The plasma membrane constitutes the interface of the cell
with its environment. This membrane thus protects the cell
against a potential harsh and hostile environment. At the same
time, nutrient transporters located at the plasma membrane
must allow controlled uptake of small molecules and essential
cofactors that may be present in that environment. The action
of these transporters relies on an electrochemical gradient across
the membrane, which is maintained by abundant ion pumps such
as the Naþ/Kþ-ATPase in mammalian cells or the proton
pumping Hþ-ATPase in yeast.