What is a membrane? A water-induced structure composed of a lipid bilayer and integral and peripheral proteins A water-induced structure composed of a lipid bilayer and integral and peripheral proteins What holds a membrane together? Hydrophobic interactions between the non-polar hydrocarbon chains of the lipids and proteins Hydrophobic interactions between the non-polar hydrocarbon chains of the lipids and proteins What proteins are in a cell membrane? Two types: integral and peripheral Are proteins capable of movement Yes, the protein float on a sea of lipid much the same as apples bobbing on water Yes, the protein float on a sea of lipid much the same as apples bobbing on water

Lipid/Water Dynamics Rule: “fats and water don’t mix, i.e., each prefers its own company” Rule: “put the two together and the two will segregate” ERGO: glycerophospolipids, sphingolipids, and sterols are virtually insoluble in water ERGO: Amphipathic compounds form microscopic lipid aggregates in a phase separate from their aqueous surroundings

Two Physical Forms for orienting amphipathic lipids Micelle (Fatty acids) Bilayer (Acylglycerolipids, sphingolipids)

Asymmetric Distribution in Human Erythrocytes Phospholipid Inner Outer Sphingomyelin 721 Phosphtidylcholine 9 23 Phosphatidylethanolamine 25 6 Phosphatidylserine 9 0 Total 50 50

Unit Membrane Hypothesis All membranes follow the same basic dimensions A membrane is composed of a lipid bilayer Membrane lipids are oriented to allow polar portions to be directed away from the bilayer Membrane Proteins can be peripheral (surface) or integral (through)

All Membranes appear Trilaminar (3 layer) When stained with OsO 4 and examined in the electron microscope, all membranes show two dense layers on the outer and inner surfaces separated by a less dense central region

Peripheral proteins Integral Proteins Bilayer Angstroms Unit Membrane hypothesis J.D. Robertson All membranes have the same general dimensions, the same general features.

Fluid-Mosaic Model Membrane is fluid Proteins float on a sea of lipid Membrane proteins diffuse laterally Membrane proteins do not cross the membrane Proteins are in constant motion Singer and Nicolson

Photobleaching Lazer Unbleached molecules enter Cell

FAST SLOW Flip-Flop (Flipases) Lateral Lipid Mobility