Working through osmosis and water potential

This is a Pearson LabBench activity on Diffusion and Osmosis:

The first few pages of this activity will be review after our diffusion lab, but it is worth reading through it.

Some quick questions to keep in mind as you work through the activity (answers are faded out to hide text – just highlight to see properly).

– – Why do they keep saying “net movement” of the solute?

– – What is hypotonic? Hypertonic? Isotonic?

– – Why do we learn about osmosis? We already know about diffusion, don’t we?

– – Why do cells have selectively permeable membranes? What does this means for life on earth?


Some quick questions to keep in mind as you work through the activity:

– – Why do they keep saying “net movement” of the solute? Remember, water molecules or solute molecules, do NOT sense a concentration gradient or differential, and do NOT move from an area of higher concentration to and area of lower concentration. Molecules will move due to Brownian motion, and as a result of random motion, will have a net movement from an area of higher concentration to an area of lower concentration. Another way to look at it is to consider one molecule: there is a 50% chance it will go in one direction, and a 50% chance it will go in the opposite direction (although it is more 3-D than that). So if half of the molecules in the concentrated area move in one direction and half of the molecules in the less concentrated area move in one direction, the net movement will be from the higher ——> lower concentration.

– – What is hypotonic? Hypertonic? Isotonic? Hyper is always “more than”, so a hypertonic solution has more solute molecules than whatever solution it is being compared to. Hypo is always “less than” and has less solute molecules. Iso means the “same” or “in balance” and means both solutions have the same concentration of solutes, and there will be no net movement of water or solute molecules.

– – Why do we learn about osmosis? We already know about diffusion, don’t we? Osmosis is a special form of diffusion, and is about the passive transport of water across selectively permeable membranes. The “passive” is important because some cells can actively transport – use energy – water across membranes. The “Selectively permeable” is also important, because this means – usually – that most molecules, especially larger ones cannot pass through the membrane without active transport.

– – Why do cells have selectively permeable membranes? What does this means for life on earth? Well, the direct answer to this question is that membranes are important for gas exchange, food delivery to cells, and more. But, more fundamentally, a cell membrane allows a cell – think back to the beginning, to bacteria and viruses – to maintain an environment different from the environment. It allows a cell to impose order on the chaos of Earth’s early environment.

Nice blog post from an entomologist who has videos and drawings of cell membranes:

Back to the LabBench activity and WATER POTENTIAL!
Why am I capitalizing those words? Because it is a tough concept to wrap your head around, no doubt about it.
Go to the page with the red blood cell animation (bursting because distilled water is entering into the cell). There is this line:

Water always moves from an area of higher water potential to an area of lower water potential.

But what is water potential, and how do you measure it?

First of all, the word “potential” is already confusing. Let’s move to Newtonian physics, and think about a ball at the top of a hill. A physicist will say that the ball has potential energy due to gravity and if let go, will roll down the hill until it comes to rest. Hmm, maybe that potential means that the water will “do’ something if its potential energy is positive or negative?? How about a rubber band? If you twist it many times, you are increasing its’ potential energy, right? Or a spring that has been stretched out? If you release either the rubber band or the spring, it will move to return to its original state. You can even say that the spring or band “moves” from a higher potential energy to a lower potential energy.

Okay, so water potential says something about the tendency of water molecules to move up or down a concentration gradient, or a membrane. What does it depend on?

(you can come back to the LabBench activity after watching the video below; the rest of the pages are mini-labs, with some animations and questions. I’ll leave it to you to decide if you need the extra practice in diffusion and osmosis.)

Let’s go to the video lectures!

Here’s a good starting place to begin learning about water potential (or explaining what you read in the textbook) from Mr. Anderson at Bozeman:

He also has one on osmosis if you would like to see his explanation (there is also some information about osmoregulation):

Some thinking questions as you work through the video:

– – Can you ever have a positive solute potential? What is the highest possible solute potential you can ever have? (answer – highlight to see: the highest value you can have for a solute potential is 0, which is pure water, I.e., no solute molecules present. In other words, you cannot have a positive solute potential, because anytime you add solutes (any molecule of any substance, other than water molecules) to a solution of water, you are decreasing the solute potential. And if you think about it, can you ever have a positive water potential, or is it always about the battle of negative water potentials?


Nice blog post from an entomologist who has videos and drawings of cell membranes:

Mr. Bob Kuhn’s Water Potential document
Here is a link to a worksheet from an AP Biology teacher names Bob Kuhn:  b-kuhn-water-potential-notes-questions

Mr. Kuhn goes over water potential (Ψ) and says that adding solute lowers the Ψ, while adding pressure raises the Ψ. By convention (i.e., a group of scientists got together and decided this) pure water at atmospheric pressure has a Ψ of 0.

Remember, Ψs = solute potential and Ψp = pressure potential.
Ψs + Ψp = Ψ

If water moves into a cell (because there is a higher solute concentration inside the cell or osmosis), pressure will build up in the interior of the cell (and might even burst an animal cell, but the cell walls of plant cells prevent this from happening) as the water pushes against the cell wall or cell membrane. This is how the pressure potential can be generated even in animals or plants at sea level (and thus at atmospheric pressure, or Ψp = 0).

