Osmosis and Water Potential (Updated)

Osmosis and Water Potential (Updated)


Captions are on! Click CC at bottom right to turn off. You can stay up to date with us on Twitter (@AmoebaSisters) and Facebook! 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 soil. While the effect may not be instant, this
can eventually actually kill plants- including trees- that had originally survived the hurricane. Why? Do plants just dislike salt that much? Well it’s actually related to this awesome
term: osmosis. When you are talking about osmosis, you are
talking about the movement of water thru 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—-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 in that water molecules can squeeze through it—the molecules are quite small—but
salt can’t. Salt molecules are too large. 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. Remember, the salt molecules are too large
to pass thru this semi-permeable membrane. 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. The water level on side B will be higher in
the U-tube. 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. Here’s some vocabulary to add in here—we
call side B hypertonic. This 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 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 areas of higher solute concentration—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. Or 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
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 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’d
be a salmon. No question. 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 generally have a higher
concentration of solutes within them than the solute concentration in the saturated
soil. The water travels into the root 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 that water? That brings us to our next osmosis topic and
why plant cells 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=pressure
potential + 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. 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 starts 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 mean 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.

100 thoughts on “Osmosis and Water Potential (Updated)

  1. We like to pin clarifications & corrections at the top of our videos! We are so appreciative of feedback we receive as it helps us continue to improve. In this video, we say the word "molecules" several times. Chemically speaking—the term "molecule" may be ok when referring to water but not salt as we did in 2:54. Salt is formed by ionic bonds [not molecular (covalent) bonds like you would see in water]. We also notice in the comments that people have questions about the diagram at 5:34. Many textbooks, in our experience, focus on the solution the cell is placed in only. Our diagram at 5:34 actually labels both the cell (as either hypertonic or hypotonic) AND the solution (as either hypertonic or hypotonic). If assuming no added pressure, look for the area that is hypertonic. It may be the cell that is hypertonic or it may be the solution that is hypertonic. But as the diagram shows, the hypertonic area (assuming no pressure) is where the net movement of water will go. We try to include addressing the cell as well as its surrounding solution, because Pinky noticed her students otherwise tended to memorize terms corresponding to swelling or shrinking, which turned out to hurt understanding. Hope that helps! 🙂

  2. So over time if you place fresh water in a saltwater tank(not all at once, but over the time of days), the cells in the fish will adapt to less salt and over time will be able to live in fresh water, right?

  3. Did anyone notice that at 1 min 30 second the arrow which is supposed to be high water concentration is actually showing high solute concentration. There is a boo boo there.

  4. Incredible video.
    Fun fact, my teacher was the one who recommended your channel to my class.
    Good job on making this insane quality and easy to understand videos.

  5. My hs biology teacher emailed me the link to one of your videos… I think ever since that day I've SEEN every video, LIKED every video and subscribed. You two remind me of my sister and I 😉 I think that's why our hs science teacher recommended you <3 STAY CURIOUS!

  6. One thing in my bucket list is to find out who you are 😂🍻
    I totally love your videos and I am a Fan of your unique style of teaching !
    I love your voice too ❤

  7. I just want to say I am a Biochemistry student who is currently a ICU nurse and your videos are awesome! I have been looking for a channel that can dumb things down to my level lol! This is not an insult! I hope that much is understood! Thank you sooo much!

  8. Ok I’m a little confused. At 5:26 you say the red blood cells are “hypertonic” and would swell and burst. But at 5:36 you show that the red blood cells are “hypotonic” and they swell. So do they swell in hypertonic or hypotonic solutions?

  9. Help me understand, if liquids move from a lower to higher solute concentration , how does Brining work in cooking?
    shouldn't all the water in the (let's say) chicken be drawn out?
    Please, i would appreciate it if someone could explain the mechanics of it to me.
    Amoeba Sister?
    Thanks.

  10. Absolute gold. I study for medical exams and I got confused by the term osmosis. I love when something is explained so simply that even a kid could understand. Thanks!

  11. A fun way to demonstrate osmosis at home is with Umibudo (sea grapes.) Umibudo is packed in super salty water, which makes them shrivel up (by osmosis.) When you rehydrate them in fresh water, you can see them swell up as the water rushes in by osmosis! Osmosis right before your eyes!

  12. really really good. finally made sense – concept clarified by whole video, really for me around minute 9

  13. this video is so good if you are strugeling in class when learning about difusion or osmosis watch this vid i have my test tomorrow and it just saved me a load of time for studying

  14. Thank you for teaching me something my teacher attempted but failed at teaching us in the span of 40 minutes in a 10 minute video.😫

  15. My teacher is forcing me to watch this I decided to just go on Snapchat and play this in the background so she thought I was on it sksksksksksksks #edgytingz

    Rarw rarw rarw Xd EDGE RArw

  16. Now im confused, hypotonic and hypertonic here and in the book provided by our school is literally the OPPOSITE, but as usual ill stick with the book

  17. hey, i've been watching your videos for a long time and they really REALLY help me with biology. I have an EOC in three days and decided to just study with your videos. however, on 5:34, i found something. i don't know if it's just me or i'm looking at the diagram wrong, but aren't hypotonic cells the swelling cells? in the diagram, the swelling cells are labeled as hypertonic instead of hypotonic. the same with the shrinking cells–they are labeled as hypotonic instead of hypertonic. if this isn't a mistake, could somebody explain to me why this is the case i would gladly appreciate it

  18. You are science Ling Lings
    Ling ling 40 hrs… Oh and there's a tumbleweed snowman at 0:11. Tee Hee Hee

  19. I totally wish you were my biology teacher! You explain something meant to be complicated so simply and make it so easy to understand. Thank you!! 🙏🏻❤️

  20. Now when hypertonic and hypotonic are the side A and B does it have anything to do with club soda (Tonic Water)?

  21. I didnt understand at 5:28 wherein it is labeled as hypertonic which SWELLS, but in the diagram, it SHRINKS

  22. thanks to you guys, I can now understand SOME sort of biology… I've always been bad at memorizing, and the teachers at our school aren't that good with making us UNDERSTAND rather than telling us to memorize them.
    anyways, its 3 am here, and I'm studying biology for my exam tomorrow. wish me luck! 😀 <3

  23. You could talk about 1topic for just almost 10min and its easy to understand
    out proctor is teaching this for almost 3hours and we still find it hard

  24. Salt water affects plants because salt water has high concentration of solute. So, when the salt water comes in to contact with plants, the tendency of the water inside the cell goes out making it shrinks and dried out.

  25. My college class made this so much more complicated than it needed to be. You guys do such a great job at explaining!!! I've been watching your videos since high school and you never disappoint amoeba sisters!

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