What can cross a membrane?

Change the openings in a model membrane. Track the larger particles and test whether opening size affects where they can go.

Starting notebook…

Simulation · Cells & Systems

How does opening size affect the number of larger particles on the right side of a model membrane?

Particles move randomly. If they can cross a membrane, they tend to spread from a more crowded area to a less crowded area overall. Individual particles can travel both ways. A selectively permeable membrane lets some substances cross more easily than others. This model uses gaps and particles of different sizes. Real cell membranes also depend on the substance and on proteins that help substances cross. This is diffusion, not a model of water moving by osmosis.

Review the related lesson →

Science words you will use

A hypothesis is a possible explanation. A prediction is what you expect to observe. The independent variable is what you change or compare; the dependent variable is what you measure. Controlled variables stay the same in a fair test. Qualitative observations describe what you notice; quantitative observations use numbers and units.

Accuracy means closeness to a trusted value. Precision describes how closely repeated readings agree. Repeating a result does not prove it is accurate. A pattern between two quantities does not, by itself, show that one caused the other.

What good work includes

Compare two opening settings using the same starting setup and counting rule.
Record actual model times and whole-number counts, with each run labelled.
Support a conclusion with counts from both settings, and explain one limit of this model.

Plan your investigation

My scientific notebook

Plan your investigation

Use your own words. You can return and change your ideas as you collect evidence.

My research questionWhat can you investigate using this equipment or record?My hypothesisGive a possible explanation: I think… because…My predictionIf… then I predict… because…What I change or compareName the independent variable. In an observation study, name what you compare.What I measureName the dependent variable and its units.What stays the sameHow will you keep the starting setup, time points, particle type and counting rule the same? How will you reproduce each opening setting?My procedure and changesUse repeatable steps. Record changes to the plan and why you made them.Reading my equipmentWhat is the smallest marked division? What units and rounding will you use?

Your apparatus

Explore a model membrane

Use the Concord Consortium model below. The large green particles start on the left; the smaller blue particles start on the right. Pause to count the green particles on the right. Use the model clock, not a wall clock. The notebook does not read the simulation for you.

Open Concord in a new tab

On a phone, turn sideways and open the model in a new tab for larger controls. Your notebook stays on this page.

Press play, then pause to count. Reset starts a new run. Check the pore slider after every reset. If the model is hard to use here, open it in a new tab.

The Concord Consortium · Next-Generation Molecular Workbench · Diffusion Across a Semipermeable Membrane.

Materials & safety

Concord Consortium membrane model
Digital notebook
A partner to check particle counts, if available

This is a computer model. It does not use living cells or require handling chemicals.

Suggested procedure — adapt and record changes

Before playing, write a prediction. Find the large green particles, the small blue particles and the barrier. The grey gaps represent openings in the model membrane.
Choose two repeatable settings: drag the Pore size slider fully left for closed and fully right for wide open. Describe these settings in your plan. The slider does not give you a real cell-pore measurement.
Reset the model, set the slider fully left and label this run Closed 1. Record the initial count of green particles on the right at model time 0 s.
Start the model. Pause near 5 s, 10 s, 15 s and 20 s. Record the actual model time shown and count green particles on the right. Check the count before restarting. If a particle overlaps the barrier, use one rule: count it only when its centre is to the right of the barrier.
Reset and repeat with the slider fully right, labelled Open 1. Keep the recording times, particle types, starting setup and counting rule the same. Use the actual displayed times if you pause late.
Check one condition by resetting and repeating it once. Label the check Open 2 or Closed 2 and keep the same settings. Record whether the results match. Replaying this model is a check of the model, not new evidence from real cells.
Compare counts at similar times. Keep each run separate in the graph. Explain whether the evidence supports your prediction. Try a middle opening as a separate follow-up, after saving this notebook.

Collect · check · compare

Your data and graph

Use whole-number counts from 0 to 10. Zero is a valid count. Use a different group name for each run; mean view only combines identical group names at exactly the same time.

Graph titleGraph view

The table keeps your exact readings. On a small screen, scroll inside the table or graph to see the rest. With a keyboard, focus it and use the arrow keys.

Add readings to enable data and graph downloads.

Explain your results and reflect

Think like a scientist

Explain, question and reflect

Patterns in my resultsCompare the green-particle counts for closed and open settings at similar times. Does a count ever fall? Use numbers from more than one run to support your claim. The right-hand space is larger than the left. Why would evenly spread particles not require five green particles on each side?Limits and possible errorsCould a moving or overlapping particle be miscounted? Did you pause at comparable times? If a reset gives the same results, what does that tell you about repeating this model? Explain one way real cell membranes are more complex than this barrier.My conclusionAnswer your research question. Your result does not have to match your prediction.The evidenceUse measurements, units and trial numbers to support your conclusion.Why the evidence supports my conclusionExplain how opening size and random particle motion help explain your counts. Use this frame if helpful: At ___ s, I counted ___ on the right in ___ and ___ in ___. This supports ___ because ___. The model cannot show ___.What I would improve or test nextWhat would you change? Why? Revisit your hypothesis and prediction.

A new question to explore

Try a middle opening. Can small blue particles cross while large green particles cannot? Use observations to explain selective permeability: some substances cross more easily than others.

Keep your work

Your lab report

Preview your notes, measurements, calculations and graph. Choose Print / Save as PDF in the report, or download an HTML copy. Submit that PDF in Google Classroom using your teacher’s instructions. This website does not submit your work.

Student name (optional)Class (optional)Investigation date (optional)

Restore notebook backup

A backup is a JSON file for restoring this notebook. It is different from your finished report. Keep your own copy before clearing browser data or changing devices.