Lactose permease is a protein found in the cell membranes of certain bacteria, such as Escherichia coli. Its primary function is to transport lactose, a sugar found in milk, into the cell. This process is essential for bacteria that utilize lactose as a source of energy. The protein operates through a mechanism known as secondary active transport, which means it uses the energy from the movement of protons (H⁺ ions) across the membrane to help move lactose against its concentration gradient. This allows bacteria to efficiently absorb lactose from their environment for metabolism.