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. What we need to recall from our previous lesson videos is that biological membranes are semi-permeable, meaning that some things can cross the membrane, but others cannot. If the solutes in a solution cannot diffuse across the membrane from areas of high concentration to areas of low concentration, then osmosis will occur. Recall osmosis is the diffusion of water across the membrane. Water will always move from hypotonic solutions towards hypertonic solutions, allowing water to move towards the more concentrated solution of solute to dilute that solution until it becomes isotonic.
Essentially, water is moving to help 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?" Here, it's important to keep in mind that hypotonic and hypertonic refer to solute concentration. However, water is not the solute; it's the solvent, and it moves from areas of higher concentration of water towards lower concentrations of water. Hypotonic solutions have lower solute concentrations but higher water concentrations, whereas hypertonic solutions, despite having higher solute concentrations, have lower water concentrations. Water is the solvent, and these terms hypo and hyper refer to the concentrations of solute.
In areas that are hypotonic, they have lower solute concentrations, but they have higher water concentrations. Hypertonic solutions have higher solute concentrations which means they have lower water concentrations. The brackets you see just mean the concentration of. So when you see something in brackets like H₂O, it means concentration of water, and solute in brackets means concentration of solute. Water will always move from hypotonic towards hypertonic solutions. If you remember that water flows from hypo to hyper, you'll be good on most of your osmosis questions. We'll be able to get some practice applying the concepts that we've learned here. Water always flows from hypo towards hypertonic solutions. That being said, I'll see you all in our next video.