The primary gas released by trees during photosynthesis is oxygen (O₂). This process occurs in the chloroplasts of plant cells, where sunlight, carbon dioxide (CO₂), and water (H₂O) are used to produce glucose and oxygen. The overall equation for photosynthesis can be simplified to show that water and carbon dioxide are converted into glucose and oxygen, with sunlight providing the energy necessary for the reaction. This oxygen is then released into the atmosphere, contributing significantly to the air we breathe and supporting aerobic life on Earth. Additionally, photosynthesis plays a crucial role in regulating carbon dioxide levels in the atmosphere.