text: All cells need energy to survive. Mitochondria is a double-membrane structure in the body cell that generates and transports essential metabolic products. The three layers of this structure are the outer membrane, intermembrane space, and inner membrane. The space inside the mitochondria is called the mitochondrial matrix, while the region outside is the cytosol. The outer membrane allows most small molecules to pass through. In contrast, the inner membrane transports specific molecules only, which is impermeable to many substances. Therefore, a shuttle is required for the transportation of molecules across the inner membrane. It acts as a pump to drive the substances from the inner membrane to the outside.