1. Osmotic Pressure

The movement of water caused by osmosis generates a force called osmotic pressure. Osmotic pressure explains why our gummy bear expanded. Let’s look at it in a bit more detail.

Original Situation After Osmosis
01_key osmosis diagram, v4 01a_key osmosis diagram, after osmosis

The beakers above show a time sequence. The beaker on the left shows the situation at the start: side 2 is hypotonic to side 1. As a result, water will move, by osmosis, from side 2 into side 1.

Water can freely diffuse across this membrane. However, once the water molecules diffuse across the membrane, they interact with the solute in a way that keeps them from returning. As a result, water molecules leave side 2, enter side 1, and stay there. With more water molecules on side 1, the water level has to rise. With less water molecules on side 2, the water level there has to fall. You can see the result in the beaker on the right.

2. Animal Cells and Osmosis

Let’s see how this can affect cells in hypotonic, isotonic, or hypertonic solutions.

07_red blood cell osmosis with labels
source: wikipedia

Consider the red blood cells shown above.

The cells in the hypotonic solution (on the right side of the diagram) are expanding and bursting. Why?

These cells were hypertonic to the surrounding solution. Because water flows from hypotonic to hypertonic, water will flow into the cells. This flow of water exerts pressure, much like the force you exert when you blow up a balloon. In the same way that a balloon expands, so will the cell, right up to point where it bursts. In the same way as air pressure can burst a balloon, osmotic pressure can burst a red blood cell.

By contrast, the cells in the hypertonic solution (the one on the left) are shriveled. Why? Because water flows from hypotonic to hypertonic. If the cells are in a hypertonic solution, water will leave the cells, and flow into the solution. The loss of water causes the cells to shrivel up, just like the loss of water from a grape causes it to become a shriveled raisin. In the same way that loss of air pressure causes a tire to become flat (deflated), loss of osmotic pressure can cause a cell (and living tissues made of cells) to become shriveled.

In an isotonic solution (shown in the middle), water flows equally between the cells and the solution. The cell doesn’t gain water or lose water (because water diffuses into and out of it at equal rates).

3. Plant Cells and Osmosis

Because plant cells have a rigid wall, they respond to osmotic pressure differently.

08_plant cell osmosis, with labels
source: wikipedia
  •  In a hypertonic solution, a plant cell will lose water. As it does, the central vacuole contracts. This causes the cell membrane to peel away from the wall, a condition called plasmolysis. As a result, the plant as a whole becomes wilted and droopy.
  • In an hypotonic solution, water will move into the plant cell. This will cause the central vacuole to expand. As it does, the entire cell will expand (just as the red blood cells did above). However, unlike red blood cells in a hypotonic solution, these plant cells won’t burst. Instead, the membrane will be pushed against the wall, keeping the plant cell solid and firm.

This is actually a healthy condition for plant cells and for plants as a whole. It’s why grocery stores use misters to keep their vegetables covered with water (a hypotonic solution), causing water to move into the veggies to keep them crisp and firm.

wilted 12_mister
Wilted lettuce, caused by loss of osmotic pressure A grocery mister. The hypotonic water keeps the veggies crisp.

4. Osmosis Quiz 1: Diagrams and Vocabulary

[qwiz style = “border: 3px solid black; ” qrecord_id=”sciencemusicvideosMeister1961-Osmosis Quiz 1 (M8)”]

[h] Osmosis Quiz 1: Diagrams and Vocabulary
[i]This activity tests you on your understanding of diagrams related to osmosis. Here’s how the quiz works:

  • Each question is multiple choice, but the entire quiz is like a series of flashcards.
  • If you get the question right, it comes off the deck.
  • If you get the question wrong, it goes to the bottom of the deck, so you can try it again.

[q labels = “top”]

 

[l]selectively permeable membrane

[fx] No. Please try again.

[f*] Correct!

[l]hypertonic side

[fx] No, thatís not correct. Please try again.

[f*] Good!

[l]hypotonic side

[fx] No. Please try again.

[f*] Good!

[l]solute molecule

[fx] No. Please try again.

[f*] Great!

[l]water

[fx] No. Please try again.

[f*] Great!

[q labels = “top”]A gummy bear is put into water. Gummy bears are full of sugar. Label this osmotic situation.

