| | Re: Why is Cl- transport across membrane PASSIVE?
There are two ways for ions to cross biological membranes: either through channels or by being pumped. Channels are for "passive" transport; ions flow through channels according to their electrochemical gradients. Pumps on the other hand use energy (either primary energy from ATP or secondary energy by coupling to the sym- or antiport of other ions) to actively translocate ions against their electrochemical gradient. The Na+-K+-ATPase is the classic example of a pump, which pushes sodium out of the cell and pulls potassium inwards. There are no such pumps for chloride ions, and they just flow through channels passively according, as you say, the Nernst equation. The transmembrane potential in the equation is generated by the non-equilibrium concentrations of sodium and particularly potassium. If the transmembrane potential changes, e.g. due to an action potential, chloride ions flow to equilibrate with the new potential level, and this does not require any energy to be expended by the cell (other than used for the biogenesis of the channels, obviously...). Hope this clarifies the issue.