Plants use a process called respiration in order to obtain energy from the food they produce during photosynthesis. when a plant undergoes respiration, it

The plant will use oxygen and the glucose that it has produced during photosynthesis. The oxygen and Glucose will metabolize to form ATP, which is usable energy for the cell. The plant will produce the byproducts of CO2, Water, and other chemical waste products. 

This is similar to human and animal cellular respiration, only we do not make glucose through photosynthesis.