Membranes form a major structural element in cells. Most universal role of membranes is to act as a selective barrier to the passage of molecules, allowing some molecules to cross while excluding others. The plasma membrane regulates the passage of substances into and out of the cell, whereas the membranes surrounding cell organelles allow the selective movement of substances between the organelles and the cytosol. One of the advantages of molecules across membranes is confining the products of chemical reactions to specific organelles. These are amphipathic molecules: one end has a charged or polar region, and the remainder of. All membranes consist of a double layer of lipid molecules containing embedded proteins. The major membrane lipids are phospholipids the molecule, which consists of two long fatty acid chains, is non-polar. The phospholipids in cell membranes are organized into a bimolecular layer with the nonpolar fatty acid chains in the middle.