Mitochondria are tiny organelles inside cells that are involved in releasing energy from food. This process is known as cellular respiration. It is for this reason that mitochondria are often referred to as the powerhouses of the cell.
When the breakdown products from the digestion of food find their way into the cell, a series of chemical reactions occur in the cytoplasm. This allows some of the energy locked up in these products to be released and incorporated into the universal energy supplier in cells known as ATP (adenosine triphosphate).