Photosynthesis, the process by which green plants and certain other organisms transform light energy into chemical energy for synthesising their food. This energy is first stored in organelles called chloroplasts (double-membrane organelle) in plants and a green pigment present in chloroplasts called chlorophyll then processes the sunlight into energy. During photosynthesis in green plants, light energy is captured and used to convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds. Photosynthesis is essential to living organisms because it is the number one source of oxygen in the atmosphere. Green plants and trees use photosynthesis to make food from sunlight, carbon dioxide and water in the atmosphere: It is their primary source of energy.