Photosynthesis is a chemical process that occurs in plants and other organisms, in which light energy is converted into chemical energy. This chemical energy is stored in the form of sugar molecules, such as glucose. Photosynthesis occurs when water and carbon dioxide are combined in the presence of light and the enzyme, chlorophyll. The light energy is used to convert the water and carbon dioxide molecules into glucose, while releasing oxygen as a byproduct. The process is essentially the reverse of cellular respiration, which takes place in all organisms and breaks down glucose to release energy.