Pressure potential is physical pressure on water. And whereas solute was a negative type of pressure, pressure potential can be positive or negative. Generally, that negative pressure is referred to as tension because it's kind of like a pulling force, as opposed to a pushing force. Remember, our straw example, we're pulling the liquid up with a negative pressure. It should be noted, though, that living cells have positive pressure, and that's because living cells are going to have solutes inside of them, they're also going to be filled with water, and that's important. If cells shrivel up, they usually die. We'll get to that in just a moment. Now, when membranes are present, we're usually going to see water move in response to solute potentials. Right? From high to low solute potential. When membranes are absent, we're really going to see water move from high to low pressure potential. Why do you think solute potential only has an effect really when there are membranes present? Well, why do we need membranes at all to concentrate those solutes? Right? Without a membrane present, those solutes are just going to diffuse. Meaning, there is going to be no potential difference in solute potential. There is going to be no difference in solute concentration. So, we need the membrane to have the difference in solute concentration, which is basically the same thing as saying we need the membrane to have the difference in solute potentials.
So, I want to do a couple of examples here, just to show you a few things. So, over on, all the way over on the left here, we have this U-shaped tube, it's filled with water, and those red dots are representing dissolved solutes. Those are solutes in the water, and there is a concentration difference. Hopefully, you can see there's more dots on one side than the other, meaning there's a concentration difference between the two sides. And this dotted line right here, that is our semipermeable membrane. Right? So it's going to allow water to pass through, but not the solutes. So on this side, we have a high concentration of solutes. Meaning, we have a low water potential, I'm sorry, low solute potential. So our solute potential is low. Here we have a low solute concentration, meaning our solute potential is high. Now, just for, I don't know, just for giggles, I'm going to add some numbers in here. So let's say that our low solute potential is going to be negative 2 mega Pascals. Right? Gotta have units, otherwise, that's meaningless. And let's call this negative one mega Pascals. Alright. So what's going to happen? Water wants to lose its potential. Right? So we're going to go from high potential to low potential, which is the same as saying we're going to go from a low concentration of solutes to a high concentration of solutes, meaning, the water is going to move over to this side, like that. So over time, you are going to wind up with a U-shaped tube that looks like this. Right? There's going to be a difference in the heights, as you can see, of the water levels on the two sides, but the concentrations will now be the same. Right? Even though there are more molecules of solute on this side, there's more water. So the concentrations balance. Right? This is going to be like our equilibrium point.
So this might all seem very familiar from our example that we talked about when we talked about osmosis; here's where I want to spice things up a little. Let's pretend that now in this U-shaped tube, I'm going to add a pressure potential on this side. And I'm going to make my pressure potential equal to 1 mega Pascal. What do you think is going to happen if I do that? What's going to happen if I add a pressure potential of 1 mega Pascal pushing down on this side of the tube? What actually is going to happen is we're going to end up with something like you see over here; let me jump out of the way. The water levels are going to become even again. Why is that? Because by adding a pressure potential of 1 mega Pascal over on the left side of the U-shaped tube, I've actually balanced out the water potentials between the two sides. So to recap, on the left side, we have a solute potential of negative 2 mega Pascals, and we also have a pressure potential of 1 mega Pascal. On this side, we have a solute potential of negative 1 mega Pascals, and no pressure potential. So our pressure potential is just equal to 0 mega Pascals. If we use our formula, right, that water potential equals solute potential plus potential pressure, we'll see that our water potential on this side, so this is plain old water potential, is negative 1 mega Pascals, and our water potential on this side is also negative 1 mega Pascals. Meaning, we don't have any net flow of water, and I say net flow there because in actuality, there's going to be water kind of going back and forth between both sides, but the net amount on each side is going to remain the same. So, hopefully, all of that makes sense. Now have a good understanding of what all these types of potentials are. Right? Water potential, solute potential, pressure potential. Now let's take these ideas and actually apply them to a living cell. So hopefully you remember there was that idea we talked about before, turgor pressure. That's the pressure inside the cell, due to the, usually, it's mostly the vacuole swelling, but generally speaking, it's the contents of the cell pushing against the cell wall. And usually, turgor pressure is experienced because the vacuole in the plant cell will swell up and cause the cell contents to push against the cell wall. We call those cell contents, by the way, protoplasts. That's the, all the living stuff inside the cell plus the plasma membrane, and it does not include the cell wall. So, you may remember Einstein's famous words. Right? That every action has an equal and opposite reaction. Right? Well, if turgor pressure is pushing against the cell wall, its equal and opposite reaction is wall pressure, which is the force exerted by the cell wall on the cell contents. So it's equal and opposite to turgor pressure. Now, actually, you have to turn turgidity up. You have to increase turgor pressure to induce wall pressure. Right? So if the cell is what we call flaccid, meaning there's no turgor pressure, or no pressure potential, like, you know, we see over here, we're not actually going to have wall pressure because we don't have turgor pressure. So you have to increase turgidity to induce wall pressure. Right? You have to swell up the cell contents, so that you can start experiencing those two pressures. Now in some cases, cells will become flaccid. Right? They'll have no turgor pressure. In fact, sometimes they can shrivel up. We call this plasmolysis, and you can kinda see that happening right here. The cell is all shriveled up due to water loss. I mean, look how much smaller that vacuole is as compared to this vacuole, or this vacuole over here. Now, in non-woody plants, when this happens, when turgidity is lost, sometimes what we'll see is wilting. And you know, maybe you've seen wilted plants start to droop over like this sad plant here. Well, this is wilting, and this is due to a drop in turgor pressure.