Lipid bilayer: A thin polar membrane made of two layers of lipid molecules; these layers are composed primarily of phospholipids, which provide barriers in cellular membranes to protect cells.
Hydrophilic:Refers to molecules or parts of molecules that are attracted to water and can dissolve in it, such as the phosphate head group of a phospholipid.
Hydrophobic:Describes molecules or portions of molecules that repel water and do not easily dissolve in it, like the fatty acid tails found in phospholipids.