Particles start outside the cell and move randomly. Watch the net movement
into the cell, down the concentration gradient, until the inside and outside reach the same
concentration — equilibrium.
Three factors change the rate of diffusion:
temperature, the concentration gradient, and the surface area : volume ratio.
Change one slider at a time and compare the time to equilibrium.
particle outside the cell (high concentration)particle inside the cellcell membrane (freely permeable)nucleus
Controls
—
Hotter particles move faster, so they hit and cross the membrane more often. Updates live.
—
More particles outside = a steeper gradient = faster net movement in. Starts a fresh run.
—
Drag to resize the cell and watch the SA:V ratio change. A bigger cell has more membrane
— but far more inside to fill. Release to start a fresh run.
Surface area : Volume
—
surface area — · volume —
Higher ratio → faster exchange. (2D model: surface area = membrane length,
volume = area inside the cell.)
inside
outside
concentration inside vs outside
Measurements
Total crossings
0
Net particles moved in
0
Total crossings / s
0
Net movement in / s
0
Equilibrium reached
0 %
Time to 90% equilibrium
—
Watch this: at equilibrium net movement / s falls to ≈ 0,
but total crossings / s stays high — particles never stop moving. That is
dynamic equilibrium.
Fair test: change one slider, press Restart run, and note the time to
90% equilibrium. Then change it back and try another value.