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You know a word that gets thrown around a lot but is actually pretty challenging to
understand?
Osmosis.
But understanding osmosis does help bring many answers to some great
questions like, “Why would it be deadly to give someone an IV of pure water?” or,
“Why would a saltwater fish placed in freshwater would experience major problems?”
We’ll answer these questions as well as explain
the process of osmosis.First understand that osmosis is the movement of water---and it
is really a type of diffusion.
Often when you are talking about osmosis, you are talking
about the movement of water thru a semi-permeable membrane, like a cell membrane.
A cell membrane has openings small enough for water molecules
to pass but will not allow larger things like salt molecules to freely pass (at least not
without assistance).
Osmosis is a type of passive transport----meaning that it doesn’t
require energy.
I mentioned osmosis is a type of diffusion---and similar to what you might
have learned about with diffusion---molecules (in this case: water) travel from areas of
a high concentration (of water) to a low concentration (of water).
But there’s another way to think about water movement in osmosis.
A low water concentration likely means there are a lot
of solutes.
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 (meaning less water concentration).
So if you want to easily figure out where the water will travel----look to the side
where there are is a greater solute concentration.
So let’s imagine you have a box with a semi-permeable membrane in the center.
Remember this membrane will let tiny things pass through it----like
water and gases----but not large solutes like salt.
We’ll label one side of the box A and one side of the box B.
If you had pure water on both sides of the box----those water
molecules are moving; water molecules like to move---- but the net movement of water
on the two sides is zero.
Meaning, the overall change in 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 move towards---A
or B?
The answer is B!
This area has a high solute concentration and water moves to areas
of higher solute concentration.
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 on the
two sides will be zero but remember that water still likes to move and movement still occurs.
Here’s some vocabulary to know---we call side B hypertonic.
You know that prefix “hyper” when you think of a hyper person?
Well that helps me think of a high amount of something---for
a hyper person it might be caffeine--- in our case though, it means a high 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 greater 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 lower solute
concentration) when compared to side B. Let’s do some real life examples because this box
thing is kind of boring.When someone gets an IV in a hospital---it may look like there’s
clear fluid running through the IV.
It might look like water.
But it is certainly not water.
That would be a disaster because of osmosis.
Let’s say hypothetically pure water was place 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 cells and red blood
cells are a great example.
So what do you think has a greater solute concentration?
The hypothetical pure water in this IV tube?
Or the red blood cells?
Well cells are not empty vessels---they can have organelles and
many types of solutes.
The pure water that hypothetically is running through this IV
tube has no solutes.
It’s just pure water.
So where does the water go?
It goes to the areas of higher solute concentration—inside
the cells.
The cells are Hypertonic compared to the 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 fun.
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.Say you go to a pet store, and you get a fish
that looks like Nemo from the movie “Finding Nemo.”
A clownfish.
Only, you don’t know it’s a saltwater fish.
What would happen if you placed this saltwater fish in freshwater?
Ok first ask---where is there a higher solute concentration?
In the saltwater fish cells?
Or in the freshwater that you placed the fish in?
Definitely in the saltwater fish cells.
So where does the water go?
It goes to the area where there is a greater solute concentration----the
hypertonic side----so it goes into that poor fish.
If you don’t rescue it soon, its cells would be swelling if you could see them under
a microscope.
Eventually, it could die.
Now one thing to clarify: saltwater fish and freshwater
fish are not necessarily isotonic (remember that means equal concentration) 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 freshwater.
Now---not all fish have this problem.
There are some fish that have 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 would be a salmon.
Osmosis explains how many kinds of plants get their water.
Sure, many plants have roots.
But how does the water get in the roots?
When it rains, the soil becomes saturated with water.
The root hair cells have a higher concentration of solutes within
them than the solutes in the saturated soil.
The water travels into the root cells as the root hair cells are hypertonic.
Or, if you live in an area that gets ice---you might
be used to salt trucks salting the road.
Great for icy roads.
Not so great for the plants that live right by the roadside.
If you dump a bunch of salt on these plants, you are making
the soil environment around them very salty.
Very high in solutes.
Very…hypertonic.
How does that water move into the roots now?
Well it doesn’t.
Many plants by salted roadsides die.
This same kind of thing can be tragic for slugs.
We think slugs are misunderstood, almost cute, gastropods.
We understand that many gardeners find them to be pests in their
garden.
But we really don’t like it when we hear of salt being placed on slugs.
It is death by osmosis.
Basically if salt is dumped on a slug, you are making its outside
environment hypertonic.
The slug is hypotonic to the salt.
Remember, the water will travel to the area of higher solute concentration.
To the hypertonic side.
So the water---the moisture that this slimy slug needs----travels
out of the slug.
The slug dehydrates.
You can get really technical with osmosis----in advanced biology, you explore a formula to
calculate water potential and that in osmosis, water travels to areas of lower water potential.
Basically areas of lower free energy.
We won’t be going into water potential calculations
with this short clip, but we like to think about water as loving solutes and avoiding
pressure whenever determining where the water will go.
Water is one of the most important molecules for life---so the big picture here
is in all topics of life, it is really important to understand how water travels.
Practice makes perfect and so we have some practice
problems if you would like to test out your osmosis skills.
That’s it for the amoeba sisters and we remind you to stay curious.