>>In this segment on cell membrane transport, now we are going to be focusing on osmosis.
So remember, osmosis is another form of passive transport. There’s still two more videos
to go on membrane transport, primary active transport and secondary active transport,
but let’s go ahead and focus on osmosis. So when you think about osmosis there are
some terms that are important for you to understand. And that is what is a solution? So a solution
is a solute plus a solvent. So if you mix the two together you form a solution. But
now you need to understand what a solute is versus a solvent. So the solvent is the dissolving
medium. So in human physiology, our dissolving medium will always be water, because we’re
dealing with the biological system. The solute, however, can vary. The solute is what is dissolved
in the solvent…or disassociates in the solvent. So if you look at the two beakers on the right-hand
side of your screen, it says which side is considered a solution: A or B? So the legend
is down below, and it tells you what color the water molecules are versus the solutes.
So Side B is considered your solution because it consists of both solutes and solvents.
Okay. So when we talk about osmosis then, osmosis is strictly the diffusion of water.
So any time we’re talking about osmosis you need to think about water diffusion. Water
is going to be moving from a high concentration of water…toward a low concentration of water…which
is the same thing as saying that water will move from a low concentration of solute toward
a high concentration of solute. And osmosis requires an aquaporin. So an aquaporin is
a specialized sort of channel. And so looking at the figure to the right, and the question
asks you: Which side has the highest concentration of water? And so if we look at Side A, Side
A has a higher concentration of water and a lower concentration of solute. Side B has
a lower concentration of water and a higher concentration of solute. Okay, so Side A has
the higher concentration of water.
Okay. So if you pause the video then you can answer this question. And the question is:
Which direction is water going to move, assuming that you have a non-permeating solute? A non-permeating
solute means that the solute is not permeable to the membrane. And so here you have a beaker
and it’s separated by a membrane. But the solutes are not permeable; they cannot pass
through that membrane. So we want to know well which direction is water going to move
then? So you can pause the video to answer this question. So always remember when water
moves, it moves from a high concentration of water toward a lower concentration of water.
So water then is moving from Side 1 to Side 2. The result of that is shown here, and you
can see how the water level inside 1 went down and the water level inside 2 went up.
So the volume’s changed. That’s an important concept: Volumes change when water diffuses.
Okay, so now let’s start tying this into the term “tonicity.” So tonicity describes
the behavior of cells when you place them in a solution. And so, when we say the behavior
of the cells, we’re talking about whether or not that cell may shrink or will that cell
expand and then burst? So different types of solutions can do this and can change the
behaviors of these cells. So the solutions that we want to discuss are hypotonic, hypertonic,
and isotonic. So let’s go through each of these solutions and think about how that will
change the cell size. So let’s begin with a hypotonic solution. So here you have a cell
that’s been placed inside of a beaker. When you work these problems, it’s going to be
very important for you to think through the problems and—rather than memorize, okay?
Because there’s ways that these problems can be written such that memorization won’t
do you any good. So this is how you work through the problems and how you think through them,
and, and this way you never have to memorize anything.
So we know hypo means “less.” Tonic means “solute.” Okay? And here we’re talking
about a hypotonic solution. So the solution is in the beaker. So it’s hypotonic. So
that means I have a low concentration of solute, which then tells me in the cell, because I
have to compare the solute concentration to something else, and here I’m comparing it
to the cell. So in the cell I have a high concentration of solute. Now we want to compare
that to the water concentration. If there’s less solute present, then that means there’s
more water. And then just the opposite condition within the cell, a lower concentration of
water in the cell.
So now you’ve set up your problem, and now what you can do is determine the direction
of water movement. So we would assume that there’s aquaporin present here, and if there’s
an aquaporin present, then water’s going to move from a high concentration of water
toward a low concentration of water—meaning water will move into your cell. So again,
don’t memorize “hypotonic solution means water will move into the cell,” because
then you’re just memorizing it and you’re not able to think through it and walk your
way through it. And so you’ll wind up missing questions because of the twists and turns
that can be done to these types of problems. Then there’s a hypertonic solution. So what
I would suggest is that you pause the video, set up this problem and tell me then which
direction water is going to move. Now before I forget, let’s go back to the previous
slide for a minute. If water’s going to be diffusing into the cell, how does that
affect the volume of that cell? So this cell can get bigger and bigger and bigger, right?
