carbon dioxide (CO2) to enter the leaf for photosynthesis while also enabling the release of oxygen (O2) as a byproduct. However, this process also leads to water vapor escaping from the leaf. To prevent excessive water loss, particularly in dry conditions, guard cells can detect changes in humidity and water availability, adjusting the stomatal openings accordingly.

By regulating the balance between gas exchange and water retention, guard cells help the plant to maintain homeostasis and optimize its physiological functions in response to changing environmental conditions. Therefore, they are essential for both the plant's survival and its ability to carry out photosynthesis efficiently.