The gas released into the atmosphere by plants during photosynthesis is oxygen (O2). During this process, plants convert carbon dioxide (CO2) and sunlight into glucose (a type of sugar) and oxygen. The oxygen produced is a byproduct and is essential for most life forms on Earth, as it is used by animals and humans for respiration. Photosynthesis occurs primarily in the leaves of plants, where chlorophyll captures sunlight. This natural process not only sustains plant life but also plays a crucial role in maintaining the balance of gases in the atmosphere, making it vital for life.