Self-Sustaining Chemical Systems:
Eucaryotic Cell Membrane
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
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.