Interactive Brownian motion simulation of molecular transport across a lipid bilayer. Simple diffusion: all small molecules pass freely through concentration gradient. Facilitated diffusion: molecules pass only through protein channels. Active transport: ATP-powered pump drives ions against their concentration gradient, accumulating them on the high-concentration side. Demonstrates osmosis, equilibration, and active vs passive membrane transport mechanisms.
This simulation models around 450 particles undergoing Brownian motion in two chambers separated by a lipid bilayer. Each particle takes a random walk: its velocity gains a temperature-scaled random kick every frame, is damped, then capped at a maximum speed. The net result is a diffusive drift of particles from the crowded high-concentration side towards the low-concentration side until the populations on each side equalise.
Three mode buttons set the membrane behaviour. Simple Diffusion lets every particle cross freely; Facilitated routes crossings only through five protein channel slots; Active Transport adds an ATP-powered pump that drives ions (orange) from right to left against the gradient while water (blue) still equilibrates. The Temperature slider scales the Brownian kick, and live telemetry tracks left/right counts, balance and ATP used. These mechanisms underpin nutrient uptake, nerve signalling and kidney function.
What does this simulation actually show?
It shows molecules moving across a cell membrane by Brownian motion. Two chambers, one high and one low concentration, are divided by a lipid bilayer, and you watch roughly 450 particles spread out over time. Depending on the chosen mode, the membrane is either fully permeable, channel-only, or fitted with an active pump.
What is the difference between the three transport modes?
Simple Diffusion allows all small particles to cross freely down the concentration gradient. Facilitated Diffusion only lets particles through five protein channel openings, so equilibration is slower. Active Transport uses an ATP-powered pump to push ions against the gradient from right to left, accumulating them on one side.
How is the particle motion calculated?
Each particle has a velocity that receives a random kick scaled by the temperature setting on every frame, simulating a random walk. The velocity is then multiplied by a damping factor of 0.88 and clamped to a maximum speed. Over many frames this stochastic jostling produces the characteristic spreading of diffusion.
The slider runs from 1 to 5 (labelled Cold to Hot) and scales the strength of the random Brownian kick added to each particle. Higher settings give larger, faster kicks and a higher speed cap, so particles dart about more energetically and equilibrate sooner. This mirrors how thermal energy raises diffusion rates in real systems.
Blue particles represent water and make up about 80 percent of the population; orange particles represent ions and make up the other 20 percent. The distinction only matters in Active Transport mode, where water continues to equilibrate freely while ions are selectively pumped against their gradient.
Left and Right show how many of the total particles are currently on each side of the membrane. Balance is a percentage that reaches 100 percent when both sides hold an equal share. In Active Transport mode an extra ATP used counter appears, incrementing each time the pump moves an ion against the gradient.
It is a qualitative teaching model rather than a quantitative one. The random-walk dynamics, gradient-driven net flow, channel selectivity and energy cost of active transport are all faithful in concept. However, the numbers are illustrative: it does not solve real diffusion coefficients, membrane potentials or ion-specific kinetics.
Moving ions against their concentration gradient is thermodynamically unfavourable, so it cannot happen by passive diffusion alone. Energy must be supplied, and in living cells this comes from hydrolysing ATP. The simulation tracks this by incrementing an ATP used counter every time the pump carries an ion uphill from right to left.
Osmosis is the diffusion of water across a selectively permeable membrane towards the side with higher solute concentration. In this model the blue water particles equilibrating between chambers represent that water movement, especially in modes where ions are confined and only water can balance the two sides.
These processes govern much of cell biology: oxygen and carbon dioxide diffuse passively across membranes, glucose enters cells through facilitated transporters, and the sodium-potassium pump uses active transport to maintain the gradients behind nerve and muscle signalling. Understanding them is fundamental to physiology, pharmacology and medicine.