The cell membrane is not very permeable to sodium ions, so they must enter through a process known as facilitated diffusion. In facilitated diffusion, sodium passes through the cell membrane in a voltage-gated ion channel, which is opened by a change in electrical current across the membrane.
These channels only remain open for approximately one millisecond, but more than 7,000 sodium ions pass through into the cell in this time. This type of facilitated diffusion of sodium only occurs in certain types of excitable cells, such as muscle cells and neurons.
In order to remove excess sodium from cells, energy must be expended in the form of adenosine triphosphate, ATP. In this process of active transport, each unit of ATP results in the removal of three sodium ions out of the cell and takes in two potassium ions.
There are many different types of facilitated diffusion, which are used by cells to take in large molecules that cannot otherwise pass across the cell membrane, such as sugars, minerals and amino acids. In this process, the molecules usually attempt to move from areas of high concentration to areas of low concentration. For this reason, diffusion is also fully reversible, as the molecules can move in either direction.