Kaplan Nursing Entrance Practice Exam

What do stomata in plant leaves primarily prevent the loss of at night?

1. Oxygen

2. Carbon dioxide

3. H2O

4. Glucose

The correct answer is: H2O

Stomata are small openings on the surfaces of leaves that serve several critical functions, primarily in gas exchange during photosynthesis and respiration. At night, when photosynthesis does not occur due to the absence of sunlight, stomata predominantly close to conserve water. This is particularly important because transpiration, the process of water vapor loss from plant tissues, can lead to significant water loss. By closing the stomata at night, plants effectively minimize this loss of H2O, allowing them to maintain hydration levels and ensure proper functioning for when photosynthesis resumes during the day. While oxygen and carbon dioxide levels do change, especially between day and night due to metabolic processes, the main function of stomata closing at night is primarily to prevent water loss rather than managing these gases. Glucose, being a product of photosynthesis, is not directly related to the function of stomata in terms of loss prevention. It is essential in energy metabolism but does not pertain to the regulation of gas exchange by stomata at night. Thus, the primary role of stomata's closure during the night is to prevent the loss of water.