(color online) In the biological ion channel (usually like a corrugated tube), ions are interacted inevitably by charges around the channel (a). This system is consistent with a channel, through which a particle is passing in an external potential field (b). Usually, if the charge is an electron, the external potential can be considered as a potential well (c). On the contrary, if positive, the potential field is a potential barrier (d). The two ends of the channel are the particle reservoirs which maintain the concentration of the ions.