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(MUSIC)

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When we were kids, growing up in West Texas, our winters would be cold, but rarely experienced

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snow.

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But we did have ice, which resulted in the roads being salted.

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As the salt mixes in and dissolves into water on the road, this can lead to a lower freezing

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point, which can help prevent the roads from icing over.

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And while this is great for making the roads more safe, it wasn’t so great for the plants

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that lived right along the roadside.

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It often caused them to die.

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Now winter can be hard for many plant species, but I’m talking about this salt affecting

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even some hardy plant life.

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This issue with salt and plants isn’t limited to winter.

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During hurricanes near the coast, salty ocean water can be dumped in large quantities into the

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soil.

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This can eventually kill plants- including

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trees- that had originally survived the hurricane.

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Why?

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Do plants just dislike salt that much?

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Well, it's actually related to a term called osmosis.

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When you are talking about osmosis, you are talking about the movement of water through a

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semi-permeable membrane, like a cell membrane.

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Water molecules are so small that they can travel through the cell membrane unassisted,

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or they can travel in larger quantities through protein channels like aquaporins.

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The movement of water molecules traveling across a cell membrane is passive transport,

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which means, it does not require energy.

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In osmosis, water molecules travel from areas of a high concentration (of water molecules)

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to a low concentration (of water molecules).

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But there’s another way to think about water movement in osmosis.

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A low water concentration likely means there is a greater solute concentration.

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Solutes are substances like salt or sugar that can be dissolved within a solvent like

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water.

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Water has the tendency to move to areas where there is a higher solute concentration, which

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would mean less water concentration.

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So, if you want to easily figure out where the water will travel in osmosis, look to the side

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where there is a greater solute concentration.

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Unless we bring in another variable, like pressure, water will generally have a net

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movement to the area of higher solute concentration.

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So, let’s bring out a U-tube!

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Ha, U-tube.

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That’s funny.

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There’s a semi-permeable membrane in the middle of it.

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Let’s assume that it is similar to a cell membrane and that water molecules can squeeze

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through it—the molecules are quite small—but salt can’t.

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Right now, there is just water in this U-tube.

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The water levels on side A and side B are equal.

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That doesn’t mean that the water molecules aren’t moving---water molecules like to

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move---but the net movement across the two
sides is zero.

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That means, the overall change in the direction of movement is zero.

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Now let’s imagine on side B, you dump a huge amount of salt there.

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So, which direction will the water initially move towards, A or B?

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Think about what we mentioned with osmosis.

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The answer is B!

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Side B has a higher solute concentration than side A. Water moves to areas of higher solute

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concentration, which is also the area of lower water concentration.

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You will also see the water level on side B rise as the water moves to that area.

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You can almost think of the water as trying to equalize the concentrations diluting

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side B. Once equilibrium is reached, the net movement of water across the two sides will

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be zero, but remember that water still likes to move and movement still occurs.

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Now here’s some vocabulary to add in here---we call side B hypertonic.

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That means higher solute concentration!

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But we can’t just say something is hypertonic without comparing it to something else.

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We say side B is hypertonic to side A because it has a higher solute concentration than

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side A. In osmosis, water moves to the hypertonic side.

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We say side A is hypotonic (hypo rhymes with low which helps me remember that it is the low solute concentration)

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when compared to side B. Let’s get a little more real life now instead

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of just the U-tube.

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As you know, water is important for your body and many processes that occur in the body.

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When someone gets an IV in a hospital, it may look like the fluid in the IV is just

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pure water.

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But it is certainly not pure water.

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That would be a disaster because of osmosis, let’s explain.

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Let’s say hypothetically pure water was in an IV.

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Now an IV tube typically runs through a vein, so that you have access to your blood stream,

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really useful for running medication through.

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Blood actually consists of many different types of components and red blood cells are

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a great example.

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So, what do you think has a higher solute concentration,

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the hypothetical pure water in this IV tube

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or the red blood cells?

