Getting through membranes
Water is a small, polar molecule that can get through the cell membrane pretty easily due to the presence of water channels (aquaporins), which are always open and act like a tube connecting one side of the membrane to the other. Other larger or charged molecules find it harder to cross the cell membrane, but they are not completely blocked.
In fact, there are two major ways that an ion like Na+ can make it through the membrane:
1. 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.)
2. By 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 only — whichever direction the pump mechanism works. This allows ions to be pushed around AGAINST a gradient.