Living things require energy in order to grow, reproduce, move–in other words, for all of their functions. The ultimate source of this energy is the sun. Some sunlight energy is converted to chemical energy by the pathways of photosynthesis. Electrons in chlorophyll molecules are excited by the light. Their energy is used to build large carbohydrate molecules from smaller carbon dioxide molecules. Some of the light energy is now stored in the bonds of the carbohydrates. Stored energy, which can be used later for work, is called potential energy. Plants use carbohydrates as an energy source for maintenance and growth and as building blocks for new tissues. Plant tissues represent stored, or potential, energy that is available to the plant, and to herbivores that eat the plant. In every energy transformation some of the energy is lost as heat. When carbohydrates are metabolized by cells, the stored energy is released and is available for work such as muscle contraction. Energy of motion is kinetic energy.