The membrane is a picky gate. Some things drift across for free (going with the crowd), some need a protein doorway, and some must be pumped uphill using energy. Pick a method and watch molecules cross.
Simple diffusion
Simple diffusionfree, downhill
Pick a method
📷 Reference image
A real labeled figure to compare with the interactive above.
Membrane Transport — OpenStax, CC BY 4.0 (via Wikimedia Commons)
