Now one of the things to consider with a lipid
bilayer, I've mentioned that it's fairly impermeable,
but we can actually measure permeability of
ions across a semipermeable membrane, and
that's illustrated what we see right here.
Membranes are not absolutely impermeable but
they're pretty impermeable to a lot of things.
When we discuss and think about permeability,
we have to think about some considerations
about the tendency of molecules. Molecules
in solution will tend to move from a high
concentration to a low concentration. And
if I have a circumstance such as I've set
up here, with the chambers on the left, you
see that the chambers on the left have been
divided in two by a semipermeable membrane.
In the left chamber of the vessel on the left
we see has a high concentration of molecules,
and on the right we see that there is a lower
concentration. The molecules can slowly move
across that membrane, and if we give things
sufficient time, what will happen is that
the two concentrations will equal out, that
is the high will move to the low.
Now the rate of diffusion or the rate of movement
of a molecule from the left to the right for
example, is related to several things.
First it's related to the concentration
difference between the two. The greater the
concentration difference, the more likely and
the easier it is for a molecule to move from
the left to the right. Second is charge.
Now charge is a little harder to envision, but
you could imagine that if something had a
great charge difference that there would actually
be an electrical potential that would occur
across that membrane and that electrical potential
might be driving a process to happen and that's
exactly what happens when you have a charge
difference across the membrane. The third
function of course is the permeability of
the membrane. The more permeable a membrane
is, the easier this process can happen and
the faster this process can happen. The less
permeable the membrane is, the less likely
and the slower that process will occur.
Now, membranes like this that are poorly permeable,
as I said, can provide a barrier but will
not absolutely stop the movement of molecules
Now the first movement across membranes that
I want to talk about is illustrated by this
example here. This movement across membranes
is occurring in the process we call facilitated
diffusion. Now as its name suggests, diffusion is
the movement of molecules from high concentration
to low concentration to even each other out,
like we saw in the previous illustration.
Facilitated part means that something is helping
that movement to occur. Well since the lipid
bilayer is fairly impermeable, it's not very
much allowing these molecules to move across
on their own. If the cell has a need to get
these molecules across the membrane, the cell
will typically have a protein embedded in
the membrane that will allow that to occur.
Now there's two typical types of proteins
that allow this to occur in this process.
Now when I emphasize when I say diffusion
and I say facilitated diffusion, I'm talking
about a process that does not require the input
of energy. Because the process is naturally
happening as molecules would normally move, from
a high concentration to a low concentration,
and these proteins that I will describe are
simply allowing that process to occur.
There are two types of proteins as you can see on
here. One is a carrier protein that’s shown
in the yellowish color. The carrier protein
has a chamber as that you can see, that is allowing
the yellow molecules to bind into it and then
the carrier is changing shape to allow it
to move across. This change as I'm illustrating
with my hands, this change that can occur
doesn't require any input of energy and is
driven simply by the movement of the molecule
from high concentration to low. As this occurs
over time, then the concentrations of the
two will equalize. A good example of this is in
our blood cells we have a GLUT, I mentioned
the GLUTs earlier, that will allow glucose
to move across the membrane of the blood cell
according to the concentration difference.
So blood cells will have a lower concentration
of glucose than the surrounding fluid around
them. With the GLUTs, the glucose will move
freely into those cells and it doesn't require
the expenditure of energy to get the glucose in.
Another type of protein that's involved in
these exchanges is an ion channel. Now I’ll
say more about ion channels later in this
talk, but ion channels typically are specific
for ions, charged molecules, like sodium or
potassium. And the remarkable thing about
them, that is the ion channels, is that they're
very specific, typically, for one ion and
rejecting other ions.