Pirl Experimental · Biology Virtual Labs
What gets through
the cell membrane?
Change the particles on each side. Control the transport proteins. Follow what crosses—and explain why.
Cell Membrane Transport · Interactive ModelHover or focus on a tab for a snapshot. On touchscreens, tap a tab.
Transport snapshot
Three routes across one membraneSmall molecules can pass through the cell membrane. Other particles need a protein. Some pumps use ATP to move particles against a concentration difference.
Compare the pathwaysSimplified picture · arrows show net transport in this example. Press Esc or tap outside to close.Transport snapshot
Straight through the cell membraneOxygen and carbon dioxide move both ways. Net movement is from higher to lower concentration.
No direct ATP use · O₂ and CO₂Simplified picture · arrows show net transport in this example. Press Esc or tap outside to close.Transport snapshot
A protein provides the pathwaySelective channels help certain molecules cross the cell membrane in this simplified picture. Net transport follows the concentration gradient, from higher to lower concentration.
No direct ATP use · Selective channelsSimplified picture · arrows show net transport in this example. Press Esc or tap outside to close.Transport snapshot
A pump uses energyThis model’s pump uses an ATP token to move sodium out. It can move sodium toward the side that already has more sodium.
ATP-driven pump shown hereSimplified picture · arrows show net transport in this example. Press Esc or tap outside to close.Focus changes what you see. All particles continue to follow the same model; switching views preserves your setup.
Schematic cross-section · equal-size, fixed compartments · particles and speeds are not to scale. On a small screen, swipe across the model.
Start with a prediction, then select Start motion.
Observe · Compare · Explain
Where are the particles?
| Particle | Outside | Inside | Crossing now | Outside → inside | Inside → outside | Current comparison |
|---|
Because this model uses equal compartment sizes, counts provide a relative concentration comparison. Equal counts at one moment do not establish dynamic equilibrium. Compare movement in both directions over time.
Set equal counts rearranges all particle types as a new starting condition and clears crossing totals. It does not simulate a cell forcing equilibrium. Odd totals differ by one particle.
Three investigations to try
- Can movement continue with equal counts? Load the diffusion example, choose Set equal counts, and start motion. Watch individual particles and both crossing totals.
- Is a gradient enough? Load the blocked-protein example. Start motion, compare the counts, then make the glucose channel available. Explain the role of the protein.
- What does energy make possible? Load the pump example. Compare sodium counts, then run several pump cycles. Identify the direction of transport relative to the count difference.
What this model simplifies
- Particle colors, sizes, paths, collision rules, and speeds are illustrative. This is a teaching model, not a calibrated measurement of cell transport.
- O₂ and CO₂ cross the cell membrane directly. Water tracers use aquaporins here; slower movement through the cell membrane’s fatty layer is omitted.
- This is not a full osmosis or tonicity model: water potential, osmotic pressure, cell volume changes, and molecule–water interactions are not calculated.
- Transport proteins are shown as selective channels to simplify the model. In real cells, glucose transport involves a protein changing shape rather than an open passageway. “Example molecule” is a fictional molecule used to explore selective transport.
- The sodium pump illustrates ATP-dependent export. One token pays for one illustrated transfer; the real sodium–potassium pump’s 3 Na⁺ out / 2 K⁺ in cycle is not represented. Electrical gradients and sodium channels are omitted.
- Active transport includes mechanisms powered indirectly by ion gradients. This activity focuses on a pump powered directly by ATP.
Science background: OpenStax: Passive Transport and Active Transport. These references support the concepts, not the numerical settings of this model.