Cell membranes & transport

• Why can't most molecules not freely diffuse across plasma membranes?


  Not soluble in lipids

  Too large

  Polar

  Of the same charge as the charge on the protein (repelled).
• Why is are plasma membranes referred to as a fluid-mosaic model?


  Fluid- because the individual phospholipids molecules can move relative to one another, it's flexible and constantly changing shape.

  Mosaic- because the proteins embedded in the phospholipid bilayer vary in shape, size and pattern in the same way as the tiles of a mosaic.