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Energy is stored in the covalent bonds of fuels, and it can be released and transformed. You will recall from Chapter 8 that energy transformations can involve light, heat, chemical, mechanical, and electrical energy. For example, when you burn wood, it releases a great deal of its energy as heat and light. In cells, molecules used as fuels release chemical energy that is used to make ATP, which in turn drives endergonic reactions. Photosynthetic cells and organisms use energy from sunlight to synthesize their own fuels, as we will describe in Chapter 10. In nonphotosynthetic cells, the most common chemical fuel is the monosaccharide glucose. Other molecules, including other carbohydrates, fats, and proteins, can supply energy to the whole organism. However, to release their energy they must be converted into glucose or intermediate compounds that can enter into the various pathways of glucose metabolism.