Osmosis

Video Transcription

In this lesson today, we are going to discuss the process and roles of osmosis. Osmosis is the movement of water across a selectively permeable membrane. A couple of things I'm going to touch base on right here.

First of all, when we're talking about osmosis, we're talking about water. If you think back to diffusion, diffusion is the movement of-- could be any substance-- from an area of high concentration to low concentration. But osmosis, we're focusing specifically on water, and we're talking about water moving across a selectively permeable membrane.

This means that the membrane can determine what passes through it. Some membranes are not going to allow solutes to pass. There's certain solutes that are not going to be allowed to pass a cell membrane. Instead of those solutes being able to pass back and forth to even out concentrations, water is going to have to pass back and forth in order to even out concentrations. So the purpose of osmosis is to even solute concentrations across the membrane by moving water.

Let's take a look briefly about what I mean when we talk about concentration. I'm going to draw a picture of two beakers right here. And we're going to put an equal amount of water in each of the beakers.

Now, let's say we drop in one of the beakers two tablespoons of salt. And we're going to use these dots to represent molecules of salt. In this one, we have quite a few molecules of salt filling up that water. Let's say in this one, we only drop in half a tablespoon. Maybe, it looks something like that.

You'll notice that we have the same amount of water in each of our beakers, but this one has a significant amount more salt than this one does. So therefore, this one would have a higher concentration of salt than this beaker would. Think of concentration as the ratio of solutes to solvent.

Let's discuss what tonicity means, because it relates to concentration. Tonicity describes the concentration of solutes across a membrane. When we have a concentration of solutes equal across the membrane-- the concentration of solutes and one side of the membrane is equal to the concentration of solutes on the other side of the membrane-- we refer to that as being isotonic.

Most of the time, there is an equal amount of water moving into and out of cells. You're not going to have a huge net movement of water. Most cells generally have equal amounts of water moving into and out of it.

Our next one here says the side of the membrane with a higher concentration, and we refer to that as being hypertonic. So the side of the membrane that has a higher concentration of solutes is going to be referred to as hypertonic. While the side of the membrane with a lower concentration of solute is going to be hypotonic.

Osmosis is the form of passive transport. What this means is that it does not require the use of ATP, or cellular energy, in order for it to happen. It just occurs naturally, because water will be moving from an area of high concentration to an area of low concentration to even out the concentration across a membrane.

Let's take a look at some examples and see if you can figure out which direction water would flow in order to even out the concentrations. We have a cell right here that has a high concentration of solutes inside of the cell and a lower concentration of solutes outside of the cell. If the solutes cannot pass through the membrane, water needs to move. So which direction would water move in this case to even out the concentration?

Water would move into the cell. You'll notice, this side of the cell would be referred to as hypotonic-- having a lower concentration-- while the inside of the cell would be hypertonic-- having a higher concentration. Water is going to have to flow into the cell to even out that concentration.

How about this example right here. The outside of the cell is hypertonic-- having more solutes than the inside of the cell. The inside of the cell is hypotonic-- having less solutes. Which direction is water going to flow in this case, if the solutes can't flow back and forth? Water is going to have to flow out of the cell to even out those concentrations.

And our last one right here, you'll notice we have the same number of solutes inside and outside the cell. So water is going to move equally into and out of the cell in this case. We're not going to have a net movement of water-- the same amount of water flowing in as flowing out naturally.

So this cell in this case, as we said, would be in a hypotonic solution. The outside of the cell has fewer solutes, so the cell is in a hypotonic solution. This cell would be in a hypertonic solution. The conditions outside of the cell are hypertonic. And this cell would be in an isotonic solution, because the number of solutes outside of the cell equals the number of solutes inside of the cell.

The type of conditions that a cell is in can have an affect on the cell. When water is moving into the cell-- let's take a look-- I'm going to backtrack here just for a second. We're going to use this information as an example about red blood cells.

Red blood cells cannot actively take in or get rid of water. When red blood cells are in certain conditions, it's going to affect what the cell looks like. This cell right here-- we're going to pretend this is a red blood cell-- that is in a hypotonic solution.

Remember, hypotonic solutions have fewer solutes outside than inside. And what that's going to do is cause water to move into the cell. Water is going to move into the blood cell, and it's going to cause the blood cell to expand. When blood cells are in hypotonic conditions, they will generally explode.

This cell is just a normal-looking red blood cell. So this would be an example of a red blood cell that was in an isotonic condition. We have equal amounts of water moving into and out of the cell, not having an effect on what that cell looks like.

This is an example of a blood cell that would be in a hypertonic solution. Remember, hypertonic solutions have a higher concentration of solutes outside than inside. So water is going to move out of the cell. And it would cause this red blood cell to shrivel up. So you'll see that the conditions that a cell is in can have an effect on what the cells looks like-- whether it's hypotonic, hypertonic, or isotonic conditions.

This lesson has been an overview on the process of osmosis, as well as the role of osmosis in our bodies.