 

[l]hypotonic

[fx] No, thatís not correct. Please try again.

[f*] Correct!

[l]hypertonic

[fx] No. Please try again.

[f*] Correct!

[!] Question 3++++++++++++++[/!]

[q labels= “top”]Label this osmotic situation.

[l]hypotonic

[fx] No, thatís not correct. Please try again.

[f*] Good!

[l]hypertonic

[fx] No. Please try again.

[f*] Excellent!

[!] Question 4++++++++++++++[/!]

[q labels = “top”]Label this osmotic situation

[l]hypotonic

[fx] No. Please try again.

[f*] Good!

[l]hypertonic

[fx] No, thatís not correct. Please try again.

[f*] Great!

[!] Question 5++++++++++++++[/!]

[q labels = “top”]Label the osmotic situation of the cell below, which is 95% water.

[l]hypertonic

[fx] No, thatís not correct. Please try again.

[f*] Correct!

[l]hypotonic

[fx] No, thatís not correct. Please try again.

[f*] Excellent!

 

[!] Question 6++++++++++++++[/!]

[q labels = “top”]Label the osmotic situation of the cell below, which is 98% water.

[l]hypotonic

[fx] No, thatís not correct. Please try again.

[f*] Great!

[l]hypertonic

[fx] No, thatís not correct. Please try again.

[f*] Good!

[!] Question 7++++++++++++++[/!]

[q labels = “top”]Label the osmotic situation of the cell below, which is 98% water.

[l]hypertonic

[fx] No, thatís not correct. Please try again.

[f*] Great!

[l]hypotonic

[fx] No. Please try again.

[f*] Great!

[!] Question 8, ++++++++++++++[/!]

[q labels = “top”]Drag the labels to complete the sentence: the cell is in a(n) ______ environment.

 

[l]isotonic

[fx] No, thatís not correct. Please try again.

[f*] Great!

[l]hypotonic

[fx] No. Please try again.

[f*] Great!

[l]hypertonic

[fx] No. Please try again.

[f*] Good!

[!] Question 9++++++++++++++[/!]

[q labels = “top”]Drag the labels to complete the sentence: the cell is in a(n) ______ environment.

 

[l]isotonic

[fx] No, thatís not correct. Please try again.

[f*] Great!

[l]hypotonic

[fx] No, thatís not correct. Please try again.

[f*] Excellent!

[l]hypertonic

[fx] No. Please try again.

[f*] Excellent!

 

[!] Question 10, ++++++++++++++[/!]

[q labels = “top”]Drag the labels to complete the sentence: the cell is ______ to its environment.

 

[l]isotonic

[fx] No, thatís not correct. Please try again.

[f*] Great!

[l]hypotonic

[fx] No. Please try again.

[f*] Great!

[l]hypertonic

[fx] No. Please try again.

[f*] Good!

[!] Question 11++++++++++++++[/!]

[q labels = “top”]Drag the labels to complete the sentence: the cell is ______ to its environment.

 

[l]isotonic

[fx] No, thatís not correct. Please try again.

[f*] Great!

[l]hypotonic

[fx] No, thatís not correct. Please try again.

[f*] Excellent!

[l]hypertonic

[fx] No. Please try again.

[f*] Excellent!

[!] Question 12++++++++++++++[/!]

[q labels = “top”]Drag the labels as if you were completing the sentence “These elodea cells are ____________ to their environment.”

[l]hypotonic

[fx] No. Please try again.

[f*] Correct!

[l]hypertonic

[fx] No. Please try again.

[f*] Good!

[!] Question 13++++++++++++++[/!]

[q labels = “top”]Drag the labels as if you were completing the sentence “These elodea cells are in a(n) ____________ environment.”

[l]hypotonic

[fx] No. Please try again.

[f*] Correct!

[l]hypertonic

[fx] No. Please try again.

[f*] Good!

[!] Question 14++++++++++++++[/!]

[q]Label the parts of these elodea cells. The cells start out in fresh water, and then are exposed to salt water.

[l]cell membrane

[fx] No. Please try again.

[f*] Correct!

[l]cell wall

[fx] No. Please try again.

[f*] Excellent!

[l]chloroplast

[fx] No, thatís not correct. Please try again.

