Osmosis and Water Potential (Updated)

(MUSIC)

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

snow.

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

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

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

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

that lived right along the roadside.

It often caused them to die.

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

even some hardy plant life.

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

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

soil.

This can eventually kill plants- including

trees- that had originally survived the hurricane.

Why?

Do plants just dislike salt that much?

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

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

semi-permeable membrane, like a cell membrane.

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

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

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

which means, it does not require energy.

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

to a low concentration (of water molecules).

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

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

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

water.

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

would mean less water concentration.

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

where there is a greater solute concentration.

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

movement to the area of higher solute concentration.

So, let’s bring out a U-tube!

Ha, U-tube.

That’s funny.

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

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

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

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

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

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

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

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

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

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

Think about what we mentioned with osmosis.

The answer is B!

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

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

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

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

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

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

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

That means higher solute concentration!

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

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

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

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

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

of just the U-tube.

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

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

pure water.

But it is certainly not pure water.

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

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

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

really useful for running medication through.

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

a great example.

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

the hypothetical pure water in this IV tube

or the red blood cells?

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

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

So, where does the water go?

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

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

a greater solute concentration,

the cells would swell and possibly burst!

Exploding red blood cells are not good.

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

blood plasma.

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

blood cells.

Another example, let’s talk about the aquarium.

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

But I’ve only had freshwater tanks

so far.

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

my freshwater tank?

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

relates to osmosis.

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

In the saltwater fish cells

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

Definitely in the saltwater fish cells.

So, where would the water go?

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

it goes into the cells of that poor saltwater fish.

If not rescued, it could die.

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

their surroundings.

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

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

Now---not all fish have this problem.

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

they have to deal with this osmosis problem.

Salmon for example.

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

Osmosis

explains how many kinds of plants get their water.

Sure, many plants have roots.

But how does the water get into the roots?

When it rains, the soil becomes saturated with water.

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

the solute concentration in the saturated soil.

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

the hypotonic soil.

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

water that is going in them.

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

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

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

Water potential considers both solute potential AND pressure potential.

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

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

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

potential to lower.

Water will travel to areas of lower water potential.

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

the total water potential.

So, let’s give a quick example.

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

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

the water potential formula.

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

cells start to gain water.

You’d expect that.

The water is moving towards the higher solute concentration.

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

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

of lower water potential.

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

against the plant cell walls from inside the plant cells,

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

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

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

to grow upright and not wilt.

Turgor pressure is definitely something to explore.

In summary, where would living organisms be without osmosis?

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

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