Mitochondria are also involved in controlling the concentration of calcium (Ca2+) ions within the cell. They work very closely with the endoplasmic reticulum to limit the amount of calcium in the cytosol.
The main job of mitochondria is to perform cellular respiration. This means it takes in nutrients from the cell, breaks it down, and turns it into energy. This energy is then in turn used by the cell to carry out various functions.