Final answer:
The statement about guard cells and stomata is true; guard cells regulate the opening and closing of stomata to maintain homeostasis based on water and carbon dioxide levels within the plant.
Therefore, this statement is true.
Step-by-step explanation:
Stomata are bordered by guard cells, which can open or close the stomata. The statement that guard cells close the stomata when water levels in the plant's cells become too low is true. Guard cells swell or shrink by taking in or losing water through osmosis, which in turn opens or closes the stomata. Stomata must open to allow air containing carbon dioxide and oxygen to diffuse into the leaf for photosynthesis and respiration. However, when stomata are open, water vapor is lost to the external environment, which can potentially lead to a water deficit in the plant. Hence, to maintain homeostasis within the plant, guard cells regulate the stomata's opening and closing in response to environmental cues such as leaf water status and carbon dioxide concentrations. When carbon dioxide levels in the cells become too low, it is also true that the guard cells will open the stomata to allow more carbon dioxide to enter for photosynthesis.