[f*] Correct!

[q]The movement of a substance from higher concentration to lower concentration is ________________

[hangman]

[c]diffusion

[f]Genius! The movement of a substance from higher concentration to lower concentration is diffusion.

[!] Card 2+++++++++++++++++++++++++++++++++++[/!]

[q]The diffusion of water is

[hangman]

[c]osmosis

[f]Correct! The diffusion of water is osmosis.

[!] Card 3+++++++++++++++++++++[/!]

[q]A mixture in which one thing is dissolved in another is a(n) ________________

[hangman]

[c]solution

[f]Yes! A mixture in which one thing is dissolved in another is a solution.

[!] Card 4+++++++++++++++++++++[/!]

[q]In a solution, the thing that does the dissolving is the

[hangman]

[c]solvent

[f]Perfect! In a solution, the thing that does the dissolving is the solvent.

[!] Card 5+++++++++++++++++++++[/!]

[q]In a solution, the thing that gets dissolved is the

[hangman]

[c]solute

[f]Nice! In a solution, the thing that gets dissolved is the solute.

[!] Card 6+++++++++++++++++++++[/!]

[q]In lemonade (that you make from a mix), the lemonade mix is the _____________.

[hangman]

[c]solute

[f]Great! In lemonade (that you make from a mix), the lemonade mix is the solute.

[!] Card 7+++++++++++++++++++++[/!]

[q]In lemonade (that you make from a mix), the water is the _____________.

[hangman]

[c]solvent

[f]Correct! In lemonade (that you make from a mix), the water is the solvent.

[!] Card 8+++++++++++++++++++++[/!]

[q]Solution “A” has more dissolved solute than solution “B.” Solution “A” is __________ to solution “B.”

[hangman]

[c]hypertonic

[f]Yes! Solution “A” has more dissolved solute than solution “B.” Solution “A” is hypertonic to solution “B.”

[!] Card 9+++++++++++++++++++++[/!]

[q]Solution “A” has less dissolved solute than solution “B.” Solution “A” is __________ to solution “B.”

[hangman]

[c]hypotonic

[f]Perfect! Solution “A” has less dissolved solute than solution “B.” Solution “A” is hypotonic to solution “B.”

[!] Card 10+++++++++++++++++++++[/!]

[q]Solution “A” has the same amount of dissolved solute as solution “B.” Solution “A” is __________ to solution “B.”

[hangman]

[c]isotonic

[f]Nice! Solution “A” has the same amount of dissolved solute as solution “B.” Solution “A” is isotonic to solution “B.”

[!] Card 11+++++++++++++++++++++[/!]

[q]Solution “A” has a higher percentage of water than solution “B.” Solution “A” is __________ to solution “B.”

[hangman]

[c]hypotonic

[f]Great! Solution “A” has a higher percentage of water than solution “B.” Solution “A” is hypotonic to solution “B.”

[!] Card 12+++++++++++++++++++++[/!]

[q]Solution “A” has a lower percentage of water than solution “B.” Solution “A” is __________ to solution “B.”

[hangman]

[c]hypertonic

[f]Good job! Solution “A” has a lower percentage of water than solution “B.” Solution “A” is hypertonic to solution “B.”

 

[!] Card 13+++++++++++++++++++++[/!]

[q]Diffusion is the movement of a substance ______ its concentration gradient.

[hangman]

[c]down

[f]Awesome! Diffusion is the movement of a substance down its concentration gradient.

[!] Card 14+++++++++++++++++++++[/!]

[q]Osmosis is the diffusion of _____.

[hangman]

[c]water

[f]Genius! Osmosis is the diffusion of water.

[!] Card 15++++++++hypotonic+++++++++++++[/!]

[q]If solution “A” has ________  solute than solution “B,” then solution “A” is hypertonic to solution “B.”

[hangman]

[c]more

[f]Yes! If solution “A” has more than solution “B,” then solution “A” is hypertonic to solution “B”

[!] Card 21+++++++++++++++++++++[/!]

[q]_____ always flows from hypotonic to hypertonic.

[hangman]

[c]water

[f]Great! Water always flows from hypotonic to hypertonic.

[!] Card 22+++++++++++++++++++++[/!]

