Every living organism, animals and plants, need a continual supply of energy to perform the processes necessary for reproduction and survival. Some of the processes like metabolism occur continually, while others only occur at certain times, like muscle contractions and cellular movements.
Living organisms represent a rich store of free energy. The primary source of energy comes from the sun. However, despite the fact that the sun is an excellent source of energy, not every life form can directly utilize the sun's energy. Plants utilize energy from the sun to manufacture sugar in the form of starch. Other living organisms use the sugar as the primary source of energy.