Page 160 - The Miracle in the Cell Membrane

The Miracle in the Cell Membrane

nerve cell has an elec tri cal poten tial of approx i mate ly 70
mil li volts. This is called the mem brane poten tial or rest ing
poten tial. Though this appears very small, it means that a
tiny cell pro du ces up to 1/20th of the volt age of a flash light
bat tery, and the axon holds the poten tial for elec tri cal activ -
i ty along the mem brane. How does this rest ing poten tial
come about? How is it con served?
Outside the axon, there are sodi um (Na+) and chlo ride
(Cl-) ions. Inside are charged pro teins and potas si um (K+)
ions. The elec tri cal imbal ance between the cell mem brane and
the exte ri or forms a rest ing poten tial along the mem brane, an
imbal ance formed by the charged ions that lets the cell mem -
brane be select ive ly per me a ble for dif fer ent ions. Sodium,
potas si um and chlo ride ions pass through the cell mem brane,
but the pas sage of high ly-charged pro teins is restrict ed—and
thus, so is the for ma tion of elec tri cal poten tial.
However, select ive per me a bil i ty can not be the sole
answer, because the num ber of potas si um (K+) ions in the
cell is always great er than the num ber of sodi um (Na+) ions.
Furthermore there are more sodi um (Na+) ions out side the
cell than there are potas si um (K+) ions. For the req ui site ion
bal ance to be estab lished, the den si ties inside the nerve cell
must be reversed.
The cell achieves this by using the kind of ion pump we
touched on ear li er. The sodi um-potas si um pump, a pro tein
mol e cule, forms a chan nel in the cell mem brane, tak ing its
ener gy from ATP (or ade no sine tri phos phate, the cel lu lar
ener gy mol e cule used direct ly by liv ing things) and takes in
potas si um (K+) ions as it expels sodi um (Na+) ions. In this
way, the cor rect ion pro por tions are main tained inside and

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