What do mitochondria do in the cells?

Supply cell energy. Mitochondria are where most cell energy is produced. This is used to perform functions in the cell and the body. Many cells, like muscle/heart muscle have a lot of mitochondria in them so they can actively do things like contract and move the blood/bones/etc. They are connected to. Sperm cells have a lot of mitochondria that supply enrergy to their flagella/tail so they can travel in fluid.