Living things cannot exist without continued inputs of energy. The ultimate source of this energy is the sun. In general, energy exists in two forms, kinetic and potential. Kinetic energy is the energy of motion, such as muscle contraction. Potential energy is stored energy, such as exists in photons from sunlight or in molecules such as carbohydrates. Complex molecules with many bonds, such as carbohydrates, have more potential energy than simple molecules, such as carbon dioxide.