Video Transcript
The diagram provided shows a
simplified outline of an axon membrane during resting potential. Which area is more positive? (A) The extracellular space or (B)
the cytoplasm of the axon.
When an action potential is not
traveling along an axon, the axon is said to be at rest. The difference in charge between
the space inside the axon and the extracellular space outside the axon is known as
the potential difference. When this is measured for an axon
at rest, it is known as the resting potential. The diagram shows the movement of
ions across the axon membrane during a resting potential. This movement of ions is
facilitated by the two ion transport proteins shown on the right of this
diagram. Let’s consider the function of each
one and at the same time note the number of charges moving in and out of the axon so
we can work out which area is more positive.
First is the sodium–potassium pump,
which uses energy in the form of ATP to transport three sodium ions out into the
extracellular space and two potassium ions into the cytoplasm of the axon, both
against their concentration gradients. Because both sodium and potassium
ions are positively charged, we can add two positive charges to the in column and
three positive charges to the out column.
Now, let’s turn our attention to
the potassium ion channel. This channel is always open, and it
allows potassium ions to diffuse down their concentration gradient. Due to the action of the
sodium–potassium pump, there is a higher concentration of potassium ions inside the
axon than outside, so they diffuse out into the extracellular space. We can therefore add some more
positive charges to the out column.
If we now look at the overall
difference in charge between the outside of the axon and the inside, it is clear
that the extracellular space contains many more positively charged ions than the
cytoplasm of the axon. We can therefore conclude that the
correct answer is (A). During resting potential, the
extracellular space is more positive than the cytoplasm of the axon.