[q]If I want to make a solution more hypertonic, I add more _____________.

[hangman]

[c]solute

[f]Nice! If I want to make a solution more hypertonic, I add more solute.

 

 

[x]

[restart]

[/qwiz]

5. Osmosis Quiz 2: Multiple Choice

[qwiz style = “border: 3px solid black; ” qrecord_id=”sciencemusicvideosMeister1961-Osmosis Quiz 2 (M8)”]

[h] Quiz: Osmosis
[i]This quiz tests you on your understanding of osmosis, and your ability to explain phenomena like the one below.

[!] Question 1, QUIZ 2++++++++++++++[/!]

[q]A solution that has a higher solute concentration than a solution on the other side of the membrane is ___________

[c]hypotonic

[c*]hypertonic

[c]isotonic

[f]No. Hypotonic means “less solute.” What prefix is associated with “more” or “too much.”

[f]Excellent. A solution that has a higher solute concentration than a solution on the other side of the membrane is hypertonic.

[f]No. Isotonic means “same amount of solute.” What prefix is associated with “more” or “too much.”

[!] Question 2++++++++++++++[/!]

[q]A solution that has a higher water concentration than a solution on the other side of the membrane is ___________

[c*]hypotonic

[c]hypertonic

[c]isotonic

[f]Yes. Hypotonic means “less solute.” Less solute means higher water concentration.

[f]No. Hypertonic means higher solute concentration. If there’s more solute, there has to be less water. You’re looking for the term that means “lower solute concentration.”

[f]No. Isotonic means “same solute (or water) concentration.” Higher water concentration means “lower solute concentration.” Which term means “lower solute concentration?”

[!] Question 3++++++++++++++[/!]

[q]A solution that has the same solute concentration as a solution on the other side of the membrane is ___________

[c]hypotonic

[c]hypertonic

[c*]isotonic

[f]No. Hypotonic means “less solute.” You’re looking for a term that means “the same solute concentration.” What prefix means “the same?” Think of a triangle that has two sides that are the same…

[f]No. Hypertonic means higher solute concentration. You’re looking for a term that means “the same solute concentration. “What prefix means “the same?” Think of a triangle that has two sides that are the same…

[f]Yes! Isotonic means “same solute concentration.” Just remember the isosceles triangle.

[!] Question 4++++++++++++++[/!]

[q]A solution that has a lower solute concentration than a solution on the other side of the membrane is ___________

[c*]hypotonic

[c]hypertonic

[c]isotonic

[f]Yes. Hypotonic means “less solute.”

[f]No. Hypertonic means higher solute concentration. The term that means lower solute concentration begins with a prefix that means “lower.”

[f]No. Isotonic means “same amount of solute.” The term that means lower solute concentration begins with a prefix that means “lower.”

[!] Question 5++++++++++++++[/!]

[q]A solution that has a lower water concentration than a solution on the other side of the membrane is ___________

[c]hypotonic

[c*]hypertonic

[c]isotonic

[f]No. Hypotonic means “less solute.” Less solute means higher water concentration. What term means “higher solute concentration?”

[f]Yes. Hypertonic means higher solute concentration. If there’s more solute, there has to be less water.

[f]No. Isotonic means “same solute (or water) concentration.” Lower water concentration means “higher solute concentration.” Which term means “higher solute concentration?”

[!] Question 6++++++++++++++[/!]

[q]A solution that has the same water concentration as a solution on the other side of the membrane is ___________

[c]hypotonic

[c]hypertonic

[c*]isotonic

[f]No. Hypotonic means “less solute.” You’re looking for a term that means “the same solute concentration” (because if the solute concentration is the same, the water concentration also has to be the same). What prefix means “the same?” Think of a triangle that has two sides that are the same…

[f]No. Hypertonic means higher solute concentration. You’re looking for a term that means “the same solute concentration” (because if the solute concentration is the same, the water concentration also has to be the same). What prefix means “the same?” Think of a triangle that has two sides that are the same…

[f]Yes! Isotonic means “same solute concentration.” If the solute concentration is the same, the water concentration will also be the same.

[!] Question 6a, QUIZ 2++++++++++++++[/!]

[q]A gummy bear is placed in water. The gummy bear is made mostly of sugar, held together by gelatin. The gummy bear is ___________ to the water.

