A protein that penetrates or spans a cell membrane to permit the passage of a substance through the membrane. Some transport proteins form pores, or channels, through which particular ions or molecules can pass. These channel proteins are often gated, enabling them to open and close in response to signals; categories include ligand-gated ion channels and voltage-gated ion channels. Other types of transport protein bind the substance on one face of the membrane, then change shape so that the substance is carried by the protein through the membrane to be released at the other face. These include the uniporters, which transport just one substance, and the cotransporters, which transport two or more different substances. Transport proteins often require energy to drive the transport process; this is provided by hydrolysis of ATP or by an existing concentration gradient. See active transport.