The Cell Membrane


The cell membrane consists of three classes of amphipathic lipids:

  • phospholipids
  • glycolipids
  • sterols

The amount of each depends upon the type of cell, but in the majority of cases phospholipids are the most abundant.


The fatty chains in phospholipids and glycolipids usually contain an even number of carbon atoms, typically between 16 and 20. The 16- and 18-carbon fatty acids are the most common. Fatty acids may be saturated or unsaturated, with the configuration of the double bonds nearly always “cis”. The length and the degree of unsaturation of fatty acid chains have a profound effect on membrane fluidity as unsaturated lipids create a kink, preventing the fatty acids from packing together as tightly, thus decreasing the melting temperature (increasing the fluidity) of the membrane.
The membrane phospholipids incorporate fatty acids of varying length and saturation. Lipids with shorter chains and ones with more double bonds are less stiff and less viscous.

Incorporation of particular lipids, such as cholesterol and sphingomyelin, into synthetic lipid membranes is known to stiffen a membrane. Such membranes can be described as “a glass state, i.e., rigid but without crystalline order”.

  • Types of phospholipids
    • Phosphatidic acid (phosphatidate) (PA)
    • Phosphatidylethanolamine (cephalin) (PE)
    • Phosphatidylcholine (lecithin) (PC)
    • Phosphatidylserine (PS)
    • Phosphoinositides:
      • Phosphatidylinositol (PI)
      • Phosphatidylinositol phosphate (PIP)
      • Phosphatidylinositol bisphosphate (PIP2) and
      • Phosphatidylinositol triphosphate (PIP3).

Phospholipids are a major component of cell membranes. They form a lipid bilayer in which their hydrophillic (attracted to water) head areas spontaneously arrange to face the aqueous cytosol and the extracellular fluid, while their hydrophobic (repelled by water) tail areas face away from the cytosol and extracellular fluid. The lipid bilayer is semi-permeable, allowing only certain molecules to diffuse across the membrane.

Cholesterol is another lipid component of cell membranes. It helps to stiffen cell membranes and is not found in the membranes of plant cells.

Glycolipids are located on cell membrane surfaces and have a carbohydrate sugar chain attached to them. They help the cell to recognize other cells of the body.

There are 10 main types of lipids in cell membranes. Each type of cell or organelle will have a differing percentage of each lipid, protein, and carbohydrate. The main types of lipids are:

  • Cholesterol
  • Glycolipids
  • Phosphatidylcholine
  • Sphingomyelin
  • Phosphatidylethnolamine
  • Phosphatydilinositol
  • Phosphatidylserine
  • Phosphatidylglycerol
  • Diphosphatidylglycerol (Cardiolipin)
  • Phosphatidic acid

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