CHAPTER 7 Membrane Structure and Function Overview: Life at the Edge The plasma membrane is the boundary that separates the living cell from its surroundings The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins Phospholipids are the most abundant lipid in the plasma membrane Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions The fluid mosaic model states that a membrane is a fluid structure with a ?mosaic? of various proteins embedded in it Membrane Models: Scientific Inquiry Membranes have been chemically analyzed and found to be made of proteins and lipids