Praxis II Biology (5235) Practice Test

Guard cells are specialized epidermal cells that surround the stomata in leaves. These cells play a crucial role in the regulation of gas exchange and water loss. They control the opening and closing of the stomatal pores, which are essential for allowing carbon dioxide to enter the leaf for photosynthesis, while also facilitating the release of oxygen and transpiration of water vapor.

The unique structure of guard cells, which can change shape by taking up or losing water, enables them to open or close the stomata in response to environmental conditions such as light, humidity, and carbon dioxide concentration. This ability to manipulate the size of the stomatal opening is vital for maintaining the plant's water balance and overall health.