Mitochondria
From The School of Biomedical Sciences Wiki
Another Function of Mitochondria is non-shivering thermogenesis which is the production of heat by the body without the muscles shivering. This occurs due to H+ ions moving into the mitochondrial matrix by facilitated diffusion, allowing the exploitation of the energy produced from the movement of the H+ ions down an electrochemical gradient to be used as heat energy. Mitochondria are also useful because they store Ca2+ ions which can be used in all sorts of other functions such as muscle contraction and blood clotting. Additionally the mitochondria by storing Ca2+ ions they help towards maintaining homeostasis within the cell by acting as a cytosolic buffer.