MathBench > Cellular Processes

Nernst Potential


Getting through membranes

In the Osmosis module, we acted as if water was the only thing that could get through the cell membrane. But that's not actually true -- if it was, our cells would be in big trouble!

It would be closer to the truth to say that water is a small, uncharged molecule, and so it can get through the cell membrane pretty easily. Other larger or charged molecules have a harder time. But they are not completely blocked.

channelsIn fact, there are 2 major ways that an ion like Na+ can make it through the membrane:

Through a channel: Membrane channels can open and close like gates. Once a channel is open, ions are free to move in both directions -- but the NET movement of ions will be down a gradient. (More on that later.) So, opening a channel allows ions to move to wherever they have the lowest potential energy.

pumpsBy a pump: Membranes also contain tiny ATP-driven protein pumps. When these pumps operate, the energy stored in ATP is used to move ions in ONE direction -- whichever direction the pump mechanism works. This allows ions to be pushed around AGAINST a gradient.