Traffic Control: How Cells Manage Molecular Movement – Student Data Guide
Name: ___________________________
Date: ____________ Class: ___________________________
Simulation link: https://phet.colorado.edu/sims/html/membrane-transport/latest/membrane-transport_all.html
Introduction
In this interactive activity, you’ll use a simulation to see how molecules move in and out of cells. You’ll test
different types of transport—passive, facilitated, and active—by adding solutes, opening channels, and using
energy (ATP). As you explore, you’ll collect data, look for patterns, and figure out how cells use proteins and
energy to stay balanced. By the end, you’ll be able to explain how real cells control the “traffic” of molecules to
keep everything running smoothly.
Learning Objectives
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Describe how solute concentration affects diffusion across a membrane.
Identify differences between passive, facilitated, and active transport.
Collect and analyze data on how transport changes with membrane proteins and energy (ATP).
Explain how cells use energy to maintain concentration gradients.
Before You Begin — Activating Prior Knowledge
Discuss or write your ideas:
1. What do you already know about how materials move in and out of cells?
2. Do you think all molecules move the same way? Why or why not?
3. When might a cell need energy to move something?
Share your ideas with a partner, then start the simulation.
Part 1: Passive Diffusion
1. Open the Passive Diffusion tab in the simulation.
2. Click on one solute type and click the ⏩for the
outside of the cell only.
3. Watch what happens for 30 seconds. Record your
results in Table 1.
4. Click the Eraser and repeat this process for the
remaining types of solutes.
5. Click on one solute type and click the ⏩for the inside of the cell only.
6. Watch what happens for 30 seconds. Record your results in Table 1.
7. Click the Eraser and repeat this process for the remaining types of solutes.
Click to add solute to the
Click to add solute to the
Table 1: Results of Adding Solutes to the Inside and Outside of the Cell
Solute
Location Added
O2
outside
CO2
Na+
K+
Glucose
O2
inside
Result
CO2
Na+
K+
Glucose
Which solutes were able to cross the cell membrane? Why do you think this is?
Part 2: Concentration of Solutes During Passive Diffusion
1. Continue in the Passive Diffusion tab in the simulation.
2. Click on one solute type that was able to pass through the membrane in part 1 and record your solute
below.
3. Click the ⏩for the outside of the cell only.
4. Pause ⏸️ the simulation after 10 seconds.
Look at the cell diagram or the Solute
Concentrations bar at the bottom of the
simulation. Record relative concentrations and direction of flow in Table 2.
5. Click ⏩again for the outside of the cell only.
6. Pause ⏸️ the simulation after 10 more seconds. Record relative concentrations and direction of flow in
Table 2.
7. Click ⏩again for the outside of the cell only.
8. Pause ⏸️ the simulation after 10 more seconds. Record relative concentrations and direction of flow in
Table 2.
Solute:____________________________
Table 2: Relative Concentration of Solute and Direction of Net Flow During Passive Diffusion
Time (s)
Inside Concentration
(Low, Medium, High)
Outside Concentration
(Low, Medium, High)
Direction of Net Flow
(Outside→ Inside OR Inside→
Outside)
10
20
30
What trend (pattern) do you see in how the solute moves?
Part 3: Facilitated Diffusion
1. Switch to the Facilitated Diffusion tab.
2. Add a Na+ Leakage Channel to the membrane.
3. Click on Na+.
4. Click the ⏩for the outside of the cell only.
5. Record whether or not the solute is able to cross the
cell membrane in Table 3.
6. Remove the solute by clicking the ⏩button.
7. Click the ⏩for the inside of the cell only.
8. Record whether or not the solute is able to cross the cell membrane in Table 3.
9. Remove the solute by clicking the ⏩button.
10. Click the Eraser and repeat this process for the remaining types of channels and solutes. Follow the
combinations in Table 3.
Click and
Drag these
to add
channels
Table 3: Results of Adding Solutes to the Inside and Outside of the Cell with Protein Channels
Solute
Protein Type
Location Added
Did Solute Pass Through the Cell
Membrane? (Y/N)
Na+
Na+ Leakage Channel
outside
K+
Glucose
Na+
inside
K+
Glucose
Na+
K+ Leakage Channel
outside
K+
Glucose
Na+
inside
K+
Glucose
Solute
Protein Type
Location Added
Na+
Na+ Voltage-gated Channel
outside
K+
Glucose
Na+
K+
inside
Did Solute Pass Through the Cell
Membrane? (Y/N)
Glucose
Na+
K+ Voltage-gated Channel
outside
K+
Glucose
Na+
inside
K+
Glucose
Na+
Na+ Ligand-gated Channelwithout Ligand
outside
K+
Glucose
Na+
inside
K+
Glucose
Na+
K+ Ligand-gated Channelwithout Ligand
outside
K+
Glucose
Na+
K+
Glucose
inside
Solute
Protein Type
Location Added
Na+
Na+ Ligand-gated Channelwith Ligand
outside
Did Solute Pass Through the Cell
Membrane? (Y/N)
K+
Glucose
Na+
inside
K+
Glucose
Na+
K+ Ligand-gated Channel-with
Ligand
outside
K+
Glucose
Na+
inside
K+
Glucose
Summarize your findings about which solutes are allowed in by each channel protein.
Na+ Leakage Channel
K+ Leakage Channel
Na+ Voltage-gated Channel
K+ Voltage-gated Channel
Na+ Ligand-gated Channel
K+ Ligand-gated Channel
Part 4: Concentration of Solutes During Facilitated Diffusion
1. Continue in the Facilitated Diffusion tab in the simulation.
2. Choose one solute/channel combination from Part 3 that resulted in a flow across the membrane.
Record your combination below.
