Photosynthesis is the process of converting light energy to chemical energy. Photosynthesis evolved shortly after life began. There was little or no oxygen in the atmosphere before photosynthesis began. The conversion of carbon dioxide to organic compounds such as sugars during photosynthesis is called carbon fixation.
Chemical energy is stored in the molecular bonds of the sugar.
Both photosynthetic and non-photosynthetic organisms undergo respiration. Some of the plant material dies and is burned by bacteria and other decomposers. Stored photosynthetic energy can also be released by burning. The equation for fire is the same as for respiration, except that the release of energy is almost entirely as heat and the reaction is uncontrolled. Respiration is under tight control in living organisms.