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When we were children in West Texas, winters were cold, but it very rarely snowed. But we did have ice, which caused the roads to be salted. Salt mixes and breaks down or dissolves in water on the road. This can lead to a lower freezing point, which can help prevent roads from freezing. Although this was great for making roads safer, it wasn't great for the plants that lived on the roadside. It often caused their death.
Now, winters can be tough on many plants, but I'm talking about salt and how it affects even some tough plants (metaphorically). This problem with plants and salt isn't limited to winter. During tropical storms near the coast, salty seawater can be pumped in large quantities into the soil. Although the effect may not be immediate, this eventually leads to the death of plants—including trees—that survived the storm. Why? Do plants hate salt that much?
In reality, this has to do with a wonderful phrase: osmosis. When you talk about osmosis, you're talking about the movement of water across a selectively permeable membrane, like a cell membrane (plasma membrane). Very small water molecules can cross the plasma membrane without help, or they can cross in large quantities through protein channels like aquaporins. The movement of water molecules when they move across a cell membrane is passive transport (not active). This means it doesn't require energy.
In osmosis, water molecules move from areas of high concentration (of water molecules) to areas of low concentration (of water molecules). But there's another way to think about how water moves in osmosis. Low water concentration often means high solute concentration. Solutes are substances—like salt or sugar—that can break down or dissolve in a solvent like water. Water is attracted to areas where there is a higher concentration of solutes, which means a lower concentration of water.
So, if you want to easily know where water will move—look to the side where there is a higher concentration of solutes. Except for another variable, like pressure, water will have a net movement to the area of higher solute concentration. So, let's bring out the U-tube. Haha, U-tube is funny. There's a selectively permeable membrane in the middle. Let's assume it's similar to a cell membrane in its ability to let water molecules pass through—water molecules are small—but salt cannot pass.
Now, there's only water in the tube. The water level on side A and side B is equal. This doesn't mean water molecules aren't in motion—water molecules tend to move—but the total movement across both sides is zero. This means the total change in direction of movement is zero. Now, let's imagine you've poured a large amount of salt into side B. In which direction will the water move—A or B? Think about what we've discussed about osmosis. The answer is B! Side B has a higher solute concentration than side A. Water moves to the higher concentration of solutes, which is also a lower concentration of water.
The water level on side B will be higher in the tube. You can think of it as water trying to equalize the concentrations—diluting side B. When it reaches equilibrium, the total movement of water across both sides will be zero, but remember that water still tends to move, and movement still happens. Here are some terms to add—we describe side B as being hypertonic. This means a higher solute concentration! But we can't just say something is hypertonic without comparing it to something else. We say side B is relatively hypertonic or hypertonic compared to side A because it has a higher solute concentration than A. In osmosis, water moves to the hypertonic side.
"We say side A is hypotonic (hypo rhymes with low, which helps me remember it has a lower solute concentration)" when compared to side B. Let's move to real life now instead of just a U-tube. As you know, water is important for your body and many of the processes that happen in your body. When someone is given an IV (intravenous fluids) in the hospital—the fluid in the IV might look like pure water. But it's definitely not pure water. It would be disastrous due to osmosis—let's start explaining.
Let's assume, for the sake of argument, that pure water was in the IV. Now, the IV tube runs through the veins, all the way to your bloodstream. It's very useful for delivering medications through it. Blood actually consists of many types of components, and red blood cells are a great example. What do you think has a higher solute concentration? The hypothetical pure water in the IV tube? Or the red blood cells? Well, cells aren't empty vessels—cells contain solutes. The pure water hypothetically flowing through the IV tube contains no solutes. So, where does the water go? It goes to areas of high solute concentration—inside the cells.
The cells are hypertonic compared to the pure water in the IV tube because the cells have a higher solute concentration. The cells would swell and possibly burst! Red blood cell bursting is not good. If someone needs fluids, they usually receive an isotonic solution relative to their blood plasma—isotonic meaning equal osmotic pressure—so that red blood cells don't swell or shrink.
Or let's talk about an aquarium. I've always wanted a saltwater aquarium since I was a child. But I've only ever had a freshwater aquarium. Still. I used to wonder as a child, why can't a saltwater fish live in a freshwater aquarium? Well, let me explain one reason why it would be dangerous for a saltwater fish and how it relates to osmosis. First, let's ask—where is the higher solute concentration? In the cells of the saltwater fish or in the freshwater that the fish will be placed in? Most certainly in the cells of the saltwater fish. So, where will the water go? It goes to the area where there is a higher solute concentration—the hypertonic side—so water goes into the cells of the poor saltwater fish. If it's not rescued, it might die.
One thing to clarify: freshwater and saltwater fish aren't necessarily isotonic to their surroundings, but they have special adaptations that allow them to live in their environment, and they generally can't make a drastic change from a saltwater environment to a freshwater environment. Now—not all fish suffer from this problem. There are some fish that have amazing adaptations to switch between freshwater and saltwater, and they have to deal with this osmosis problem. Salmon, for example. I think if I could be any fish, I'd be a salmon. No questions.
Osmosis explains how many plants get water. Many plants have roots, certainly. But how does water get into the roots? When it rains, the soil becomes saturated with water. The root hair cells generally have a higher solute concentration inside them than the solute concentration in the moist soil. Water moves into the root cells because the root cells are hypertonic compared to the hypotonic soil. By the way, you might wonder—well, why don't the root hair cells burst with all this water? This brings us to our new osmotic topic and why the plant cell wall is amazing!
So, let's bring in a new variable that can affect osmosis: pressure potential. This is where it becomes very useful to understand how water potential (Ψ) can be measured. Water potential takes into account both solute potential (Ψs) (or osmotic potential) and pressure potential (Ψp). In osmosis, water moves to areas of lower water potential. So, the equation is: Water Potential (Ψ) = Pressure Potential (Ψp) + Solute Potential (Ψs). Adding a solute actually makes the solute potential value negative and lowers the overall water potential value. Water moves to areas of lower water potential.
But applying pressure can raise the pressure potential, a positive value, thus raising the overall water potential. Let's give a quick example. In a water potential experiment with potato tuber samples—many copies of this experiment are online and meticulously done—you can calculate the water potential of the potato tuber using the water potential equation. When the potato tuber is placed in distilled water—pure water—the potato tuber begins to gain water. You'd expect that. Water moves towards the higher solute concentration. Because of the high solute concentration, the potato tuber has a lower solute potential. This means a lower overall water potential than its surroundings, and water moves to areas of lower water potential.
But as time passes and the potato tuber cells gain water, the water entering the cells exerts pressure against the plant cell wall from inside the cells. Thus, it raises the overall water potential in the potato tuber cells. We want to point out that the turgor pressure caused inside the plant cell, due to osmosis and the plant cell wall, is extremely important for plant structure and the ability of plants to grow upright and not wilt. Turgor pressure is something definitely worth exploring.
In summary, where would living things be without osmosis? It's involved in the movement of one of our most valuable resources: water. "Well, that's all for the amoeba sisters, and we remind you to stay curious. Translation by Abbas Jawad."