6. Cell Membrane & Transport
Osmosis
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So now that we've introduced tonicity in our last lesson video, in this video we're going to talk about how tonicity affects the direction of osmosis. And so what we need to recall from our previous lesson videos is that biological membranes are semi permeable, which means that some things can get across the membrane, but other things cannot get across the membrane. And so if the solutes that are in a solution cannot diffuse across the membrane from areas of high concentration to areas of low concentration, then osmosis is going to occur. And recall, osmosis is the diffusion of water across the membrane. And so here's what you guys need to know. Water will always move from hypotonic solutions towards hypertonic solutions. And so this is the direction of water movement, from hypo towards hypertonic solutions. And so this is going to allow water to move towards the more concentrated solution of solute in order to dilute that solution and it will continue to move towards that solution until it becomes isotonic. And so essentially the water is moving to help to try to create equal solute concentrations in both solutions. Now, you might be thinking, wait a second, doesn't that go against the natural tendency of diffusion? Don't substances always diffuse from high concentration to low concentration? But here you're telling me that it's diffusing from low concentration to high concentration. So how does that make sense? Well, what's important to keep in mind is that hypotonic and hyper tonic are terms that refer to the solute concentration. However, water is not the solute, water is the solvent. And so what's important to keep in mind is that water is still moving from areas of higher concentration of water towards lower concentrations of water. So let's take a look at this in a little bit more depth here to clear this up. Hypotonic solutions, they do have lower solute concentrations like we described in our last lesson video. However, hypotonic solutions, even though they have lower solute concentrations, they actually have higher water concentrations. And hypertonic solutions, they do have higher solute concentration just like what we described in our last lesson video. However, hypertonic solutions, although they have higher solute concentrations, they have lower water concentrations. And so what's really important to just, realize here is that water is the solvent, whereas solutes are not the solvent. And so, these terms hypo and hyper are referring to the concentrations of solute. However, water is not the concentration of solute and water is still going to be moving from areas of higher concentration of water towards lower concentrations of water. So once again, let's take a look at our image down below to try to clear this up even further. And so notice what we have here down the middle is a biological membrane, which is semipermeable, meaning that some things can cross the membrane but other things cannot cross the membrane. And so in a scenario where the solutes cannot cross the membrane. Although the solutes would love to diffuse from higher concentration to lower concentration across the membrane, they can't because they're being blocked. And so in some scenarios, if the solutes cannot diffuse across the membrane, instead of the solutes diffusing from high to low concentration, because they can't, what's going to happen is water is going to move instead. And in areas that are hypotonic, yes, they have lower solute concentration because hypo tonic is referring to the lower solute concentration, but it turns out that they have actually have a higher water concentration. They have higher water. And hypertonic solutions, yes, they have a higher solute concentration because, remember, hypertonic is referring to the solute concentration being higher. However, higher, solute concentration means that it's going to have lower water concentration. And by the way, the brackets that you see here, just mean the concentration of. And so when you see something in brackets like h two o in brackets, it means concentration of water, and solute in brackets means concentration of solute. And so remember what we said, what you need to know is right here. Water will always move from hypo towards hypertonic solutions. And so over here on the left we have hypo, over here on the right we have hyper. So water is always going to flow in this direction. And so if you're able to remember, water flows from hypo towards hyper, then you'll be good on most of your osmosis questions. And so we'll be able to get some practice applying the concepts that we've learned here, but once again, keep in mind, water always flows from hypo towards hypertonic solutions. So that being said, I'll see you all in our next video.
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