Chapter 11 Membrane Structure and Membrane Transport

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Chapter 11 Membrane Structure and Membrane
Transport

• Alexs TA Section
• Bioc 441
• 2 Feb 2006

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Basic Membrane Structure

• Lipids with polar headgroups and long hydrophobic
  tails
• Will spontaneously form micelles and bilayers.
  Bilayers will turn into liposomes / vacuoles.
• Significant protein mass Up to 75!
• Lipid layer 3 nm thick
• with Proteins 5-8 nm thick

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Membrane Structure
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Membrane Composition

• Different lipid composition in each cell type
• Inside and outside bilayers not homogenous in
  lipid composition(maintained by flippases)
• Different protein fractions
• Different protein compositions

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Membrane Proteins

• Protein types
• Integral protein in membrane
• Lipid Linked covalent bond with a lipid in the
  membrane
• Peripheral electrostatic interaction with polar
  head groups or integral proteins
• 10-20 of genome is integral proteins, by
  hydropathy analysis
• Proteins are often glycosylated on the outside of
  the membrane. This is usually as a recognition
  sequence for signal transduction.

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Membrane Protein Spanning Structures
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Fluid Matrix Model

• 2 Dimensional Liquid
• Free lateral diffusion within the bilayer for
  both proteins and lipids (soft of)
• Bilayer itself highly flexible, think of amoebas
• Splits dividing cells
• Fuses vesicle transport
• Reduced freedom of movement (to 2D) greatly
  increases collision rate of membrane bound
  molecules

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FRAP - fluorescence recovery after photo-bleaching

• Measuring diffusion rates of the membrane for
  both lipids and proteins
• Make lipid bilayer with fluorescent headgroups
• Bleach them with a laser! (destroys fluorescent
  probes)
• Measure rate of recovery, I.e. diffusion rate
  back into bleached area.
• Rates of over 1um/sec lipids can circumnavigate
  an ecoli in 1 second.

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Not quite free diffusion
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Fluidity Maintenance

• Sterols -gt less fluid
• Unsaturated lipids -gt more fluid
• Cells manipulate membrane composition to regulate
  fluidity, such as being grown in different
  temperatures.

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Transporters
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More Transporters