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Well, cells are not empty vessels, they contain solutes.

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The pure water that hypothetically is running through this IV tube has no solutes.

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So, where does the water go?

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It goes to the area of higher solute concentration which in this case is inside the cells.

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The cells are hypertonic compared to the pure water in the IV tube because the cells have

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a greater solute concentration,

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the cells would swell and possibly burst!

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Exploding red blood cells are not good.

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If a person needs fluids, they typically will receive a solution that is isotonic to their

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blood plasma.

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Isotonic means equal concentration, so you won’t have any swelling or shrinking red

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blood cells.

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Another example, let’s talk about the aquarium.

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I have always wanted a saltwater fish tank, ever since I was a little kid.

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But I’ve only had freshwater tanks

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so far.

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I did often question when I was a kid, why is it that a saltwater fish can’t be in

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my freshwater tank?

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Well, let me explain one reason why this would be dangerous to a saltwater fish and how it

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relates to osmosis.

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First ask---where is there a higher solute concentration?

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In the saltwater fish cells

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or in the freshwater that the fish would be hypothetically placed in?

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Definitely in the saltwater fish cells.

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So, where would the water go?

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It goes to the area where there is a higher solute concentration----the hypertonic side----so

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it goes into the cells of that poor saltwater fish.

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If not rescued, it could die.

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Now one thing to clarify: saltwater fish and freshwater fish are not necessarily isotonic to

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their surroundings.

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But they have special adaptations that allow them to live in their environment and usually

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cannot make a major switch from a saltwater environment to a freshwater one.

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Now---not all fish have this problem.

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There are some fish that have this amazing adaptations to switch between fresh and salt water, and

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they have to deal with this osmosis problem.

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Salmon for example.

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I think if I could pick to be a fish, I’d be a salmon.

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Osmosis 

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explains how many kinds of plants get their water.

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Sure, many plants have roots.

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But how does the water get into the roots?

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When it rains, the soil becomes saturated with water.

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The root hair cells generally have a higher concentration of solutes within them than

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the solute concentration in the saturated soil.

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The water travels into the root hair cells as the root hair cells are hypertonic compared to

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the hypotonic soil.

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By the way, you may wonder---well, why don’t those root hair cells burst with all the

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water that is going in them.

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That brings us to our next osmosis topic and why plant cell walls are amazing!

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So, let’s bring in another variable that can influence osmosis: pressure potential.

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This is when it’s very useful to understand how one can calculate water potential.

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Water potential considers both solute potential AND pressure potential.

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In osmosis, water travels to areas of lower water potential.

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So, the formula is water potential is equal to the pressure potential plus the solute potential.

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Adding solute actually causes the solute potential to have a negative value and the overall water

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potential to lower.

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Water will travel to areas of lower water potential.

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But exerting pressure can raise the pressure potential, a positive value, therefore raising

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the total water potential.

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So, let’s give a quick example.

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In the popular water potential in potato cores lab---all kinds of neat variations of this

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lab procedure exist online---you can calculate the water potential in potato cores using

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the water potential formula.

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When a potato core is first put into distilled water—that’s pure water---the potato core

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cells start to gain water.

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You’d expect that.

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The water is moving towards the higher solute concentration.

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Thanks to their higher solute concentration, they have a lower solute potential.

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That means a lower total water potential than the surroundings and water travels to areas

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of lower water potential.

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But over time as the potato core cells gain water, the water that has entered exerts pressure

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against the plant cell walls from inside the plant cells,

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therefore raising the overall water potential in the potato core cells.

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We want to point out that this turgor pressure that results in plant cells, thanks to osmosis

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and plant cell walls, is critical for overall plant structure and the ability of plants

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to grow upright and not wilt.

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Turgor pressure is definitely something to explore.

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In summary, where would living organisms be without osmosis?

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After all, it involves movement of one of our very valuable resources: water.

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Well, that’s it for the Amoeba Sisters and we remind you to stay curious!