Microbiology Demystified

The heads always face a watery fluid such as the extracellular fluid on the

outside of the cell and the intracellular fluid inside the cell. Tails align back to

back preventing the watery fluid from crossing the cytoplasmic membrane.

The Fluid Mosaic Model

In 1972, S. J. Singer and G. L. Nicolson developed the Fluid Mosaic Model,

which describes the structure of the cytoplasmic membrane. They called this a

mosaic because proteins within the cytoplasmic membrane are arranged like

tiles in a mosaic artwork. The term fluid is used to imply that membrane proteins

and lipids flow freely within the cytoplasmic membrane. There are two kinds of

proteins within the cytoplasmic membrane. These are:

• Integral proteins. An integral protein extends into the lipid bilayer.
  Integral proteins are typically glycoproteins that act like a molecular sig-
  nature that cells use to recognize each other. Glycoproteins have a carbo-
  hydrate group attached to them. Two examples are:
  
  
  • Transmembrane protein. A transport protein that regulates the move-
    ment of molecules through the cytoplasmic membrane.
  
  
  • Channel protein. A channel protein forms pores or channels in the
    cytoplasmic membrane that permit the flow of molecules through the
    cytoplasmic membrane.
  
  
  
  

(^74) CHAPTER 4 Prokaryotic and Eukaryotic Cells
Integral
protein
Glycolipid Oligosaccharide
Hydrophobic
α helix
Integral
protein
Hopanoid
Phospholipid
Peripheral
protein
Fig. 4-5. The cytoplasmic membrane enables some substances to pass into and out
of the cell.