 Today we're going to be talking about diffusion, osmosis, and water balance. Diffusion is the movement of the solute in solution from an area of high solute concentration to an area of low solute concentration. So, for example, if we have a solution and we dump in some salt, we're going to have a high solute concentration in the corner of the beaker. As time goes by, we're still going to have a relatively high concentration of solute molecules in the corner. But we're also going to have some solute molecules diffusing, moving from an area of high solute concentration to an area of low solute concentration until eventually they reach equilibrium where they're randomly and evenly placed throughout the solution. In this example, we have solute molecules in the extracellular space moving across a cell membrane or lipid bilayer to an area of low solute concentration. So over time, again, we have molecules of solute moving from an area of high concentration to an area of low concentration until we have equivalent solute concentration on both sides of the lipid bilayer. So there are 14 solute molecules on this side of the bilayer and 14 on this side in the same volume, giving them equal concentration. So that's diffusion, whether diffusion happens as it did in these beakers without a membrane or in your cells where diffusion may happen across the lipid bilayer. Diffusion is movement of a solute from an area of high solute concentration to an area of low solute concentration. In contrast, osmosis is the movement of solvent, usually water, across a membrane from an area of high solvent concentration to an area of low solvent concentration. Let's take a look at an example of osmosis. On the right side of the membrane, we have a high concentration of solute molecules and a low concentration of solvent. So since osmosis deals with the movement of solvent, we're going to write down that we have low solvent concentration on that side of the beaker. And on the other side of the membrane represented in red, we have a high solvent concentration and a relatively low solute concentration. So again, we're going to look over time at what happens in osmosis. So we're still going to have the same number of solute molecules on each side of the membrane because the solute molecules are not moving. The solvent is going to move. And so over time, a movement of high solvent concentration to an area of low solvent concentration until they reach an equilibrium. Talking about osmosis, especially as it relates to our cells, we need to know three more definitions. Hypertonic, hypotonic, and isotonic. It's important to remember when we're looking at hypotonic solutions, hypotonic solutions, or isotonic solutions, they are always hypotonic to another environment, hypotonic to another environment, or isotonic to another environment. You always are comparing the solute and solvent concentrations from one solution to the next. So if a solution is hypotonic to a second solution, it has a high solute concentration relative to the other solution. Hypotonic solutions have low solute concentration relative to another solution. And isotonic solutions have equal solute concentrations relative to each other. Tinnicity and osmosis are very important when it comes to cells. In a hypotonic solution, the solute concentration outside of the cell is greater than the solute concentration inside of the cell. So that means that the solvent or water concentration is low outside of the cell and high inside of the cell. So in this case, water is going to flow from high water concentration to low water concentration. In hypotonic solution, water is going to flow out of the cell. In a hypotonic solution, the water concentration outside of the cell is high and the water concentration inside of the cell is low. So we're going to have water flowing from its high concentration to its low concentration. Water is going to flow in to the cell. In isotonic solution, solute concentration and solvent concentration are equal. So it's tempting to say that no water would move across the cell membrane. However, water is always flowing across the cell membrane. It's just going to flow so that the solvent concentration remains equal. It's going to flow in to and out of the cell at the same time. So let's take a look at how that might affect plant cells. So we said in a hypotonic solution, we're going to have water going from an area of high water concentration to an area of low water concentration, leaving the cell. In plant cells, we can see that they have this cell wall and the cell membrane, which surrounds all their cytosol and cytokodin. And so if water leaves a plant cell, that cell membrane pulls away from the cell wall. Let's look at an isotonic solution and see how that affects plant cells. So as we said, water would be entering and exiting the cell. And again, we see that the cell membrane of this plant cell has pulled away a little bit from the cell wall. That leads to a wilted or flaccid looking plant cell. However, if we're in a hypotonic solution, if water is entering the cell, water fills up this vacuole inside the plant cell, and the water pressure pushes the cell membrane against the cell wall, creating a more rigid or healthy appearance for the plant. So for plant cells, the ideal solution is hypotonic. And let's take a quick look at some plants that are either wilted. So we just said plant cells tend to be wilted in an isotonic solution, because there's less water inside of the cell, inside of those vacuoles pushing against the cell wall. So those cells tend to be wilted. Healthy plant cells exist in a hypotonic solution. Hypotonic, again, we said has a lower solute concentration outside of the cell than inside of the cell. So the way to get from an isotonic to a hypotonic solution, that's why if your plants are looking wilted and you water them, you can get a healthier looking plant cell. So plant cells ideally are in hypotonic solutions. But what about animal cells? Let's take a look at what happens to some animal cells. These are red blood cells in different solutions, and determine maybe what's the best for animal cells and whether that's different from plant cells. In a hypotonic solution, we see water leaving red blood cells. This gives these red blood cells sort of shriveled appearance. In an isotonic solution, water is entering and leaving red blood cells in an equivalent rate. They have their typical donut shape. They look pretty healthy. But let's take a look at hypotonic and determine if that might be the best, because that's the best for plant cells, remember. In a hypotonic solution, water is going to enter the red blood cells. So when water enters the red blood cells, they swell up, kind of looking more like balloons. What happens to the balloon when you put too much in it? It bursts. So this is called lysis. Cells are placed in a hypotonic solution, they burst. So obviously, that's not ideal. For our red blood cells and animal cells, an ideal solution is going to be isotonic. So animal cells are a little bit different than plant cells when we're thinking about water balance and tenacity.