Question

How does a lipid bilayer form a barrier to molecules?

Answer 1

Lipid bilayers are 5nm thick structures primarily composed of phospholipids. The molecules are amphiphilic containing a hydrophilic phosphate head and a pair of hydrophobic hydrocarbon tails. The cage around these is made by the covalently bonded phosphate heads.

Complete answer:
This characteristic is significant to the structure of a cell wall because, in water, phospholipids tend to become arranged with their hydrophobic tails facing one another and their hydrophilic heads facing out. During 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 opposite side.
In fact, phospholipids heated in absolution tend to spontaneously form small spheres or droplets (called micelles or liposomes), with their hydrophilic heads forming the outside and their hydrophobic tails on the within.
The lipid bilayer has been firmly established as the universal basis for cell-membrane structure. It's easily seen by microscopy, although specialized techniques, like X-ray diffraction and freeze-fracture microscopy, are needed to reveal the small print of its organization. The bilayer structure is due to the special properties of the lipid molecules, which cause them to assemble spontaneously into bilayers even under simple artificial conditions.

Note:
The foremost abundant membrane lipids are the phospholipids. These have a polar head group and two hydrophobic hydrocarbon tails. The tails are usually fatty acids, and that they can differ long (they normally contain between 14 and 24 carbon atoms).