The gas primarily used in photosynthesis is carbon dioxide (CO2). During this process, green plants, algae, and some bacteria convert carbon dioxide and water into glucose and oxygen using sunlight as energy. The chlorophyll in plant leaves captures sunlight, which drives this chemical reaction, making it fundamental to the survival of plants and, by extension, to all life on Earth. Photosynthesis not only produces the oxygen we breathe but also forms the basis of the food chain, as plants serve as the primary producers.