Chapter 8 Membrane Structure and Function

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Chapter 8 Membrane Structure and Function

• Membrane Structure

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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

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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

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• 1950s electron microscopy allowed for the
  visibility of the plasma membrane and support the
  DD Model
• But.

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Problems with the DD 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)

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1972 Singer and Nicholsons 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

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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)

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SATURATED less fluid/closer packing
UNSATURATED more fluid
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• Cholesterol modulates membrane fluidity
• Less fluid at warmer temp
• More fluid at cooler temp

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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

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• 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.

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Carbohydrates

• Important in cell-to-cell recognition
  (sorting/rejection of foreign particles)
• External portions of cells
• Oligosaccharides (lt15 monomers)
•  some covalently bonded to lipids (glycolipids)
•  most covalently bonded to proteins
  (glycoproteins)

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DIFFUSION
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OSMOSIS
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OSMOSIS
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OSMOSIS
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Facilitated Diffusion
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Active Transport
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Endocysosis and Exocytosis