An endothermic reaction needs to absorb energy in order for it to be able to proceed and an exothermic reaction releases heat as a result of this chemical reaction. If you consider photosynthesis, light or radiant energy is required for this reaction to occur. It must be absorbed by the chlorophyll pigment in the chloroplasts of the plant cells. After that occurs, the light reaction can proceed and water molecules are split into hydrogen and oxygen atoms. Then, the dark reactions allow carbon fixation to occur, forming glucose. Oxygen and water are also released. This would not be possible without the radiant energy being absorbed. Cellular respiration, on the other hand is exothermic. Glucose and oxygen combine in the mitochondria of cells and the end result is the release of carbon dioxide and water which are waste products, the production of A.T.P, a high energy molecule, and heat is also released.