The phospholipid bilayer
Two-layered amphipathic sheet that self-assembles in water.
Every cell membrane is built around a phospholipid bilayer: two layers of phospholipid molecules arranged tail-to-tail. Each phospholipid molecule has two regions with opposite affinities for water:
- a hydrophilic head containing the phosphate group, which is polar and attracted to water;
- two hydrophobic fatty-acid tails, which are non-polar and repelled by water.
A molecule with both polar and non-polar regions is called amphipathic, and this is the property that drives bilayer formation.
When phospholipids are placed in water they spontaneously arrange themselves so that the hydrophilic heads face outwards into the aqueous environment on both sides of the bilayer (cytoplasm and extracellular fluid), while the hydrophobic tails face inwards, away from water. This is the lowest-energy configuration — it minimises the unfavourable interactions between water and the non-polar tails (the hydrophobic effect). No energy input is required.
The resulting bilayer is about 7-10 nm thick and behaves as a two-dimensional fluid: molecules can move sideways within their layer, and small breaks in the membrane reseal spontaneously. The bilayer is selectively permeable: small non-polar molecules (, , steroids) diffuse through easily; polar / charged molecules (ions, glucose) cannot cross and require transport proteins.
- Phospholipid = hydrophilic head + hydrophobic tails = amphipathic.
- Heads outwards (face water on BOTH sides); tails inwards (away from water).
- Spontaneous self-assembly driven by the hydrophobic effect.
- ~7-10 nm thick; selectively permeable; self-sealing.