Membrane Structure Function Chapter 7 Membrane Structure fluid

Membrane Structure & Function Chapter 7

Membrane Structure • fluid mosaic model – a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids

Phospholipids • amphipathic – hydrophobic & hydrophilic regions • arrange themselves so that the hydrophobic tails face away from water • bilayer is held together by weak hydrophobic interactions

Membrane Fluidity • most of the lipids & some of the proteins drift laterally • lateral movement of phospholipids is rapid • lateral movement of proteins is slow • flip-flopping of phospholipids is rare • fluidity is enhanced by unsaturated hydrocarbon tails of phospholipids • cholesterol molecules reduce fluidity but help prevent solidification at low temperatures by preventing tight packing of phospholipids

Membrane Proteins • mosaic – collage of different proteins embedded in the fluid matrix of the lipid bilayer • integral proteins – penetrate the hydrophobic core of the lipid bilayer; many are transmembrane • peripheral proteins – bound to the surface of the membrane • proteins determine most of the membrane’s specific functions

Membrane Protein Functions

Glycoproteins & Glycolipids • carbohydrates attached to membrane proteins & lipids • function as “name-tags” to help distinguish cells (cell-cell recognition) • important in sorting cells into tissues & organs during embryonic development • basis for rejection of foreign cells

Membrane Synthesis • begins in ER • vesicles fuse with plasma membrane – the inside layer of the vesicle becomes the outside layer of the cell

Form Fits Function • the plasma membrane controls traffic into and out of the cell • exhibits selective permeability – allows some substances to cross more easily than other • the fluid mosaic model explains how the plasma membrane is able to do this