Why do phospholipids spontaneously form bilayers

The hydrophilic head group consists of a phosphate-containing group attached to a glycerol molecule. The hydrophobic tails, each containing either a saturated or an unsaturated fatty acid, are long hydrocarbon chains. This characteristic is vital to the structure of a plasma membrane because, in water, phospholipids tend to become arranged with their hydrophobic tails facing each other and their hydrophilic heads facing out.

In this way, they form a lipid bilayer—a barrier composed of a double layer of phospholipids that separates the water and other materials on one side of the barrier from the water and other materials on the other side. In fact, phospholipids heated in an aqueous solution tend to spontaneously form small spheres or droplets called micelles or liposomes , with their hydrophilic heads forming the exterior and their hydrophobic tails on the inside Figure 3.

Figure 3. In an aqueous solution, phospholipids tend to arrange themselves with their polar heads facing outward and their hydrophobic tails facing inward. Improve this page Learn More. Skip to main content. Module 5: Cell Membranes. Search for:. These have a polar head group and two hydrophobic hydrocarbon tails. The tails are usually fatty acids and they can differ in length. Hydrophilic molecules dissolve readily in water because they contain charged groups or uncharged polar groups, that can form either favourable electrostatic interactions or hydrogen bonds with water molecules Hydrophobic molecules are insoluble in water because all or most of their atoms are uncharged and non polar.

They cannot form energetically favourable interactions with water molecules. If dispersed in water , they force the adjacent water molecules to reorganise into ice like cages that surround the hydrophobic molecules.

For the above reason, lipid molecules spontaneously aggregate to bury their hydrophobic tails in the interior and expose their hydrophobic heads to water.

Being cylindrical phospholipid molecules spontaneously form bilayer in aqueous environments. In this energetically most favourable arrangement, the hydrophilic heads , face the water at each surface of the bilayer and the hydrophobic tails are shielded from the water in the interior. Structure of Phospholipids :. Arrangement in Membranes :. Properties of the Phospholipid Bilayer :. Brent Cornell. Cell Introduction 2. Cell Structure 3.

Membrane Structure 4. Membrane Transport 5. Origin of Cells 6. Cell Division 2: Molecular Biology 1. Metabolic Molecules 2. Water 3. Protein 5. Enzymes 6. Cell Respiration 9. Photosynthesis 3: Genetics 1. Genes 2.