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