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