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A membrane channel is a membrane protein forming a selective passage through which appropriate substances can move when the passage is open. Ion channels are an important class. Passive channel transport follows the relevant electrochemical gradient; the channel does not need to bind and carry each ion through a full alternating-access cycle. ^[ref]

For one channel population, the approximation $I=N{p}_{o}i$ separates channel number $N$, open probability $p_o$ and current per open channel $i$. Doubling channel number can double mean current without changing single-channel conductance. Conversely, a ligand can change opening probability while the number of channels stays fixed. This explains why a current measurement alone does not identify which molecular property changed.

A membrane transport protein enables selected substances to cross a cell membrane. A membrane channel forms a permeation pathway, whereas a carrier binds its cargo and changes conformation. Transport can follow an electrochemical gradient or be coupled to an energy source to move material against it. For example, GLUT4 supports facilitated glucose transport, whereas the sodium-potassium ATPase couples transport to ATP hydrolysis. Selectivity and transport mechanism are separate properties. ^[ref]