Mitochondria are organelles found in cells that act as the powerhouse by converting sugar to ATP through oxidative phosphorylation. They have an outer and inner membrane, with the inner membrane folded into cristae to increase surface area for ATP production. Mitochondrial diseases occur when the mitochondria do not produce enough energy from birth due to defects, leading to early death. Mitochondria also naturally decline in function over time, explaining why older individuals have less energy than younger ones.