Then we could have a situation where the proteins
are not embedded in the membrane. And you have
a high concentration of stuff on one side of the
membrane and a low concentration of stuff
on the other side of the membrane. What happens
when this is a situation and this stuff,
the molecules cannot pass through the membrane
because there are no channel proteins or
carrier proteins. They want to. Everything in
life wants to come to equilibrium.
So here's a U tube. There is a membrane in the
middle. And in this tube we have a lot of solute
on one side of the membrane. And then we
have less solutes on the other side.
The solutes are sitting in a solvent. The solvent
here is just an aqueous environment like we
would see in a cell. So when water can pass
through a cell membrane, you'd think well,
should it be able to? Wait a moment. Why would
water be able pass through the cell membrane.
Isn't water a polar molecule? Don't we have
that huge area of hydrophobicity, right?
With the hydrophobic tails of the phospholipids.
So water happens to be so small that it can actually
sneak in between those somehow. There is some
other mechanisms that we're learning about now
that help water pass through. Aquaporines and
such. We're not going to dig into those quite yet.
So either way let's just assume for simplicity
that water can so small it can just sneak
right between the phospholipids. Not exactly
true but it can. So what's going to happen
to bring everything into equilibrium. Water is
then going to pass through the membrane
to the area of high concentration of solutes in
order to make it equal with the concentration
of solutes on the other side of the membrane.
Now this all makes sense when we're looking at it
in a U tube here where the water is moving from
one side back to the other side in order to
create an equal solution on both sides of the
membrane. So, then let's take it to a cellular
environment. Bring some reality to this. When
we consider how water moves, we are considering
a concept of osmotic pressure, right. Osmotic
pressure is driving the water to the area of
higher concentration in order to dilute the
solution on the other side of the membrane.
Tonicity is the concept, and red blood cells
give us a great example for exploring that.
Let's say we have the environment just as it
should be, right. Our blood cells, these are
red blood cells. They are floating around in our
blood. The solution inside the red blood cell
should have the same tonicity or osmolarity as
the solution that the red blood cells are floating
around in. That again is one of those matters of
homeostasis. So we say these solutions are isotonic
or isosmotic. If that's the terminology you prefer.
So normal cells are floating around in normal
extracellular fluid which should have the
same tonicity. They are isotonic solutions.
What happens though if somehow we end up putting
our red blood cells in a hypertonic solution?
You have to keep in mind here what's really
important is which is hypertonic.
Is it hypertonic inside the cell or is it
hypertonic outside the cell. For reference here,
we are putting the cells which are normal tonicity,
normal amount of solute that we would have in
our human body. And we're putting them in a
glass full of water that is hypertonic.
We're putting them in the ocean. There is
lots and lots of solute in there.
What is going to happen to the nett movement of
water? Osmosis is going to go in which direction
in this case?
So because the solution outside the cell is
hypertonic relative to the solution inside the cell
the water in the cell is going to move out to
try and dilute the external environment.
Well, that's not going to be a good situation
for the red blood cell because it's going to
shrivel up and lose all of its water because it
can never dilute the rest of the world.
So, with reference, we're looking at the external
environment here. So the external environment
is hypertonic to the internal environment of
the red blood cell which would be hypotonic
relative to the external environment. I know this
can get a little bit confusing but let's try
again by looking at the opposite situation. Now,
we've put our red blood cells that belong
in a normal solution that we find in our body.
Normal tonicity. It should be isotonic
but somehow we are running out of solutes for
the blood, right. We take our red blood cells
and we put them in a hypotonic solution. So
the hypotonic solution has less solutes
than the solution inside the red blood cells.
What's going to happen to water?
Water is it going to move into the cell or is
water going to move out of the cell?
Well because the red blood cell has more
solutes in it than the surrounding environment,
water is going to rush into that cell in order
to try and dilute the environment inside the cell
and make it equal with that outside of the cell.
We're headed towards isotonic.
But the red blood cell is sort of a finite
sized thing and so what we have is a lot of
movement of water in creates extra pressure,
pressure, pressure. Finally the red blood cells
would burst. So it's really critical when you're
thinking about osmolarity or tonicity that
you consider which place is hypertonic, hypotonic.
Are we talking about the surrrounding environment
or are we talking about the internal environment?
Generally in this conversation we will be
talking about putting cells into a hypotonic or
hypertonic solution. Here is the review of that.
Again we have isotonic. That's ideal. That's
what it should be like inside the human body.
If homeostasis gets thrown off, we may see that
there is a hypertonic environment. Too much salt
in the environment. We've seen that. Too much
salt will cause those cells to shrivel up because
water from the cells is coming out of the
cells to try and dilute the environment.
And then when we have our blood cells put into
a hypotonic solution, the water from the outside
is trying to dilute the inside of the cell.
And because so much water is rushing in,
that will cause the cell to burst or lyse.
To lyse is to break apart.