'Glycerophospholipids' or 'phosphoglycerides' are
glycerol-based
phospholipids. They are the main component of
biological membranes.
Structures
The term glycerophospholipid signifies any derivative of
sn-glycero-3-phosphoric acid that contains at least one O-
acyl, or O-
alkyl or O-alk-1'-
enyl residue attached to the glycerol
moiety and a
polar head made of a
nitrogenous base, a glycerol, or an
inositol unit.
It contains a glycerol core with
fatty acids. They can be the same or different subunits of fatty acids.
★ Carbon 1 (tail, apolar) contains a fatty acid, typically ''saturated''
★ Carbon 2 (tail, apolar) contains a fatty acid, typically ''unsaturated'' and in the
cis conformation, thus appearing "bent"
★ Carbon 3 (head, polar) contains a
phosphate group or an
alcohol attached to a phosphate group
Nomenclature and stereochemistry
Glycerophospholipids generally use a "sn" notation which stands for
stereochemical numbering. When the letters "sn" appear in the nomenclature, by convention the
hydroxyl group of the second carbon of glycerol (sn-2) is on the left on a
Fischer projection. The numbering follows the one of Fischer's projections, being sn-1 the carbon at the top and sn-3 the one at the bottom.
The advantage of this particular notation is that the spatial conformation (R or L) of the glycero-molecule is determined intuitively by the residues on the positions sn-1 and sn-3.
For example sn-glycero-3-
phosphoric acid and sn-glycero-1-
phosphoric acid are
enantiomers.
Examples of glycerophospholipids
Lecithin and
cephalin are more common than the others in most human membranes, but
cardiolipin is quite common in the inner membranes of
mitochondria.
Uses
Use in membranes
One of a glycerophospholipid's functions is to serve as a structural component of
cell membranes. The cell membrane seen under the
electron microscope consists of two identifiable layers, or "leaflets", each of which is made up of an ordered row of glycerophospholipid molecules. The composition of each layer can vary widely depending on the type of cell.
★ For example, in human
erythrocytes the ''
cytosolic'' side (the side facing the
cytosol) of the
plasma membrane consists mainly of
phosphatidylethanolamine,
phosphatidylserine, and phosphatidylinositol.
★ By contrast, the ''exoplasmic'' side (the side on the exterior of the cell) consists mainly of
phosphatidylcholine and
sphingomyelin, a type of
sphingolipid.
Each glycerophospholipid molecule consists of a small
polar head group and two long
hydrophobic chains. In the cell membrane, the two layers of phospholipids are arranged as as follows:
★ the
hydrophobic tails point to each other and form a fatty, hydrophobic center
★ the
ionic head groups are placed at the inner and outer surfaces of the cell membrane
This is a stable structure because the ionic hydrophilic head groups interact with the aqueous media inside and outside the cell, whereas the hydrophobic tails maximize hydrophobic interactions with each other and are kept away from the
aqueous environments. The overall result of this structure is to construct a fatty barrier between the cell's interior and its surroundings.
Use in emulsification
Glycerophospholipids can also act as an
emulsifying agent to promote
dispersal of one substance into another. This is sometimes used in
candy making.
External links
★
Diagram at uca.edu
★
Diagram at ualr.edu
See also
★
Phospholipid
★
Biological membrane
External links
★
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