why do the phospholipids form a bilayer

Because the heads are polar molecules they (form hydrogen) bonds with water molecules found outside the cell and in the cytoplasm and they arrange on the outer layers (the outside of each side) of the cell membrane. In contrast the tails are nonpolar and (cannot form hydrogen bonds) are found in between the polar heads in the middle of the cell membrane. The tails bond to each other. (Phospholipids form a double layer because heads, water loving, are attracted to the water in the cytoplasm inside the cell and the watery fluids outside the cell.

The tails are water repelling and they are between the or in the middle of the heads. )
Phospholipids spontaneously form lipid bilayers, which generate biological membranes. Diglycerides contain two fatty acids linked to glycerol. Many diglycerides contain a phosphate group attached to the third -OH group of glycerol, producing a phospholipid. Phospholipids often contain additional charged groups attached to the phosphate.

In phospholipids, the two fatty acids are hydrophobic, or insoluble in water. But the phosphate group is hydrophilic, or soluble in water. When phospholipids are mixed with water, they spontaneously rearrange themselves to form the lowest free-energy configuration. This means that the hydrophobic regions find ways to remove themselves from water, while the hydrophilic regions interact with water. The resulting structure is called a lipid bilayer.

All biological membranes (except for those found in certain unusual bacteria, members of the Archaea) contain lipid bilayers, as well as proteins, which provide membranes with stability and specialized functions. The image above is based on original work by H. Heller, M. Schaefer, K. Schulten, "Molecular dynamics simulation of a bilayer of 200 lipids in the gel and in the liquid-crystal phases", J. Phys. Chem. 97:8343-60, 1993.

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