Energy is released when cells use oxygen to break down the sugars stored as starch in the mitochondria of the cells. Carbon dioxide and water are produced as waste products.
plant cells use cellular respiration, just like our cells. That is, they use oxygen with food to get the energy they need for growth, repairs, and reproduction.
Yes, plants carry out aerobic cellular respiration to get energy from food just like we do. So when people say that oxygen is a waste product of photosynthesis they are incorrect. Plants need oxygen...