Again, solute potential is inversely related to total water potential. As [solute] goes up, Ψ goes down. And vice versa. Note: those brackets are shorthand for “concentration”.


Using our potato cores as an illustration:

Mr. Kuhn’s example of potato cells is a good illustration as to why potato cells don’t keep absorbing water when put into distilled (pure) water until they completely burst. Click here for some pictures of the potato lab that we did (it was already set up so we just weighed the cores the next day). Ignoring the fact that potato cells have cell walls (and are thus more resistant to bursting), if the process of osmosis continued until the concentration of solutes were the same in the cell and in the outside solution (of course, because it is pure water, they will never be the same), the potato cells would keep absorbing water until it completely burst.

Why does the process of osmosis – the net movement of water across the cell membrane into the cell – stop BEFORE the concentration of solutes is the same on both sides of the membrane?

Because as water enters into the potato cell, the pressure of the water inside also increases. This increases the pressure potential value of the water, which increases the total Ψ of the cell. Since water travels from high to low Ψ, this means water will tend to leave the cell instead of entering it. At some point, the solute potential (Ψs) and the pressure potential (Ψp) even each other out. And there is no more net movement of water, in or out of the cell.

Solute Potential Formula: 

Ψs = -iCRT

Work through the equation in the Kuhn document, and email me any questions. Remember, animal cells (with only a cell membrane) cannot really build up any pressure inside their cells. Plant cells, with their cell walls, can.

See below for work-throughs and answers to the first 4 problems in Mr. Kuhn’s document.

Simple Osmosis Simulation

Here is a simple simulation that will graph the change in water potential of a cell and the surrounding environment:

To start, set the pressure potential for both to 0.
Then set the solute concentration to around 2-3M for the cell, and close to zero for the environment.
Set the temperature to the same for both.
Hit the play button, and use the arrows in the upper right hand corner to go through the graphs. The second one should show the change in water potential for both the cell and environment. Notice that the lines don’t meet, why?

Now you can play with NaCl (remember to change the ionization constant to 2) and/or glucose if you haven’t already. There isn’t too much else to play with, but it will serve as a reinforcement of the water potential equations.

Working through the first 4 problems (try to do them on your own first!):

1. Solution inside bag has a Ψ of -6.25 bar (Ψs = -6.25 bar, Ψp = 0).
Solution surrounding bag has a Ψ = -3.25 bar  (again, Ψs = -3.25 bar, Ψp = 0).

Which direction will water flow?
Answer: it will move to enter the bag, or inside.

2. Plant cell has lower Ψ than its surrounding environment, and if Ψp = 0, is the plant cell hypertonic or hypotonic in terms of [solute] to its surroundings? Will the cell gain or lose water?

Answer: The cell must be hypertonic, or have more solutes inside the cell than the environment. Water always travels from a higher potential to lower potential. The plant will gain water. This is exactly how plant roots absorb water and manage to transport it all the way to top of a tall tree: water potential gradients. But how do plants construct and maintain that gradient?? Hmm….

3. A cell is in equilibrium with its surroundings. The surrounding liquid has a molarity of 0.5M. Calculate the Ψs of the surrounding solution. What is the Ψ of the surrounding solution? The cytoplasm inside the cell? Assume 22 degrees C (RT) and an ionization constant of 1.

So, what are our assumptions? That we are at atmosphere and thus the Ψp for both = 0.

The formula for Ψs = -iCRT (see handout for explanation if you haven’t gone through it already).

So, Ψs = -(1)(0.5 moles/liters)(0.0831 liters bars/mole K)(22+273 K)
            = -12.26 bars

Important: notice how all the units cancel out! Always, always, always pay attention to your units so you know that you have plugged in the correct values and units.

Answers: The Ψs for both the cell cytoplasm and the surrounding liquid = -12.26 bars (they are in equilibrium and the Ψp = 0)
The total Ψ for the cell = Ψs + Vp = -12.26 bars + 0 bars = -12.26 bars

4. Another name for solute potential is osmotic potential. If you think about it, the presence of solutes in a cytoplasm helps to predict where the water will go (net movement, as always). It makes sense that biologists would think about osmotic potential as a way to predict water movement in or out of a cell. So, don’t let that confuse you!

Back to problem: Calculate the osmotic potential of a 2.4M solution of sucrose at 24 degrees Celsius. It doesn’t specify a cell or a dialysis bag, but again, the thought is that if you increased the [sucrose] inside of a cell with a semi-permeable membrane to 2.4M, it would definitely drive the net movement of the water to inside the cell. Right? And sucrose does not ionize in pure water, so the ionization constant = 1.

Again, Ψs = -iCRT
Ψs = -(1)(2.5 moles/liters)(0.0831 liters bars/mole K)(24+273 K)
Ψs = -61.7 bars.

Thinking question: if you place a dialysis bag with 2.5M sucrose solution in a solution of 0.5M sucrose solution, which way will the water travel? Is the solution inside the bag hypotonic or hypertonic? Will the water continue to travel until the solution inside the dialysis bag is isotonic with the surrounding solutions? Why or why not? What would happen if you placed the 2.5M sucrose solution inside of a plant cell? (magically, since it is difficult to do that without hurting the cell!).

Try to do #5 and #6 on your own – I can post my answers in a few days if needed.



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