While the plant cell has a rigid cell wall, an animal cell membrane is a flexible lipid bilayer. The lipid molecules (mostly phospholipids) that make up the membrane have a polar, hydrophilic head and two hydrophobic hydrocarbon tails. When the lipids are immersed in an aqueous solution the lipids spontaneously bury the tails together and leave the hydrophilic heads exposed. Thus this is a handy membrane to use, because it can automatically fix itself when torn.
There are three different major classes of lipid molecules - phospholipids, cholesterol, and glycolipids. Different membranes have different ratios of the three lipids. What makes the membrane truly special is the presence of different proteins on the surface that are used for various functions such as cell surface receptors, enzymes, surface antigens, and transporters. Many of the membrane-associated proteins have hydrophilic and hydrophobic regions. The hydrophilic regions are used to help anchor the protein inside of the cell membrane. Some proteins extend across the lipid bilayer, others cross the bilayer several times.
