Video narration
If you breathe in oxygen and breathe out carbon dioxide, why does a plant give off oxygen and take it in too? Plants exchange gases with air, though they have no lungs. Tiny pores, mostly beneath leaves, let gases move in and out; these are stomata. Carbon dioxide enters through those same pores. In sunlight, leaves use water and carbon dioxide to make sugar. This is photosynthesis. Oxygen leaves, and water also escapes as vapor. Here's the twist: plants use oxygen, too. Day and night, respiration uses sugar and oxygen to release energy, giving off carbon dioxide. By day, both processes happen, but photosynthesis releases more oxygen than respiration uses. At night, photosynthesis stops; respiration continues. Roots absorb water. That's why: in daylight, oxygen released exceeds oxygen used, while respiration never switches off.