The most familiar active-transport mechanism is the "sodium pump," by which Na is expelled from the cell and K is brought in. One molecule of ATP is used for every three Na ejected and two K brought in.

The enzyme that helps to accomplish this is embedded in the cell membrane. It receives ATP from inside the cell and releases ADP back to the inside, so only the ions being transported actually cross the membrane.

The diagram at the right of the page shows the transport enzyme picking up ions on one side of the membrane, rotating, and dropping them off at the other side. It is not likely that the enzyme physically rotates, but the net effect is the same.

The cell membrane is a part of the active chemical machinery of the cell, controlling what goes in and out and actively pumping some substances one way or the other. Unlike bacterial membranes, it has no respiratory or photosynthetic roles. In eucaryotes these roles are played by special organelles.