[c]hypotonic

[c*]hypertonic

[c]isotonic

[f]No. Hypotonic means “less solute.” You’re comparing a gummy bear, with all of its sugar, to mostly pure water. What word means “more solute?”

[f]Excellent. A gummy bear, with all of its sugar, has a higher solute concentration than the water that it’s in.  That makes the gummy hypertonic to the water.

[f]No. Isotonic means “same amount of solute.” You’re comparing a gummy bear, with all of its sugar, to mostly pure water. What word means “more solute?”

[!] Question 7++++++++++++++[/!]

[q]In this diagram, the cell is ________ to the solution outside the cell

[c]hypotonic

[c*]hypertonic

[c]isotonic

[f]No. Hypotonic means “less solute.” Look at how much more solute is inside the cell than outside. What term means “higher solute concentration?”

[f]Yes. Hypertonic means higher solute concentration, and the cell is clearly hypertonic to its environment.

[f]No. Isotonic means “same solute (or water) concentration.” Look at how much more solute is inside the cell than outside. What term means “higher solute concentration?”

[!] Question 8++++++++++++++[/!]

[q]In this diagram, the cell is ________ to the solution outside the cell

[c*]hypotonic

[c]hypertonic

[c]isotonic

[f]Yes. Hypotonic means “less solute,” and the cell is clearly hypotonic to its environment.

[f]No. Hypertonic means higher solute concentration. Just by looking, you can tell that there’s a lower concentration of solute inside the cell than outside the cell.  What term means “lower solute concentration?”

[f]No. Isotonic means “same solute (or water) concentration.” Just by looking, you can tell that there’s a lower concentration of solute inside the cell than outside the cell.  What term means “lower solute concentration?”

[!] Question 9++++++++++++++[/!]

[q]In this situation, water will

[c*]flow into the cell.

[c]flow out of the cell

[c]flow into and out of the cell at equal rates, with no net change.

[f]Yes. Water always flows from hypotonic to hypertonic. Because the cell is hypertonic to its environment, water will flow into the cell.

[f]No. Water always flows from hypotonic to hypertonic. Is this cell hypertonic or hypotonic to its environment?

[f]No. Water flow would be equal if the cell were isotonic to its environment, and that’s clearly not the case. Keep in mind that water always flows from hypotonic to hypertonic, and figure out the answer.

[!] Question 10++++++++++++++[/!]

[q]In this situation, water will

[c]flow into the cell.

[c*]flow out of the cell

[c]flow into and out of the cell at equal rates, with no net change.

[f]No. Water always flows from hypotonic to hypertonic. The hypotonic side is the one with less solute. Now figure out the answer.

[f]Yes. Water always flows from hypotonic to hypertonic. Because the cell is hypotonic to its environment, water will flow out of the cell.

[f]No. Water flow would be equal if the cell were isotonic to its environment, and that’s clearly not the case. Keep in mind that water always flows from hypotonic to hypertonic, and figure out the answer.

[!] Question 11++++++++++++++[/!]

[q]Assume that this cell is an animal cell (without a cell wall). In this situation, the cell will

[c*]increase in size

[c]decrease in size

[c]remain the same size.

[f]Yes. Water always flows from hypotonic to hypertonic. As water flows into the hypertonic cell, it will increase in size.

[f]No. Water always flows from hypotonic to hypertonic. Is this cell hypertonic or hypotonic to its environment? Figure that out, then figure out the direction of water flow.

[f]No. The cell would stay the same size if it were isotonic to its environment. It’s not. Keep in mind that water always flows from hypotonic to hypertonic, and figure out the answer.

[!] Question 12++++++++++++++[/!]

[q]Assume that this cell is an animal cell (without a cell wall). In this situation, the cell will

[c]increase in size

[c*]decrease in size

[c]remain the same size.

[f]No. Water always flows from hypotonic to hypertonic. Is this cell hypertonic or hypotonic to its environment? Figure that out, then figure out the direction of water flow.

[f]Yes. Water always flows from hypotonic to hypertonic. As water flows out of the hypotonic cell, it will decrease in size.

[f]No. The cell would stay the same size if it were isotonic to its environment. It’s not. Keep in mind that water always flows from hypotonic to hypertonic, and figure out the answer.

