Cell Membrane

• Phospholipid bilayer
  • The membrane is composed of a pairing of phospholipids, which if you remember correctly, are made of a hydrophilic head and a hydrophobic tail. A model is posed in 1.a.
  • Halcyon&on did a great job of describing this under fluid mosaic model
• Cholesterol inside cell wall
• Proteins extrude
  • on cytoplasm side, additional protein molecules are bound to proteins protruding from bilayer
  • On outside, short carbohydrate chains are linked to protruding proteins, in combination with the cell wall they play a role in cell interactions and transfers
• Fluid-mosaic model says structure of cell membrane can move freely
• Two basic embedded protein configurations have been found so far:
  • relatively simple rodlike structure consisting of an alpha helix embedded in the hydrophobic interior of the membrane, with less regular hydrophilic portions extending on either side. (See ill. 1.b)
  • The other is found in large, globular molecules with complex tertiary or quarternary that would result from repeated "passes" through the membrane.
• Cell membrane of bacterial cells is much the same, except that it doesn't contain cholesterol
• Eukaryotic membranes have similar membranes but different types of lipids.
• Most membranes are 60% protein and 40% lipid, proteins have many different functions

1.a
  _  _  _  _  _  _
 |_||_||_||_||_||_|  -Hydrophilic Head
  I  I  I  I  I  I
  |  |  |  |  |  |   -Hydrophobic Tail

  |  |  |  |  |  |
  I  I  I  I  I  I
 |_||_||_||_||_||_|  -Hydrophilic Head

1.b

  _  _  _       _  _  _
 |_||_||_| \   |_||_||_|  -Hydrophilic Head
  I  I  I   \   I  I  I
  |  |  |    |  |  |  |   -Hydrophobic Tail
            /
  |  |  |  /    |  |  |
  I  I  I  |    I  I  I
 |_||_||_|  \  |_||_||_|  -Hydrophilic Head

            ^Alpha Helix