How can a cell change size without eating or growing?
Imagine a cell placed in a new liquid. Minutes later, its volume changes. Where did the water go, and what does the surrounding solution have to do with it?
Your mission: test three solution conditions, collect model evidence, and explain water movement across a cell membrane.
How does the environment affect water movement across a cell membrane?
WICOR focus: Inquiry: predict, compare conditions, and justify a claim with evidence.
- I can distinguish an observation from an inference.
- I can compare surrounding solutions while keeping the model cell the same.
- I can explain net water movement and identify a model limitation.
The case of the changing cell.
Two copies of a model cell begin with equal size and equal internal solute concentration. They are placed into different surrounding solutions. Later, one is larger than the other.
Notice: What could you observe about the two cells? Wonder: What evidence would you need before claiming that water moved?
Observation or inference?
“After the model runs, Cell A has a larger outline than Cell B.”
Change the solution. Track the cell.
Testable question: In this model, how does the concentration of dissolved particles outside a cell affect its net water movement?
Predict the results for a solution with less, equal, or more non-penetrating solute outside than inside. Which two settings would make the clearest comparison?
Observe: direction of net water movement and illustrated cell size.
Keep consistent: starting cell, internal solute, membrane properties, and modeled time.
Run the model to see the result.
Run the model to see the result.
Choose two different settings to compare.
Model rules: With a membrane permeable to water but not the pictured solute, lower solute outside produces net water entry, equal effective solute produces no net movement, and higher solute outside produces net water exit. All size changes are illustrative, not measured. The cell shown is a simplified animal-like cell, not a plant cell with a cell wall.
Follow the water, not the label.
- Compare lower solute outside with equal concentration.
- Swap Cell A and Cell B settings, then run again.
- Compare higher solute outside with lower solute outside.
Your simulated observation log
| Run | Cell A setting | A result | Cell B setting | B result |
|---|---|---|---|---|
| Run a comparison to record model outputs. | ||||
The log stays on this open page only; reset clears it. Results are programmed illustrations, not original biological measurements.
Need a hint for spotting the pattern?
Compare each outside solution with the cell interior. Water can move both ways across the membrane, but the net movement favors the side with more effective solute in this simplified case. Swapping A and B tests whether the result follows the setting.
Does the cell “want” more water?
A student says, “The cell chooses to drink when it needs water.” Another says, “A difference in effective solute concentration causes net water movement.” Which claim is scientifically testable?
After investigating: reveal the life science connection
Osmosis is the net movement of water across a selectively permeable membrane due to a difference in water potential. In the simplified model, water moves toward the side with more solute that cannot cross the membrane.
Hypotonic surroundings: net water enters and an animal cell may swell. Isotonic surroundings: water moves in both directions with no net volume change. Hypertonic surroundings: net water leaves and the cell may shrink.
Structure and function: the plasma membrane helps regulate exchange between the cell and its environment. The real effects depend on membrane permeability and solute type. Plant cells have walls that change how swelling works.
Important limitation: the model does not calculate water potential, show individual molecules, or predict the timing or amount of volume change. It cannot demonstrate that osmosis occurred in a real specimen.
Design a real investigation
Imagine using equally sized potato pieces in different saltwater concentrations. What would you measure before and after soaking them? How would you keep soaking time, piece dimensions, and temperature consistent? Why would multiple pieces per condition improve the evidence?
Further reading: OpenStax Biology 2e: Passive Transport.
Bring your evidence to the comments.
Write one comment below this post with all four parts:
- Prediction: Which surrounding solution did you think would make the cell larger or smaller?
- Evidence: Describe two model runs, including a swap of A and B. Give the setting and the result.
- Explanation: Use the word osmosis to explain why net water movement changes.
- Next question: Name one limitation and describe an experiment with real materials that could test the pattern.
Respond to a classmate: Ask a question about their evidence or suggest one way to improve the experiment.
Submit your response in the website’s comments section below, not in the model. The activity does not collect student names or written answers.
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4 Responses
I predicted that In Run A, Cell 1 had a higher solute outside and After I swapped the settings, it was the same thing just inverted This pattern supports it is what happened because. A limitation of the model is the thing that is important. Next, I would test why it forms or what it looks like at a microscopic level.
The cell grew with lower solute outside. And the one that has higher solute outside shrank because water exits. With lower solute outside the water enters and does not leave which makes the cell grow way more than the higher solute outside. A limnation of the model Is a. Next I would test b to make it bigger.
I predicted both A and B would change. In Run lower solute outside, Cell A had swelled water solute outside and inside have the same amount of dots. After I swapped the settings both cell A and cell B, got smaller circles . This pattern supports that different weather changes can impact on the surrounding solution because it changes the sizes of the circle. A limitation of the model is the amount of swelled water comes in and out Next, I would test the different water levels.
I predicted that when the cell gets more water it gets bigger and when it gets less water it gets smaller. In the simulation when I added more water it swelled and when the water exited it shrunk. Osmosis is the net movement of water molecules across a semipermeable membrane from an area of higher water concentration is lower solute concentration to an area of lower water concentration is higher. And the next thing I would test is how sensitive water is to vibrations and other things.