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The Cell Membrane

• All  cells are separated from the external environment by a Plasma Membrane

•  Membrane is pliable and semipermiable only allowing certain materials in/out of cells.

• It is composed of many components.

  • Phospholipids

  • Cholsterol

  • Proteins

•  Phospholipids

  • Very similar to triglycerides however, one of the fatty acid chains has been replaced by a very polar Phosphate group

• This poses a problem for the molecule since the molecule is both polar and non-polar simultaneously.

• It is therefore very unstable in water, polar end is attracted to it, non polar tails are repelled by it.

• This forces the phospholipids to take an arrangement such that their tails face each other.  This accomplishes 2 things..

  • The polar heads can be adjacent to polar water molecules

  • The non-polar tails can be adjacent to each other.

• As the layer enlarges it forms a bubble shape formed of a lipid bilayer which is the foundation of the plasma membrane

• The membrane requires a few more components to make it complete:

• Cholesterol

  • Functions to hold the membrane together like glue.  Adds strength to the membrane.

• Proteins (2 types)

  • Intrinsic Proteins

    • Extend into or through lipid bilayer

    • Often are channel proteins allowing large molecules in/out

  • Extrinsic Proteins

    • Exist only on surfaces of the bilayer

    • Often serve as receptor molecules

    • Glycoproteins serve as cellular markers (nametags)

• Entire model is referred to as the Fluid Mosaic model of the plasma membrane

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