Ion channels that permit ions to pass between the interior of the cell and the extra-cellular medium are an important part of a living cell. There are multiple mechanisms that happen in an ion channel, but the simplest is that ions flow through the cell preferentially in one direction as a result of diffusion arising from a difference of ion concentrations on the inside and outside of the cell.
As a first model of an ion channel, let's see how it would behave if the flux through the open channel were simply due to diffusion arising from a concentration difference. We'll model the open channel as a cylinder.
Part (a) A typical ion channel radius is about 1 nm (nanometer), and the thickness of the cell membrane is about 7 nm. The concentration of Na+ ions inside a mammalian heart cell is about 0.6 ions/nm3, and in the fluid surrounding the cell is about 6.0 ions/nm3. The diffusion constant for ions is about 109 nm2/s (nanometers2/second). If the channel were just an open cylinder, how many ions per second would flow through the channel?
Part (b) Is the flow from the inside to the outside or from the outside to the inside?
Part (c) What is responsible for the flow of the ions through the channel in this model?
Part (d) Can this "diffusion constant, D " be the same constant as in the other Fick's law equation?