Membranes
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Membranes. Membrane Functions. A Bit of History. Charles Overton—1890’s Lipid nature of membranes Lipids readily crossed the membrane Polar/charged molecules did not Irving Langmuir—1900’s Phospholipid monolayer. E. Gorter and F. Grendel—1925 RBCs Lipid bilayer

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Membranes

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Membranes


Membrane Functions


A Bit of History

  • Charles Overton—1890’s

    • Lipid nature of membranes

      • Lipids readily crossed the membrane

      • Polar/charged molecules did not

  • Irving Langmuir—1900’s

    • Phospholipid monolayer


  • E. Gorter and F. Grendel—1925

    • RBCs

    • Lipid bilayer

      Lipid bilayer was not enough to describe differential permeability

  • Hugh Davson and James Danielli—1935

    • Protein-lipid "sandwich"


  • Electron microscope—1950s

    • Organelles have membranes too

    • Membranes have trilaminar appearance

  • J. David Robertson—1960

    • "unit membrane" theory

    • Basically extension of "sandwich" to all membranes


Problems with this theory

  • Artificial membranes (no protein) look same under EM

  • Doesn't explain different ratios of lipid/protein in different membranes

  • Most membrane proteins are globular, not in sheets


Fluid Mosaic Model

  • S. Jonathan Singer and Garth Nicolson—1972

    • Lipid bilayer (fluid)

    • Mosaic of proteins floating in the lipids

  • Richard Henderson and Nigel Unwin—1975

    • Structure of membrane-bound protein

    • Went through the membrane (not just on surface)


The Lipids of Membranes

  • Phospholipids

  • Glycolipids

  • Sterols


Different membranes, different lipids

Differ between layers of the bilayer--assymetric


Lipid Movement


Evidence for fluidity


Photobleaching


Laser tweezers


Maintaining Fluidity

  • Fatty acid type/length

  • Sterols


Sterols

Moderate extremes


Membrane Proteins


Three types


Functions of membrane proteins


Properties of membrane proteins

  • Asymmetric

    • Evidence--structures

  • Diffuse laterally, rotate

    • NO flip-flop

    • Can be anchored to other things

      • Restricted mobility

  • Often glycosylated


Evidence for Protein Mobility


Glycoproteins


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