1.8. Plasma Membrane

The plasma membrane is a thin lipid layer that encases the cellular contents of all living cells, both Eukaryotic and Prokaryotic. It separates the interior of each cell from the extracellular environment and acts as a selective semi-permeable barrier. Chemically, the plasma membrane consists of a layer of lipids with many proteins embedded in it that form passages for molecules to enter and exit the cell. This structural model for plasma membrane is called the Fluid Mosaic Model – with the lipids forming the fluid in which a mosaic of proteins is floating around like icebergs. Some of these proteins allow for free diffusion of molecules, while others use energy to actively pump molecules across the plasma membrane. Some membrane proteins are specialized as receptors for specific external signals such as hormones, which help a cell respond to a changing environment.