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Biology
Membrane transport
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A cross-section of a cell membrane: add solutes on either side, place channels and pumps in the membrane, and watch which solutes cross, in which direction and through which protein.
While paused, each step advances 0.1 s of model time.
Starting the model…
OptionsLipids, glucose metabolism, highlights, pure random diffusion, follow a particle
Tap a particle in the scene to follow it; the marker never changes its path.
Tab to the protein tray, a protein in the membrane or a ligand; Enter grabs; arrows choose where; Enter releases; Esc cancels.
Model notes
- Particle counts in the scene are illustrative, for counting and comparing, not real concentrations; each solute has at most 200 particles on both sides together.
- Particles move randomly. To make the net flow visible with few particles, the original model vetoes 90 % of steps against the difference when the counts differ by more than 10 % (the “Pure random diffusion” option removes this bias). O₂ and CO₂ pass through the lipids on 90 % of contacts.
- The membrane potential is a set value (−70, −50 or +30 mV) that opens or closes voltage-gated channels after 0.25 s; it is not computed from the ions.
- The Na⁺/K⁺ pump uses 1 ATP for every 3 Na⁺ carried out and 2 K⁺ carried in, regardless of the difference; the ADP and Pi it makes fade away.
- The sodium–glucose cotransporter is secondary active transport: it works only while outside Na⁺ exceeds inside Na⁺, and brings glucose in even when there is more glucose inside.
- There is no endocytosis, exocytosis or osmosis; times and rates are chosen for observation, not biochemical measurement.