Representative membrane transporters covered in Section 3.2. The transporters shown from left to right are the ATP-dependent multidrug transporter P-glycoprotein (Pgp, PDB:5KPI), a member of the ATP-binding cassette (ABC) transporter family; the bacterial leucine transporter (LeuT, PDB:3MPN), which transports amino acids across the membrane utilizing the electrochemical gradient of Na+; the lactose permease (LacY, PDB:2Y5Y) of the major facilitator superfamily, which catalyzes the translocation of galactopyranoside using the pH gradient; the SecY translocon (PDB:3BO0), which mediates the transmembrane secretion or insertion of nascent proteins; and the β-barrel transporter FecA (PDB:1KMO), which transports ferric citrate across the outer membrane of E. coli.