The gas released from plants during photosynthesis is oxygen (O2). This process occurs primarily in the chloroplasts of plant cells, where chlorophyll captures light energy from the sun. Using this energy, plants convert carbon dioxide and water into glucose and oxygen. The glucose serves as a vital energy source for the plant and can also be stored for later use. The oxygen produced is released into the atmosphere as a byproduct, contributing to the oxygen supply essential for the survival of most life forms on Earth. This process is fundamental to the planet's ecosystem.