= 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. <Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[Chapter 11, Ion Channels and the Electrical Properties of Membranes]{ref}
For one channel population, the approximation $I=Np_o i$ 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.
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