Biology Regents Practice Exam

What structures in plant leaves regulate water loss and gas exchange?

1. Vacuoles

2. Chloroplasts

3. Guard cells

4. Mitochondria

The correct answer is: Guard cells

In plant leaves, guard cells play a critical role in regulating water loss and facilitating gas exchange. These specialized cells are located on either side of the stomata, which are small openings on the leaf surface. By changing their shape, guard cells can open or close the stomata. When guard cells take in water, they swell, causing the stomata to open. This allows carbon dioxide to enter the leaf for photosynthesis and oxygen, a byproduct of photosynthesis, to exit. However, an open stomata also increases water loss through transpiration. Therefore, guard cells help balance the need for gas exchange with the necessity of conserving water, especially during hot or dry conditions. The other structures mentioned do not serve this specific function. Vacuoles primarily store nutrients and waste products and maintain turgor pressure. Chloroplasts are involved in photosynthesis but do not regulate gas exchange directly. Mitochondria are responsible for energy production in cells through cellular respiration.