There are different proteins in the membrane, which play a major role in both active as well as passive transport. Active transport is carried out by membrane – protein. In protein pumps energy is used to carry substances across the cell membrane. These pumps can transport substances from a low concentration to a high concentration (‘uphill’ transport). Transport rate reaches a maximum when all the protein transporters are being used or are saturated. The carrier protein is very specific in what it carries across the membrane. These proteins are sensitive to inhibitors that react with protein side chains