It can expand and expand and expand until eventually the cell will burst. So this cell
will lyse. So now let’s go back to the hypertonic solution.
Okay, so if you’ve already paused the video and set up the problem then you can continue
on with the video. So the hypertonic solution then tells me that I have a high concentration
of solutes, right? Because hyper means “high” or “lots of,” tonic again means “solute.”
And we’re talking about the solution. So that’s where I’m getting a high concentration
of solutes in the beaker. All right, and then in the cell I have a low concentration of
solutes—with respect to the solution. And now we determine where we have a higher concentration
of water. Well if we have less solute, that means that we have more water. And if we have
high solute that means we have less water. And I’m going to put solvent here just so
that you remember that solvent and water are the same thing. So now which direction is
water going to diffuse, assuming an aquaporin is present?
So water then will diffuse out of the cell, moving from a high concentration of water
toward a low concentration of water. So since the cell is actually losing water, that means
that this cell is going to shrink or shrivel. Then we have isotonic solutions. So iso means
“same.” Tonic again means “solute.” So what that means then is that we have the
same amount of solute, the same concentration of solute in the solution as we do in our
cell. So if we have the same concentration of solute in both locations, then we must
have the same concentration of solvent, water, in the solution as well as the cell. And if
we assume an aquaporin is present, do we have a net movement of water? Okay the answer is
no, right? Water cannot move—there’s no net movement of water because there’s no
gradient of water to move. So in an isotonic solution, water is not diffusing; there’s
not net movement of water.
Okay, so then a little question for you: Would it be appropriate to describe a cell as hypotonic,
hypertonic, or isotonic, or can we only use those terms to describe solutions? And the
answer to that is absolutely, you can say a cell is hypertonic to a solution. You can
say a cell is hypotonic to a solution; notice I’m always comparing it to something. It’s
hypertonic. The cell is hypertonic to a solution. Okay? And so this is why I don’t want you
to memorize what happens in hypertonic solutions or hypotonic solutions, because then what
happens if I turn it around and I say, “Oh, the cell is hypotonic,” or “The cell is
hypertonic to the solution.” Now that’s going to blow your mind and now you’re going
to have to try to keep things straight. So it’s not difficult to set up these problems
and work through them, so that’s what I highly suggest for you.
Okay, so here’s a practice problem for you. And so what I would like for you to do is
pause the video and work this problem. So here we have a cell, and it’s placed in
a saltwater solution of 0.2 millimolar sodium chloride. Now in these problems it’s always
going to be important for you to draw, okay? So I’m just going to take a cell and I’m
going to put it into my solution. So here’s my cell. Okay, and I’m putting it into a
saltwater solution that has 0.2 millimoles of sodium chloride. Okay, neither sodium nor
chloride can permeate the plasma membrane, meaning those ions are not going to be diffusing.
So we want to know, well what’s going to happen to the cell? And they gave you a little
hint here: The osmolarity of sodium chloride in the intracellular fluid is less than 0.2
millimoles. Okay, so now we’re dealing with osmosis, correct? And now what we need to
do is convert to osmoles, or milliosmoles is fine as well. So sodium chloride will disassociate
in water, and when it disassociates in water, you get 0.2 sodium. You get 0.2 of chloride
ions, so that means we’re going to have less than 0.4 milliosmoles of sodium chloride.
Now inside the cell that means we’re going to have less than the 0.4 millosmoles, right,
of sodium chloride. So now my question to you is where is there more water? Okay? So
in other words—if you have less solute, you’re going to have more water, right?
So we have less solute in the cell, which means we have more water in the cell, less
water in the solution. So that means our water is going to diffuse out of that cell. In this
particular problem, we already know it’s an osmosis question. However if you were working
this on an exam or in the laboratory, you would be told that an aquaporin is present.
So we could actually add to this: “Assume an aquaporin is present.” Okay? And that
will let you know then that that water is capable of diffusing if there’s an aquaporin
present. Okay, so the answer then, if water’s moving out of the cell, the cell is going
to shrink because the saltwater—the solution—is hypertonic. There’s more solute in the solution
than the cell.