The primary gas that plants release during photosynthesis is oxygen. During photosynthesis, plants take in carbon dioxide from the atmosphere and absorb sunlight through chlorophyll in their leaves. Using this energy, they convert carbon dioxide and water into glucose (a sugar used for energy) and oxygen. The overall chemical reaction is vital for sustaining life on Earth, as it not only provides food for plants but also generates oxygen which is essential for the survival of most living organisms, including humans. This process underscores the interconnectedness of life and the importance of plants in our ecosystem.