1. Chapter 8 Membrane Structure and Function

2. Davson-Danielli Model
Protein sandwiched on either side of a phospholipid bi-layer
8nm
Surrounds cell
Selectively permeable
Polar heads towards the protein layer forming hydrophillic
Nonpolar tails oriented in between heads forming hydrophobic zone

3. Membrane Models based on research
Membranes are made of lipids.
Phospholipids can form membranes
Membrane is actually phospholipids bilayer
There is protein in membranes.
Membranes are coated on both sides with proteins

4. 1950s electron microscopy allowed for the visibility of the plasma membrane and support the D&D Model
But………….

5. Problems with the D&D Model
Not all membranes are identical or symmetrical (they have different functions and are also bifacial with a distinct inside and outside face)
A membrane with an outside layer of proteins would be an unstable structure!
(membrane proteins NOT soluble in water; hydrophobic regions would be in an aqueous environment)

6. 1972 Singer and Nicholson’s Fluid Mosaic Model
Fluid = wet; capable of flowing
Mosaic = an assemblage of particles
Proteins are individually embedded in bilayer
Ampipathic
Hydrophillic portions are maximally exposed to water
Hydrophobic portions in the nonaqueous environment inside the bilayer

7. Membranes are Fluid
Phospholipids and some proteins move laterally within the membrane
Membranes held together by weak hydrophobic attractions
Cholesterol and unsaturated hydrocarbon tails affect membrane fluidity
Saturated = no double bonds
Unsaturated = double bonds (hinder close packing of hydrocarbons)

8. SATURATED = less fluid/closer packing
UNSATURATED = more fluid

9. Cholesterol modulates membrane fluidity
Less fluid at warmer temp
More fluid at cooler temp

10. Membrane Proteins Integral Proteins Embedded Hydrophobic regions
Unilateral – reaching only partway across the membrane
Transmembrane – with hydrophobic midsections between ends exposed on both sides of membranes

11. Peripheral Proteins – attached to the surfaces.
Functions: transport, enzymatic activity, signal transduction, intercellular joining, cell-cell recognition, and attachment to the cytoskeleton and extra cellular matrix.

12. Carbohydrates
Important in cell-to-cell recognition (sorting/rejection of foreign particles)
External portions of cells
Oligosaccharides (<15 monomers)
 some covalently bonded to lipids (glycolipids)
 most covalently bonded to proteins (glycoproteins)

13. DIFFUSION

14. OSMOSIS

15. OSMOSIS

16. OSMOSIS

17. Facilitated Diffusion

18. Active Transport

19. Endocysosis and Exocytosis