Carrier proteins are involved in active transport (movement of substances against the concentration gradient, from a low concentration to a high concentration) across the cell membrane. The carrier proteins are a specific shape and will allow a particular molecule or groups of molecules to bind to it and be carried through the membrane. However this requires ATP energy (from cellular respiration), and when the enzyme ATPase breaks ATP down into ADP this releases energy that enables the carrier protein to change shape and act as a pore for the molecule to travel through the membrane without passing through the hydrophobic centre. The carrier protein releases the molecule inside the membrane and then returns to its original shape to begin the process again. Because ATP energy is needed in this process, cells that do a lot of active transport (like the muscles) have lots of mitochondria.
