01 / PARTITIONING
The core sets the barrier
Small nonpolar molecules enter the hydrophobic interior readily. Water and other small uncharged polar molecules cross more slowly. Glucose and ions have negligible passage on this model’s timescale.
Can your particle cross?
Grab it and move from side A to side B—or back again.
Polar headsNonpolar tailsHydrogen bondsIon–dipole interactionsNonpolarPolarCationAnion
Keyboard: focus the scene and use arrow keys. For NaCl, click either ion or press N to select the next ion. “Try crossing” follows the same permeability rules.
02 / HYDRATION
Water surrounds the solute
Polar solutes hydrogen-bond with water; Na⁺ and Cl⁻ have ion–dipole interactions with water. Losing favorable hydration and entering the nonpolar core costs free energy. The dotted lines are interactions, not fixed tethers. As a hydrated particle approaches the core, the water cluster briefly lags and recoils to illustrate the energetic cost of disrupting these interactions.
03 / SELECTIVITY
A protein provides a route
Channels offer selective pathways. A glucose carrier binds glucose and alternates access between the two sides. Passive transport requires no ATP; its net direction depends on the relevant gradient.
Random motion. Net diffusion.
Individual particles move both ways; the population reveals the gradient.
96
Particles on side A
24
Particles on side B
0
Crossings A → B
0
Crossings B → A
Ready to observe
Press Run. Compare the two crossing counters as the concentration difference changes.
Nonpolar O₂ particlesDot color shows particle type; teal and ochre lines on the graph show sides A and B.
Concentration over time
Counts represent concentration because compartment volumes are equal.
━ Side A ━ Side B Dashed line: 60 particles per side
Try three comparisons
- Run O₂ from 50%, 80%, and 100% on A. Compare the early net movement.
- Run glucose with the same gradient, then add its carrier. What changes?
- Watch near equal concentrations. Do individual particles stop crossing?
Scientific model, boundaries, and sources
This is a qualitative teaching model, not molecular dynamics. Particle sizes, membrane thickness, motion, carrier timing, and crossing rates are illustrative and not to scale. Temperature, membrane area, thickness, and compartment volumes stay constant. A pure bilayer’s actual permeability varies with lipid composition and temperature. “Blocked” means negligible passage on this teaching timescale, not a mathematical zero in every membrane.
Dragging is a permeability probe. Your pointer supplies external manipulation; it is not passive diffusion or a literal pulling force exerted by water. The lag for water and urea makes an energy barrier visible, not a measured viscosity. Hydration partners exchange rapidly. The brief water-cluster recoil is a conceptual cue for the hydration and partitioning barrier, not a literal elastic force. Only representative water interactions are drawn; nonpolar molecules are surrounded by water too, but do not form the depicted solute–water hydrogen bonds.
Proteins are selective. This model uses a simplified GLUT-like alternating-access carrier, aquaporin, urea transporter, and idealized separate Na⁺ and Cl⁻ channels. Real channels have specialized selectivity filters; this illustration does not resolve their atomic structures. NaCl dissociates into hydrated Na⁺ and Cl⁻ in water. Ion motion in real membranes follows the electrochemical gradient, including membrane voltage; the drag activity does not calculate voltage.
Diffusion model. Random walkers encounter the membrane from both directions with symmetric crossing probabilities. A successful passage is compressed into one crossing event; this view does not resolve the microscopic passage pathway. Adding a matching protein increases passage probability at its location and resets the trial for comparison. No ATP, pumping, carrier saturation, or osmotic volume changes are modeled. For water, dots represent a dilute labeled-water tracer in water on both sides, not unequal total water density or an osmosis experiment. Ions are intentionally omitted from this concentration-only diffusion experiment.
Sources: Alberts et al., Principles of Membrane Transport, Molecular Biology of the Cell; Cooper, Cell Membranes, The Cell: A Molecular Approach; Peterlin et al., Permeability of phospholipid membrane for small polar molecules.