3. Click the ⏩for the outside of the cell only.
4. Pause ⏸️ the simulation after 10 seconds.
Look at the cell diagram or the Solute
Concentrations bar at the bottom of the
simulation. Record relative concentrations and direction of flow in Table 4.
5. Click ⏩again for the outside of the cell only.
6. Pause ⏸️ the simulation after 10 more seconds. Record relative concentrations and direction of flow in
Table 4.
7. Click ⏩again for the outside of the cell only.
8. Pause ⏸️ the simulation after 10 more seconds. Record relative concentrations and direction of flow in
Table 4.
Solute:______________________
Channel Type:___________________________________
Table 4: Relative Concentration of Solute and Direction of Net Flow During Facilitated Diffusion
Time (s)
Inside Concentration
(Low, Medium, High)
Outside Concentration
(Low, Medium, High)
Direction of Net Flow
(Outside→ Inside OR Inside→
Outside)
10
20
30
Why did the solute flow the way it did?________________________________________________________
_______________________________________________________________________________________
Part 5: Active Transport
1. Switch to the Active Transport tab.
2. Add the Na+/Glucose Cotransporter to
the cell membrane.
3. Click Sodium and ⏩ for the outside of
the cell.
4. Click Glucose and ⏩ for the outside of the cell.
5. Observe how solute movement changes.
6. Complete Table 5.
Table 5: Sodium and Glucose Transport Across the Cell Membrane
Solute
Starting Inside
Concentration
Starting Outside
Concentration
Movement
Direction
ATP Added?
(Y/N)
Na+
Glucose
Was all the glucose able to cross the membrane? Explain why or why not.
What could you do to increase the amount of glucose that can go through?
1. Add the Na+/K+ to the cell membrane.
2. Click Sodium and ⏩ for the outside of the cell.
3. Click Sodium and ⏩ for the inside of the cell.
4. Click Potassium and ⏩ for the inside of the cell.
5. Click ATP and ⏩ for the inside of the cell.
6. Observe how solute movement changes.
7. Complete Table 6.
Table 6: Sodium and Potassium Transport Across the Cell Membrane
Solute
Starting Inside
Concentration
Starting Outside
Concentration
Na+
K+
What is the ratio of sodium to potassium that makes the channel work?
Movement
Direction
ATP Added?
(Y/N)
How are the two active transporters alike?
How are the two active transporters different?
Why does active transport require ATP?
Part 6: Challenge – Maintain a Gradient
Continue in the Active Transport tab.
Try to keep sodium (Na⁺ ) high outside the cell and glucose entering the cell.
1. Adjust pumps and channels until both goals are met.
2. Record your configuration and results.
Pumps Added
Solutes Added Outside
Solutes Added Inside
What combination of pumps and channels kept sodium and glucose balanced?
Part 7: Analysis
Compare passive, facilitated, and active transport in the table below.
Transport Type
Energy Needed?
Requires Protein?
Passive
Facilitated
Active
Which type of transport is the most energy efficient? Why?
How do cells use active transport to maintain balance?
What might happen if a cell couldn’t perform active transport?
Moves High → Low OR Low → High?
Reflection and Collaboration
Discuss with your partner or group:
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What patterns did you see across all three types of transport?
How does this simulation connect to how real cells work?
What surprised you the most about how molecules move?
Write one question you still have about cell transport:
_______________________________________________________________________________________
Part 8: Closing Activity — From Membranes to Real Life
Purpose
To evaluate how well you understand how cells manage molecular movement and how these transport
processes apply to real-world biological systems.
Step 1: Synthesis — Concept Map (Individual)
Create a concept map on a separate sheet that connects the following key terms:
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Passive Diffusion
Facilitated Diffusion
Active Transport
Concentration Gradient
Membrane Proteins
ATP
Include arrows and labels showing relationships (e.g., “requires energy,” “moves high → low,” etc.).
→ This helps you demonstrate how all the mechanisms fit together to maintain homeostasis.
Step 2: Presentation of Conclusions (Pair or Small Group)
In pairs or trios, discuss and record your group’s conclusions:
1. What is the most efficient transport process and why?
2. Why does a cell need all three types of transport?
3. What evidence from the simulation supports your conclusions?
Then, share one takeaway with the class or post it on a shared board (“Cell Transport in Action”).
Step 3: Application — Real-World Connection
Choose one real-world scenario below and explain which type(s) of membrane transport are involved and
how they maintain balance in that system.
Options:
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Nerve impulse transmission (Na⁺ /K⁺ pump)
Glucose absorption in the small intestine
Oxygen and CO₂ exchange in the lungs
Water balance in plant roots
Answer frame:
In [chosen system], (process) transport moves (substance) from (location) to (location). This
helps the organism maintain (specific function or balance) because ____________________.
Step 4: Reflection — Self-Assessment
Complete this self-assessment to measure your achievement of the learning objectives:
Learning Objective
I Can Explain It
Clearly
I Understand It
Somewhat
I Need More
Practice
How solute concentration affects
diffusion
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☐
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Differences between passive,
facilitated, and active transport
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☐
How proteins and ATP change
transport
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☐
☐
How cells use energy to maintain
gradients
☐
☐
☐
Part 10: Extension
Open the Playground tab and create your own experiment.
Design one test of how increasing the number of transport proteins changes the rate of solute movement.
Record your hypothesis, observations, and conclusion below.
Hypothesis: (If…then…because…)
Data Table:
Data Summary:
Conclusion:
This activity was developed using the Membrane Transport simulation from PhET Interactive Simulations, University of Colorado Boulder.
Licensed under CC-BY 4.0.