Grade by Grade
B. Life Systems · Cells

Osmosis: Water on the Move

B2.4 A1.2 A3.3 A1.5About 60 minutes
Learning goals
  • I can explain osmosis as the diffusion of water through a selectively permeable membrane.
  • I can predict what happens to plant and animal cells in pure water and in salty water.
  • I can carry out a fair test on osmosis in potato pieces and analyse the results.
  • I can explain how osmosis is used in food preservation, including traditional First Nations, Métis and Inuit methods, and how it affects plants beside salted roads.

Words to know

osmosis
The diffusion of water through a selectively permeable membrane, from where there is more water (less dissolved substance) to where there is less water (more dissolved substance).
solute
A substance that is dissolved, such as salt or sugar in water.
solution
A mixture in which a solute is dissolved evenly in a solvent such as water.
turgid
Firm and swollen with water. A plant cell with a full vacuole is turgid.
plasmolysis
When a plant cell loses so much water that its cell membrane pulls away from the cell wall.
contractile vacuole
A vacuole in some single-celled organisms that collects extra water and pumps it out of the cell.

Crispy celery

Leave a stalk of celery on the counter overnight and it goes limp and bendy. Stand it in a glass of water and in a few hours it is crisp again. Sprinkle salt on cucumber slices and, after a while, a puddle of water appears around them. Where does the water come from, and where does it go? The answer is osmosis.

A drooping, wilted celery stalk on a plate next to a crisp, straight celery stalk standing in a glass of water.
The limp stalk has lost water from its cells. The stalk in the glass takes in water by osmosis, its cells fill up and push against their cell walls, and it stays crisp. AI-generated image

Osmosis is diffusion of water

Osmosis is the diffusion of water through a selectively permeable membrane, like a cell membrane. Water particles are small and pass through the membrane easily. Many dissolved particles (solutes), such as salt and sugar particles, can't pass through as easily.

To predict which way water moves, think about how much water is on each side:

  • Pure water has the most water (the highest concentration of water).
  • Salty or sugary water has less water, because some of the space is taken up by dissolved particles.

Water diffuses from where there is more water (fewer dissolved particles) to where there is less water (more dissolved particles). A shortcut many students use: water moves toward the side with more dissolved stuff.

pure waterbalancedvery salty waterwater moves in:cell swellsin = out:no changewater moves out:cell shrinks
An animal cell (like a red blood cell) in three liquids. Arrows show the overall direction water moves through the cell membrane.
Surrounding liquidAnimal cell (no wall)Plant cell (has a wall)
Pure water (less dissolved stuff than inside the cell)Water moves in. The cell swells and may burst.Water moves in. The vacuole fills and pushes on the wall. The cell becomes firm (turgid) but the wall stops it from bursting.
Balanced (same amount of dissolved stuff)Water moves in and out equally. No change.No change; the cell is a bit less firm than in pure water.
Very salty (more dissolved stuff than inside the cell)Water moves out. The cell shrinks and shrivels.Water moves out. The vacuole shrinks, the membrane pulls away from the wall (plasmolysis) and the plant wilts.

Osmosis all around you

  • Watering plants: soil water has fewer dissolved substances than root cells, so water moves into roots by osmosis.
  • Road salt: in Ontario winters, salt melts ice on roads. Salty meltwater soaks into the soil beside the road. Now the soil water has more dissolved substances, so it is harder for roots to take in water by osmosis. That is one reason roadside trees and grass can look brown and dry in spring.
  • Single-celled organisms in fresh water, such as paramecia, constantly take in water by osmosis. A contractile vacuole collects the extra water and pumps it out, so the cell doesn't burst.
  • Your body keeps your blood at just the right balance of salts and water, so your blood cells don't swell or shrink. When patients get fluids through an IV in hospital, the fluid is made with a carefully balanced amount of salt for this reason.

Osmosis and keeping food safe

Bacteria and moulds that spoil food are cells too, and they need water to live and grow. People have used this knowledge for thousands of years:

  • Salting (for example, salt fish or salt pork) and sugaring (for example, jams) surround microbes with lots of dissolved particles, so water moves out of their cells by osmosis and they can't grow.
  • Drying removes most of the water microbes need. First Nations peoples across Canada have long preserved meat, fish and berries by drying and smoking them. Pemmican, dried meat pounded and mixed with fat (and sometimes dried berries), was made by First Nations of the Plains and by the Métis, and it was a key food during the fur trade; families and communities still make it today. Inuit dry Arctic char and caribou meat in the cold, dry air so it keeps for a long time.

These methods come from close observation passed down over many generations. Food scientists today explain the same results with cell biology: without enough water, microbes can't grow.

Worked example

Let's predict using osmosis

A slice of fresh potato is placed in very salty water for 30 minutes. Predict what happens to its cells and to the slice. Explain.

  1. Compare dissolved particles: the salty water has more dissolved particles than the liquid inside the potato cells.
  2. Water moves toward the side with more dissolved stuff, so water moves out of the potato cells.
  3. The vacuoles shrink, turgor pressure drops, and the membranes may pull away from the walls (plasmolysis).
  4. The slice becomes soft and bendy, and it gets a little lighter and shorter because it lost water.

The potato cells lose water by osmosis; the slice becomes floppy, lighter and smaller.

