CHEMICAL COMPOSITION OF THE PLASMA MEMBRANE

Cell membrane lipids

Lipids are the major components of the cell membrane and of membranes in general (accounting for 30 to 50% of their mass). The two other main components of plasma membranes are sugars and proteins.
For each organelle membrane, the proportion of each type of lipid is different, and this in turn results in different fonctional abilities for the organelle in question.
       
Among plasma membrane lipids:
     55% are phospholipids
     25% are cholesterol
     20% are glycolipids

Phospholipids

Each phospholipidis made up of two regions:

- a hydrophobic region (nonpolar) made up of two fatty acid chains
- a hydrophilic region (polar) - the rest of the molecule

Phospholipids have a very particular organization around water.
Depending on the relative proportion of water to lipid around them, phospholpids will form different structures, of which lipid bilayers and micelles are the most representative.

Plasma membranes are lipid bilayers where the two hydrophilic regions are outside and the two hydrophobic regions are inside.
Phospholipids are set in a parallel fashion, creating two layers with opposite orientations.
This structural organization creates a hydrophobic barrier between two hydrated compartments: in the case of the cell membrane, between the extracellular compartment and the cytosol.

They are subdivided in the following families:

1. Phosphoglycerides (major component of cell membranes) includes:

- phosphatidylcholine
- phosphatidylethanolamine
- phosphatidylserine
- phosphatidylinositol bisphosphate (PIP2)
- phosphatidic acid (minor component)

Phosphoglycerides are synthesized from phosphatidic acid, which results from the esterification of glycerols's three alcohol residues by two fatty acids (in C1 and C2 position) and by phosphoric acid (in C3).