Active Transport
 

Active transport gives the cell the ability to increase the amount of a substance either inside or outside the cell.  An example would be the ions sodium and potassium.  The membrane is "leaky."  Sodium and potassium can cross the cell membrane easily.  The problem is, the ions will go down a concentration gradient.  In the case of Sodium and potassium sodium will leak to the inside of the cell and the potassium will leak to the outside of the cell.  It is then conceivable that the concentration of sodium inside the cell could become so large that the much needed sodium could become toxic to the cell.  Same with potassium, in that the potassium is needed inside the cell.  To combat this problem the cell has a protein that uses ATP (energy) to "pump" these ions where they could cause no harm to the cell.

The sodium potassium pump works like this.  A channel protein is modified so that materials cannot pass through it passively.  For the protein to work right, it has to have ATP.  The sodium ions inside the cell bind to the protein.  ATP binds to the protein causing it to change shape exposing the sodium to the outside of the cell.  The protein in a shape that allows potassium ions to bind to it.  When this happens the protein changes it shape again and exposes the potassium to the inside of the cell.  The potassium ions can now inside the cell.  This process happens very quickly.  So fast that in one minute 300 sodium ions are pumped out of the cell and 200 potassium's are pumped into the cell. 

Active transport doesn't just work with sodium and potassium.  Proton pumps are necessary for ATP to be made from ADP in the mitochondria. 

Active transport moves material against a concentration gradient (low to high) and uses ATP.