Worked example

Let's analyse potato-strip data

Three potato strips each started at 10.0 g. After 2 hours: in tap water, 10.8 g; in slightly salty water, 9.9 g; in very salty water, 8.6 g. Find the percent change for each and explain the pattern.

  1. Percent change = (final mass − starting mass) ÷ starting mass × 100%.
  2. Tap water: (10.8 − 10.0) ÷ 10.0 × 100% = +8% (gained water).
  3. Slightly salty: (9.9 − 10.0) ÷ 10.0 × 100% = −1% (almost no change, close to balanced).
  4. Very salty: (8.6 − 10.0) ÷ 10.0 × 100% = −14% (lost water).
  5. Pattern: the more salt, the more water the potato lost. In tap water, which has fewer dissolved particles than potato cells, water moved in.

+8%, −1%, −14%. Water moved into the potato in tap water and out of it in salty water: evidence of osmosis.

+10%+5%0%−5%−10%+8%tap water−1%slightly salty−14%very saltychange in massPotato strips after 2 hours (start: 10.0 g each)
The potato data as a bar graph. Bars above 0 mean the strip gained water; bars below 0 mean it lost water. The more salt, the more water the potato lost.
Just for fun

Ever tried to learn by "osmosis" by sleeping with your textbook under your pillow? Bad news: osmosis only moves water, and your pillow is not a selectively permeable membrane.

Hands-on activity

Fair test: osmosis in potato strips

You need: 1 large potato, a knife and cutting board (adult), a ruler, a kitchen scale if you have one, 3 cups or jars, table salt, measuring spoons and cup, paper towel, a marker, a timer.

  1. Write a hypothesis about what will happen to potato strips in tap water, slightly salty water and very salty water.
  2. Mix 250 mL of each: tap water with no salt, with 2.5 mL (½ tsp) salt (slightly salty), and with 30 mL (2 tbsp) salt (very salty). Stir until dissolved. Label the cups.
  3. Ask an adult to cut 3 potato strips the same size (about 1 cm × 1 cm × 6 cm). Peel them first so the skin doesn't affect the test.
  4. Measure each strip's length and, if you can, its mass. Also bend each gently and note how bendy it is.
  5. Put one strip in each cup, fully covered, for at least 1 hour (overnight in the fridge works too, as long as all three go in).
  6. Take out the strips, pat them dry the same way, and measure again.
  7. Record your results in a table and calculate the change (and percent change for mass).
  8. Write a conclusion and explain your results using osmosis. What would make your results more reliable?

Safety: an adult does all the cutting. Don't eat the strips afterwards. Wipe up spills so no one slips.

What to look for: the strip in tap water is firmer and slightly longer and heavier; the very salty one is floppy and lighter. A strong answer says water moved toward the side with more dissolved particles, and suggests repeating with more strips.

Your turn

1

Define osmosis.

2

A red blood cell is put in pure water. What happens? Why?

3

A plant cell is put in pure water. Why doesn't it burst?

4

What is plasmolysis?

5

Why does wilted celery get crisp again in a glass of water?

6

How does salting fish keep it from spoiling?

7

What does a contractile vacuole do?

More practice: warm-up

1

Osmosis is the diffusion of which substance?

2

In salty water, does water move into or out of a cell?

3

Which cells can burst in pure water: plant or animal?

4

Which plant structure stops bursting?

5

What happens to a raisin left in water overnight?

More practice: on your own

1

Explain why trees near a busy salted road often look unhealthy in spring.

2

A potato strip went from 12.0 g to 10.8 g. Find the percent change and say whether it was in tap water or salty water.

3

Why do you sprinkle salt on cucumber slices to make them less watery before making a salad?

4

Why must hospital IV fluid have a balanced amount of salt?

5

Explain how drying meat, as in making pemmican, keeps it from spoiling.

Challenge

1

Paramecia live in fresh ponds. Predict what would happen to a paramecium whose contractile vacuole stopped working. Then predict what would happen if it were moved to salt water.

2

A student put potato strips in sugar water instead of salt water and got the same pattern. Explain why.

3

In the potato test, the strip in slightly salty water hardly changed (−1%). What does this tell you about the inside of potato cells? How could you find the "balance point" more exactly?

Check your understanding

Quiz

1.

Osmosis is the diffusion of…

2.

An animal cell placed in very salty water will…

3.

A plant cell placed in pure water becomes…

4.

A potato strip went from 10.0 g to 11.0 g. It was most likely in…

5.

Salting food keeps it from spoiling because…

6.

Why don't plant cells burst in pure water?

For parents and teachers

Big idea: osmosis is the diffusion of water through a cell membrane. Water moves toward the side with more dissolved particles. Cells swell, stay the same or shrink depending on what surrounds them.

How to help at home:

  • Do the potato fair test together (you do the cutting). Help with the percent change calculation.
  • Revive a limp celery stalk or carrot in water, and salt cucumber slices, and ask your child to explain each.
  • In spring, look for salt damage along roads and talk about it.

Safety: adults do all cutting. Don't eat test materials.

Common difficulties:

  • Getting the direction wrong. Use the shortcut: water moves toward more dissolved stuff.
  • Thinking salt moves into the potato. The main change is water moving out.
  • Forgetting that plant cells don't burst because of the cell wall.