Lipid Bilayer & Membrane Fluidity

Bilayer composition

Lipids per leaflet 60

Temperature 0.50

Cholesterol 15%

Unsaturated tails 35%

Order parameter S

–

Diffusion coeff. D

–

Membrane phase

–

Self-assembly progress

100%

How it works

Each lipid is amphipathic: a hydrophilic head (blue) and a hydrophobic tail (orange, or bent for an unsaturated kink). Press "Scramble" to fling every lipid into the surrounding water at random — hydrophobic tails then reorient away from water and pack together while heads face outward, and the two-leaflet bilayer re-forms on its own, the same driving force that assembles real cell membranes and liposomes.

Once assembled, lipids don't sit still — they diffuse laterally within their own leaflet. How fast depends on the phase of the membrane:

S = order parameter (0=disordered, 1=ordered)
S ≈ (1 − T) − 0.35·unsaturated − cholesterol·buffer(T)
D ≈ D_max · (1 − S) · (0.4 + 0.6·T)

Temperature — heat disorders tail packing and speeds up lateral diffusion (liquid-crystalline behaviour); cooling favours a tightly packed, low-mobility gel phase.
Cholesterol — a rigid steroid (green) wedged between tails. Above the transition temperature it restrains motion and orders the tails (liquid-ordered phase); below it, it disrupts tight gel packing and keeps the membrane from freezing solid. It buffers fluidity in both directions.
Unsaturated tails — a cis double bond puts a permanent kink in the tail (bent instances), preventing tight packing and increasing disorder/fluidity even at low temperature.

This is the fluid mosaic model: a two-dimensional fluid of lipids whose viscosity is tuned live by exactly these three knobs, the same three a real cell uses to keep its membranes functional across a range of temperatures.

Polar head Saturated tail Unsaturated (kinked) tail Cholesterol

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