Chapter 8 Membrane Structure and Function

• Membrane Structure

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

Membrane Models based on research

1. Membranes are made of lipids. 2. Phospholipids can form membranes

3. Membrane is actually phospholipids bilayer

4. There is protein in membranes. 5. Membranes are coated on both sides

with proteins

• 1950s electron microscopy allowed for the visibility of the plasma membrane and support the D&D Model

• But………….

Problems with the D&D Model

1. Not all membranes are identical or symmetrical (they have different functions and are also bifacial with a distinct inside and outside face)

2. 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)

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

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)

• SATURATED = less fluid/closer packing

UNSATURATED = more fluid

• Cholesterol modulates membrane fluidity– Less fluid at warmer temp– More fluid at cooler temp

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

• 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.

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)

DIFFUSION

OSMOSIS

OSMOSIS

OSMOSIS

Facilitated Diffusion

Active Transport

Endocysosis and Exocytosis