Phospholipids form a bilayer because they are amphipathic: each molecule has a hydrophilic phosphate head that interacts with water and hydrophobic fatty-acid tails that avoid contact with water. In an aqueous environment, they therefore arrange spontaneously into two layers with the heads facing the water and the tails enclosed inside.
How the bilayer forms
- The phosphate-containing heads are polar and can interact with polar water molecules through electrostatic attractions and hydrogen bonding.
- The non-polar hydrocarbon tails cannot form hydrogen bonds with water. Water molecules become more ordered around exposed tails, which is energetically unfavourable.
- The phospholipids aggregate so that the tails are brought together and shielded from water. This is called the hydrophobic effect.
- Two layers form with their tails pointing inward toward each other. The hydrophilic heads face the aqueous cytoplasm on one side and the aqueous extracellular fluid on the other.
- A continuous bilayer has no exposed edges, so it may close to form a sealed compartment, such as a cell membrane or vesicle.
The bilayer is stable but not rigid. Individual phospholipids can move laterally, contributing to the fluidity described by the fluid mosaic model. Its hydrophobic interior also has low permeability to ions, large molecules and most polar molecules, allowing the membrane to act as a selective barrier.
A common misconception is that the tails are strongly repelled by water. More precisely, non-polar tails cannot form favourable interactions with water, so the arrangement that minimizes their exposure is more stable. Another misconception is that phospholipids form a single layer; a bilayer is required because cells have water on both sides of the membrane.
Exam technique
This is SL and HL content in B2.1 Membranes and membrane transport. For an explain question, link amphipathic structure to orientation: hydrophilic heads face water, hydrophobic tails face inward, producing a stable bilayer with a hydrophobic barrier.
