Certain molecules and ions are not able to pass through cell membranes to balance the concentration gradient. Transport proteins are required to support this process of facilitated transport. Facilitated transport does not require energy. Examples are shown in the upper part of the graphic. A uniporter transports one small molecule in one direction (glucose transporter or proton transporter). Symporters allow entrance of 2 molecules in the same direction (glucose-sodium uptake). Antiporters regulate the passage of 2 molecules in opposite direction (sodium-calcium antiporter).
Transport of certain molecules or ions against their concentration gradient over a membrane can occur via active transport. An active transporter can be activated or inactivated by ligand binding. The sodium-potassium pump is a good example of an active transporter. During this process, which demands energy (ATP), two potassium ions are pumped into the cell and 3 sodium ions are expelled. Binding of a drug to this pump can cause inactivation of the transport protein. Drugs that are able to do so are the cardiac glycosides like digoxin, which decrease conduction and heart rate and increase contraction force.