Na+/K+ (Sodium/Potassium) Pump
The Na+/K+ pump is found in the membranes of many types of cells. In particular,
it plays a very important role in nerve cell membranes. Notice that 3 positive ions
(Na+) are pumped out of the cell (towards ECF) for every 2 positive ions (K+)
pumped into the cell (towards ICF). This means that there is more positive
charges leaving the cell than entering it. As a result, positive charge builds up
outside the cell compared to inside the cell. The difference in charge between the
outside and inside of the cell allows nerve cells to generate electrical impulses
which lead to nerve impulses.
The Na+/K+ pump illustrates "active transport" since it moves Na+ and K+
against their concentration gradients. That is because there is already a high
concentration of Na+ outside the cell and a high concentration of K+ inside the
cell. In order to move the ions (Na+ and K+) againts their gradients, energy is
required. This energy is supplied by ATP (adenosine triphosphate). An ATP
molecule floating inside the cell, binds to the pump transferring some energy to it.
As the energy is used, the ATP falls off and having lost its energy it is converted
into ADP (adenosine diphosphate).
Notice in the diagram that there are 3 binding sites for the 3 Na+ ions on the
inner surface of the pump and 2 binding sites for the 2 K+ ions on the outer
surface of the pump. The shape of these binding sites ensures that only Na+ and
K+ can bind and be transported.
Since the pump requires an ATP every time it works, ATP must be constantly
supplied to the cell. ATP is created during the processes called "cellular
respiration" which occur inside the cell (in the ICF). Part of cellular respiration
happens in the cytoplasm and part happens in the mitochondrion. Since these
organelles are part of the nerve cell, as the ATP is made, it floats up to the pump
and provides its energy. More ATP is made and the pump continues to do its job.
If something interferes with the production of ATP, the pump will stop working
and the nerve cell will also stop working. This can cause serious loss of nerve
function and even death. Since cellular respiration requires oxygen, if you were
to stop breathing, ATP could not be produced and you would die. Of course ATP
is needed by many processes in the body so it is not only the Na+/K+ pump that
would stop.
There are poisons or toxins that also interfere with the pump. One is called
"oubain", an arrow poison. Oubain works by attaching to the pump and blocking
its action. A branch of science called "pharmacology" deals with how drugs affect
the body. Pharmacologists have designed drugs that, if administered fast enough,
can travel to the cells and attach to the oubain removing it from the Na+/K+
pumps allowing them to function properly.
This is just one example of active transport in which ATP is used to pump
materials against a concentration gradient. Your body stores glucose (a sugar) in
your liver and muscles. In order to stockpile the glucose for when you might need
it, the glucose must be pumped into cells building up a high concentration there.
Even though it uses up ATP to do this, every glucose molecule can be broken
down by cellular respiration to produce 38 ATP's! So it's a worthwhile process.