Plants go through cellular respiration when they are not using photosynthesis for their metabolic needs; when they are not in need of food.
During cellular respiration, the cell uses oxygen to break down sugar. Breaking down sugar produces the energy the organism needs. This is very similar to wood burning in a fire. As the wood burns, it combines with oxygen and releases heat energy and carbon dioxide. When the cell uses oxygen to break down sugar, oxygen is used, carbon dioxide is produced, and energy is released.