Schematic diagram of the processes involved in the trafficking of transmembrane proteins using ion channels as a example.

Ion channels from a pool within the endoplasmic reticulum are moved to the membrane surface via shuttle proteins, a process called exocytosis. Ion channels can also be moved from the membrane surface back to the endoplasmic reticulum via different shuttle proteins, a process called endocytosis. Both endocytosis and exocytosis occur continuously, and one thus characterizes them by their rates, which are simply the number of transmembrane proteins being removed and inserted from the membrane per unit time, respectively. Both rates can be independently regulated.