Lipid bilayer

Amphipathic or amphiphilic molecules   hold   both hydrophobic (water-hating) regions and hydrophilic or waterloving.  The Membrane lipids are amphipathic molecules as they are made up of hydrophobic fatty acid chains and a hydrophilic polar headgroup. In the glycerophospholipids two hydrocarbon chains are hydrophobic while the phosphorylated headgroup and the glycerol backbone are hydrophilic. With  In the sphingolipids  the fatty  acid  chain  and  the hydrocarbon  chain of the sphingosine  are hydrophobic  while the phosphorylated  or sugar  headgroup  is hydrophilic.  In the case of cholesterol the whole molecule apart from the hydroxyl group on carbon-3 is hydrophobic in nature.

In the aqueous solution amphipathic molecules will orientate themselves in such like a way as to stop the hydrophobic region coming into contact with the water molecules.  In the case of those fatty acid salts that hold only one fatty acid chain  like  as sodium  palmitate,  a constituent  of soap  the molecules  form a spherical    micellar   structure   is diameter    usually   <  20   nm   in   that  the hydrophilic headgroups interact with the surrounding  water molecules and the hydrophobic  fatty acid chains are hidden inside the micelle. Because the two fatty acid chains of phospholipids are too bulky to fit into the interior of a micelle the special structure for most phospholipids in aqueous solution is a 2D bimolecular sheet or lipid bilayer. Like lipid bilayers, in  that  the  phospholipid   molecules  are  orientated  with  their  hydrophobic chains in the interior  of the structure  and their hydrophilic  headgroups  on the surfaces, can be relatively huge structures of up to about 1 mm2  in area. In the two layers of lipids in the bilayer are defined to as the inner, outer leafiets. In   biological   membranes   the   individual   lipid   species   are   asymmetrically

                            Figure: Structure of (a) a micelle and (b) a lipid bilayer.

distributed among  the two leafiets.  For instance, in the plasma membrane of phosphatidylcholine, erythrocytes, and sphingomyelin are preferentially situated in the outer leafiet, although phosphatidylserine and phosphatidylethanolamine are commonly in the inner leafiet.The Lipid bilayers will spontaneously   self-assemble   in aqueous solution.  The main  driving  force  following  this  is the  hydrophobic  effect  the  hydrophobic fatty  acid  chains  prevent  coming  into  contact  with  the  water  molecules.  Once it is formed, the bilayer structure is maintained through multiple noncovalent interactions involving hydrophobic interactions and van der Waals forces among the hydrocarbon chains hydrogen bonding and charge interactions among the polar headgroups, and hydrogen bonding among the headgroups and the surrounding water molecules.