<unk> s are pore-forming proteins that help to establish and control the small voltage gradient across the plasma membrane of all living cells (see cell potential) by allowing the flow of ions down their electrochemical gradient. They are present in the membranes that surround all biological cells.
Ion channel
Ion transporter
Calcium channel
Voltage-gated ion channel
Sodium channel
Receptor 
Transmembrane protein
Chloride channel