How are the concentration gradients on sodium and potassium ions maintained?
With such HUGE gradients on these ions, why don’t they just diffuse down their gradients? There is a huge driving force on them to do that. Keep in mind that in order to diffuse, they have to cross the membrane by facilitated diffusion (because charged ions or molecules cannot cross the lipid bilayer unassisted). So, there have to be transport channels in the membrane for them. If there were tons of these channels, then the ions would diffuse rapidly down their concentration gradients, and the normal distribution of ions would disappear. Neurons do not have many of these transport channels. They have very few. Here’s a drawing that shows more detail of the plasma membrane: In this drawing, five transport proteins are shown within the lipid bilayer. The two at the far left, with the question marks in them, are proteins that we will investigate next week. The other green, light blue, and purple channels are the ones we are going to work on now. The phospholipids of the lipid bilayer are
