Mitochondria are often referred to as the powerhouses of our cells, because they generate chemical energy similar to that obtained from a battery. Whether it's a brain, muscle or plant cell, nano-sized gateways control the activity of the mitochondrial battery, by carefully allowing certain proteins and other molecules to enter into our mitochondria. Some of these proteins are large and complex molecules, yet they are essentially "spirited" into from the cytoplasm into the mitochondria, while the mitochondrial membrane remains water-tight and intact. How this happens has confounded science for decades.

Source: Secrets of a cellular nanomachine revealed: Learning how any molecule passes through any membrane