Mitochondria are called the powerhouse of the cell. These membrane-bound organelles are found in the cytoplasm of all eukaryotic cells. 

Mitochondria are responsible for carrying out cellular respiration, breaking down nutrients, and producing adenosine triphosphate (ATP), which are the main energy molecules used by the cell. ATP molecules provide the energy essential for performing all cellular functions, which is why they are called the powerhouse of the cell.