AP® Biology · Unit 2 · Topic 2.3
The Phospholipid Bilayer
A phospholipid bilayer forms because phospholipids are amphipathic: their water-loving heads face the watery outside and inside of the cell while their water-fearing tails hide from water by facing each other. No protein or glue holds the two layers together; the same hydrophobic effect that built the bilayer keeps it sealed.
Drop phospholipids into water and they do not need instructions. Each phospholipid has a phosphate head that loves water and two fatty acid tails that hate it. Water molecules constantly form hydrogen bonds with each other, and anything hydrophobic that sits among them breaks up that bonding network, which costs energy. So the tails of nearby phospholipids turn away from water and cluster together, while the heads stay facing outward into the water. The result is two sheets of phospholipids, tails pointing inward at each other, heads pointing outward on both faces. That double layer, the bilayer, is simply the lowest energy arrangement available, so it forms on its own.
This same logic explains why a small tear heals itself and why nothing needs to staple the two leaflets together. A tear exposes tails to water along its edge, which is unfavorable, so the surrounding phospholipids shift and rotate to close the gap and hide those tails again. The two leaflets stay stacked for the same reason: pulling them apart would expose tails to water, so the hydrophobic effect keeps reasserting itself with no separate glue required.
Tail structure changes how tightly the bilayer packs. Saturated tails are straight and line up closely, making a stiffer, more ordered membrane. Double bonds introduce kinks that prevent tight packing, making the bilayer more fluid, looser, and easier for molecules to slide past each other. This fluidity sets up the next idea in the unit, the fluid mosaic model, where proteins and cholesterol move within this flexible, self-sealing sheet rather than sitting fixed in place.
Key terms
- Amphipathic
- Having both a water-loving (hydrophilic) part and a water-fearing (hydrophobic) part in the same molecule.
- Hydrophobic effect
- The tendency of nonpolar substances to cluster together in water to minimize disruption of water's hydrogen bonding.
- Leaflet
- One of the two single-molecule-thick layers of phospholipids that together make up the bilayer.
- Fatty acid tail
- The hydrophobic hydrocarbon chain of a phospholipid, which can be saturated (straight) or unsaturated (kinked).
- Fluidity
- How easily phospholipids and proteins move within the membrane, increased by kinks from unsaturated tails.
Watch it explained
- How it worksHow do the water loving heads and water hating tails end up facing the way they do?
- How it worksWalk me through how a small tear in a bilayer seals itself back up on its own.
- Going furtherWhat happens to the packing of a bilayer when its tails pick up extra double bonds?
- Common mistakeWhy doesn't a bilayer need anything holding its two sheets together?
Check yourself
End of topic.