Note that initially Na+ ions (orange) are inside the cell and K+ ions
(blue) are outside the cell. The cell's plasma membrane is indicated as
black. The transmembrane transport protein (yellow) is open to the inside.
Once three Na+ ions enter the transport protein and bind to their binding
sites, the transport protein can react with ATP and the transport protein
becomes phosphorylated. The phosphorylated transport protein has an
inorganic phosphate ion covalently attached to the protein. The
phosphorylation of the transport protein causes a conformational change in
the transport protein such that the ion binding sites are now open to the
outside of the cell. The three Na+ ions leave their binding sites and the
binding sites change so that they are less likely to bind Na+. At the same
time, the K+ ion binding sites become exposed so that K+ ions are free to
bind. The binding of two K+ ions causes the hydrolysis of the covalently 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attached phosphate such that the phosphate is removed from the transport
protein. The removal of the phosphate causes another protein conformational
change such that the ion binding sites are again open to the inside of the
cell. The K+ ions leave their binding sites and enter the cell. The
transport protein is now ready for another transport cycle. The Na/K pump
is an active transport system. The energy for the active transport comes
from the ATP molecule which is used to phosphorylate the transporter. Thus,
to function, this pump requires a continuous source of ATP. Note that for
every ATP molecule used, three Na+ ions are pumped out of the cell and 2 K+
ions are pumped into the cell.