Final answer:
Stomata are the structures on a leaf that allow the diffusion of carbon dioxide and oxygen, regulated by guard cells.
Step-by-step explanation:
In plants, the gas exchange of carbon dioxide (CO₂) and oxygen (O₂) is facilitated through stomata, which are small, regulated openings located predominantly on the underside of the leaf. The stomata are essential for basic photosynthetic functions, allowing these gases to diffuse directly into and out of the leaf's internal structures. Each stoma is surrounded by guard cells that regulate its opening and closing, thereby controlling the passage of gases and helping to prevent excessive water loss through transpiration.