Membrane channel

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.