[!] Question 13++++++++++++++[/!]

[q]When plant cells are in this environment, they’ll be the most solid and firm.

[c*]hypotonic

[c]hypertonic

[c]isotonic

[f]Yes. When plant cells are in a hypotonic environment, water flows into the cells. This expands the central vacuole, and pushes the membrane against the wall. That makes the cells (and the entire plant) solid and firm.

[f]No. When plant cells are in a hypertonic environment, water flows out the cells. This contracts the central vacuole, and peels the membrane away from the wall. That makes the cells (and the entire plant) droopy and wilted.

[f]No (but this is the second best answer). If plant cells are in an isotonic environment, water will be entering and leaving them at the same rate. The cells will be in reasonably good shape, but a change in their osmotic condition would make the plant even more solid and firm.

[!] Question 14++++++++++++++[/!]

[q]When animal cells are in this environment, they’ll shrink and shrivel

[c]hypotonic

[c*]hypertonic

[c]isotonic

[f]No. When animal cells are in a hypotonic environment, water flows into the cells. Study the diagram below to see what conditions will cause animal cells to shrink and shrivel.

[f]Yes. When animal cells are in a hypertonic environment, water flows out the cells. This makes them shrink and shrivel.

[f]No. If animal cells are in an isotonic environment, water will be entering and leaving them at the same rate. Study the diagram below to see what conditions will cause animal cells to shrink and shrivel.

[!] Question 15++++++++++++++[/!]

[q]When a plant is in this environment, it wilts.

wilted
Wilted lettuce

[c]hypotonic

[c*]hypertonic

[c]isotonic

[f]No. When plant cells are in a hypotonic environment, water flows into the cells. This expands the central vacuole, and pushes the membrane against the wall. That makes the cells (and the entire plant) solid and firm.

[f]Yes. When plant cells are in a hypertonic environment, water flows out the cells. This contracts the central vacuole, and peels the membrane away from the wall. That makes the cells (and the entire plant) droopy and wilted.

[f]No. If plant cells are in an isotonic environment, water will be entering and leaving them at the same rate. The cells will be in reasonably good shape. What osmotic condition would cause water to leave the cells, causing the plant to wilt?

[!] Question 16++++++++++++++[/!]

[q]When animal cells are in this environment, they’ll expand, then burst.

[c*]hypotonic

[c]hypertonic

[c]isotonic

[f]Yes. When animal cells are in a hypotonic environment, water flows into the cells. This causes them to expand, and eventually burst.

[f]No. When animal cells are in a hypertonic environment, water flows out the cells. This makes them shrink and shrivel. What osmotic condition causes them to expand and burst?

[f]No. If animal cells are in an isotonic environment, water will be entering and leaving them at the same rate. Their size remains the same. Study the diagram below to see what conditions will cause animal cells to expand and burst?

[!] Question 17++++++++++++++[/!]

[q]If you want to keep animal tissue or cells alive outside the body, you have to keep them in what kind of osmotic environment?

[c]hypotonic

[c]hypertonic

[c*]isotonic

[f]No. When animal cells are in a hypotonic environment, water flows into the cells. This causes them to expand, and eventually burst. What condition keeps them the same size (which is the healthiest condition for the cells)?

[f]No. When animal cells are in a hypertonic environment, water flows out the cells. This makes them shrink and shrivel. What condition keeps them the same size (which is the healthiest condition for the cells)?

[f]Yes. If animal cells are in an isotonic environment, water will be entering and leaving them at the same rate. Their size remains the same, which is the healthiest condition for the cells.

[!] Question 12++++++++++++++[/!]

[x]

[restart]

[/qwiz]

Next

This (at least for now) ends this series of tutorials about cell membranes and osmosis. Here are some possible next moves.

  • To extend and deepen your understanding of water flow in living organisms, use my tutorial on Water Transport in Plants. That tutorial also introduces the concept of water potential, an idea that’s part of the AP Biology curriculum (Topic 2.8) and which is interesting in its own right).
  • Proceed to Enzymes  (the next module in my biology course)
  • Use the menu above to choose another module.
  • Watch Osmosis! (Music Video) or buy Osmosis! on iTunes or Amazon