Fluid Mosaic & Membrane Transport

Cross-section of a plasma membrane. Drag to orbit, scroll to zoom, click any structure for detail.

Every cell is wrapped in a thin, two-layer film of fat molecules called phospholipids — the plasma membrane. Nothing in it is bolted down: the lipids and proteins drift around like rafts on water, which is why biologists call it the "fluid mosaic" model.

That oily core is a great barrier — it stops water-soluble things like ions from just leaking through. But cells still need those ions to move in and out, so special proteins sit in the membrane as gatekeepers: channels that let ions flow downhill on their own, and pumps that spend energy (ATP) to push them the other way.

The star of this model is the Na⁺/K⁺ pump: it burns one ATP to push 3 sodium ions out and pull 2 potassium ions in, against their natural gradients. That's what keeps the inside of your cells electrically negative and primed to fire a nerve signal instantly. Step through the pump cycle below, or block it with ouabain to see what happens when it stops.