Chapter 7 Membrane Structure and Function Overview: Life at the Edge the plasma membrane is the boundary that separates the living cell from its nonliving surroundings selectively permeable: allows some substances to cross it more easily than others Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins a phospholipid is an amphipathic molecule, meaning it has both hydrophilic and hydrophobic regions head composed of phosphate group attached to one carbon of glycerol two fatty acid tails are hydrophobic in the fluid mosaic model, the membrane is a fluid structure with a ?mosaic? of various proteins embedded in or attached to a bilayer of phospholipids Membrane Models: Scientific Inquiry phospholipids are the most abundant lipid in the plasma membrane