Cell Membrane Structure & Permeability Plasma Membrane – The plasma membrane that surrounds the cell is a fluid mosaic of phospholipids and proteins that separates the living cell from its nonliving surroundings. The membrane is selectively permeable consisting of a amphipathic (both hydrophilic and hydrophobic) bilayer of phosphate molecules. The layers are held together by hydrophobic interactions of the fatty acid tails. ~8 nm

Membrane Components Both the phospholipids and the proteins present in the plasma membrane are amphipathic – Having both a hydrophilic and hydrophobic region – All parts float freely. Often referred to as a “SEA” of phospholipids Components move laterally but do change orientation (exo- to endo-) – Movement is constant (~2nm/s for phospholipids) – Movement can be directional for components such as proteins

Cholesterol & Unsaturated Fatty Acids Cholesterol – make the membrane more rigid at higher temperatures – lowers the temperature at which the membrane solidifies Unsaturated fatty acids – Make the membrane more fluid (resists tight making)

Membrane Proteins Membrane proteins – float freely with their hydrophilic regions in the center of the molecules and their hydrophilic regions extending outwards – some directed or held rigid by internal cytoskeleton fibers – 2 classes Integral Proteins – has imbedded regions (can be transmembrane or partially imbedded) Peripheral Proteins – loosely held only at the surface of the membrane Integrins – Attach to the ECM & to the cytoskeleton to give greater strength & structure

Protein Types Protein TypeFunction: TransportHydrophilic channels and gates EnzymeActive sites are exposed to speed up metabolic pathways SignalExtracellular receptor sites cause a conformation change sending messages to the internal environment Cell-Cell RecognitionHave glycoproteins that act as identification tags Adhesion (joining)Act as gap and tight junctions Attachment to the ECM Maintains cell shape and stabilizes location