Fluid mosaic membrane: simple diffusion, facilitated diffusion and active transport animated

What it does

Every cell is wrapped in a membrane only a few billionths of a metre thick. The fluid mosaic model pictures it as a double layer of lipids (the fluid part, which keeps drifting sideways) with proteins floating in it like tiles in a mosaic. This page animates that membrane and lets you watch four different things try to cross it:

• Oxygen slips straight through the lipids: simple diffusion.
• Water goes through the lipids slowly and through water channels (aquaporins) quickly: osmosis.
• Glucose can only cross at a carrier protein, which flips to let it through: facilitated diffusion. No energy is used, so it still only goes from more to less.
• Sodium ions are charged and cannot pass the lipids at all. They leak through ion channels downhill, and the Na⁺/K⁺ pump pushes them uphill, out of the cell, splitting ATP: active transport.

It fits Class 9 Science (the cell, the plasma membrane, diffusion and osmosis) and Class 11 Biology (cell structure and the fluid mosaic model, and how substances cross membranes in plants and animals). Start with the whole cell in cell explorer, see water moving in and out of real tissues in osmosis lab, and see how the pump’s uneven ions make a voltage in membrane potential calculator.

How to use it

1. Pick a level. Class 9 keeps the picture simple; Class 11 labels each protein and adds the rate graph and one more question.
2. Pick a substance: Oxygen, Water, Glucose or Sodium Na⁺.
3. Set how many particles start outside and inside the cell with the two sliders. Tap the picture to add more particles on either side.
4. Watch the counts. The big line says which way the net movement goes. For sodium, tick or untick Pump on and Ion channels open.
5. Read the formula line underneath, filled in with your starting numbers.

Worked example

Oxygen, 30 particles outside and 6 inside. With the model’s permeability P = 0.5 per second, the starting rate is 0.5 × (30 − 6) = 12 particles per second inward. As oxygen moves in, the difference shrinks, so the flow slows, and it stops (on average) at 18 and 18.

Switch to glucose with the same numbers. Each carrier can only work so fast (Vmax = 10 per second, half speed at K = 8 particles), and carriers work both ways, so the net rate is 10 × 30/38 − 10 × 6/14 = 7.89 − 4.29 = 3.61 per second, much slower. On the Class 11 graph, with 30 outside simple diffusion gives 15 per second and the carriers give 7.89; at 60 outside it is 30 against 8.82. The carrier curve flattens because every carrier is busy: this is saturation, and it is how you can tell carrier transport from simple diffusion in an experiment.

With sodium and only the pump on, Na⁺ piles up outside even though there is already more there. After the pump has moved 30 Na⁺ out it has used 10 ATP and brought 20 K⁺ in.

Tips

• Particles never stop crossing, even when both sides are equal. “No net movement” means as many go each way.
• The lipid tails are oily, so they let through small uncharged molecules (O₂, CO₂) and stop ions and big polar ones (glucose).
• Both channels and carriers are proteins, and both need a gradient. Only pumps can go against it, and only by using ATP.

FAQ

Why is it called fluid? The lipids are not fixed: each one moves sideways, swapping places with its neighbours millions of times a second, and many proteins drift too. The drifting heads in the picture show this.

What are the little branches on the outside? Sugar chains attached to proteins (glycoproteins). They help cells recognise each other.

Is this how fast real molecules move? No. It is a teaching model with made-up numbers. Real membranes carry millions of molecules a second and are far thinner than drawn.

Last updated 2026-11-09.