In
a nervous cell, different ions (sodium, potassium, chloride, etc.) have
different concentrations on each side of the cell membrane.
But, as we saw
in the previous section, ions tend to diffuse
rapidly around the solutions which makes the inner and outer environments
of cells, and gradually become balanced, that is, their concentration becomes
the same in all parts of the solution.
How this phenomenon
happens?
The first cause is
temperature.
When ions and molecules dissolve in a solution, they move randomly about
(thermal agitation or Brownian movement), and thus travel to other parts
of the compartment. The speed of this movement is dependent on the temperature,
i.e.; the higher the temperature of the solution, the quicker it will reach
equilibrium.
The second cause
is the the difference of concentration or chemical
gradient: ions from regions of higher concentration
move toward regions of lower concentration.
The third cause is
the difference of electrical charge or electrical
gradient. Positive ions (cations) are electrically
repelled away of parts of the solution which is positively charged and
attracted to parts of the solution which are negatively charged. Negative
ions (anions) are influenced in the same way: repelled by the same charge
and attracted to the opposite charge.
Free
Diffusion
Molecules
in solution move randomly about. In this beaker full of water, an impermeable
membrane divides the beaker in half. On the left side there is a collection
of molecules. The membrane prevents movement of the water and the molecules
from crossing from one side of the beaker to the other
When
the membrane is removed, it allows
the water and molecules to move freely (diffusion) to the other side.
The speed of diffusion is proportional to the temperature of the solution.
Now, biological
membranes, such as the cell membrane. are permeable to many ions. This
is so because of the existence of pores
or passive transport channels bridging one side to the other, through which
ions can pass freely.
For
instance, if in the compartment on one side of the membrane there is a
higher concentration of common salt (made of sodium and chloride ions)
than in the other side, these ions will pass gradually to the other side,
following the difference of concentration. Eventually, both compartments
will have the same concentration of ions. The rate of passage is regulated
by the number and size of pores (it is not an instant phenomenon, because
it is a kind of slower diffusion).
Diffusion
Across a Permeable Membrane
Small
pores in the surface of the permeable membrane allow the selective passage
of ions. There are specific channels for each ion (sodium, chloride, potassium,
etc.). The rate of passage is regulated by the number and size of pores.
After a while, the concentration of both ions (green and yellow bars) will
be the same on both sides of the membrane.
