Which cell structure is mainly responsible for releasing energy from food molecules in some single-celled organisms ?

The structure that is responsible for releasing energy from food molecules in some singled celled organisms is MITOCHONDRIA. The major function of mitochondria is to carry out cellular respiration, which make energy available for the cells to use. The mitochondria is also called the power house of the cell, because it is the organelle that specializes in turning the food molecules into the energy form that is usable by the cells.