The atmospheric gas that is a by-product of the light reaction of photosynthesis essential to aerobic respiration is oxygen (O2). During the light reaction of photosynthesis, plants and other photosynthetic organisms use the energy from sunlight to split water molecules into hydrogen and oxygen atoms. The hydrogen atoms are then used to combine with carbon dioxide to form glucose and other organic molecules, while the oxygen atoms are released into the atmosphere as a waste product. Oxygen is essential for aerobic respiration, the process by which animals and other organisms use oxygen to break down glucose and other organic molecules to produce energy.