You may already know a lot about membranes, if so skip ahead to the next paragraph. Basically, the cell membrane has three layers: two layers that "like water" with one layer in between that "doesn't like water." Any particle with a charge (like an ion) can't get through the middle zone. Think about a cell membrane like a closed door. A cat may be fine on either side of the door, but it can't get through the middle. (I almost made a pun there about cations).

In order for our cells to work right, they sometimes have to move ions across the cell membrane. One way to do this is through channels. The ion channel is like a cat door. It allows the ions to move through the membrane by making a sort of tunnel. My cats' door can be set to open in both directions, only one direction, or stay closed. So I can keep them on either side of the big door. Ion channels can be like this too. They may always be open, but it is more likely that they will only be open at certain times. I think all ion channels are proteins, but I'm not sure about that.

Nerve cells work because of ion channels that let sodium and potassium move through the cell membrane. Ion channels can be controlled in many ways. The ion gates in the nerve cell membrane open because of changes in a cell's "charge" or electrical potential. Ion pumps pack potassium (K+) into nerve cells and throw sodium (Na+) out, so both will move in opposite directions when the gates open. When the sodium gates open, sodium rushes in. This makes the cell more positive. The positive charge is the trigger to open the potassium gates, and close the sodium gates. Potassium rushes out, but no more sodium can come in, so the charge gets back to normal. This makes the potassium gates close.

What would happen if a poison made all of your sodium gates stay shut? The toxic snail Conus textile makes a poison that does just that. It uses this poison to kill fish that are much larger and faster than it.

In a typical membrane, a phospholipid bilayer forms a barrier between the cell interior and exterior. Its actually pretty impermeable to ions, but a bit leaky, depending on the type of ion you are talking about. The membrane is very permeable to water, so that putting your typical cell in an ion solution that is lower in concentration than cells interior, will cause water to rush into the cell across the membrane and the cell will burst. Membrane structure determines which ions and how much of them cross the membrane. In turn structure can be altered by hormones, temperature, electrical charge of the membrane and other factors. Temperature makes the lipid more fluid and more permeable, so membranes that need to withstand high temperatures often contain more saturated fats to make them stable.

Cell membranes also contain transport proteins and channels to regulate movement of ions into and out of the cell. Some of these proteins are passive transporters that allow ions to move according to a concentration gradient. Others are active transporters that use energy to transport ions against the concentration gradient. The most common example of an active transporter is the sodium-potassium ion pump. The Na+-K+ pump transports sodium from inside of the cell, where it is in relatively low concentration to the outside, and transports potassium from outside the cell, where it is in relatively low concentration, to the interior. In voltage gated channels, like nerve cells, the cell membrane has positively charged ions on its exterior (typically Ca++) and negative on its interior, creating a voltage gradient. When the membrane is depolarized, voltage gated channels in the membrane open, and the cell is flooded with ions from the exterior. These are just a couple examples of the great number of ways cells control transport across membranes.