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TABLE 4.2 The 2005 'Comprehensive Classification System' of Fahy
et al. [5] . This System Divides all Lipids into 8 Major
Categories and Further Subdivides the Categories,
Assigning a 12-Digit Identifier to Each Lipid.
Major categories
FA. Fatty Acids
GL. Glycerolipids
GP. Glycerophospholipids
SP. Sphingolipids
ST. Sterol Lipids
PR. Prenol Lipids
SL. Saccharolipids
PK. Polyketides
The newer and more complex system ( Tabl e 4 . 2 , from Fahy, E. et al., 2005 [5] )isbased
around 'lipidomics' a loosely defined term encompassing several diverse fields of lipid
study. Lipidomics is a large-scale study of non-water-soluble molecules (lipids). The
term was first introduced in 2001 as an analog to the well-established fields of proteo-
mics (based on proteins) and genomics (based on nucleic acids). The goal of lipidomics
is to relate lipid composition to lipid physical properties and biological activities [6] .
While the lipidomics-based classification system is comprehensive and compatible
with informatics, from a membrane structure perspective it is basically similar to the
simpler system of Hauser and Poupart and is unnecessarily complex for the membrane
studies outlined in this topic. Only a small fraction of the lipids classified by Fahy et al.
have a significant role in membranes. Here we focus only on the lipids that significantly
affect membrane structure and function. This topic will not generally consider non-
membrane lipids or many other important aspects of lipids such as metabolism and
nutrition.
The discussion of membrane lipids will begin with a lengthy description of fatty acids and
then will be followed in Chapter 5 by the most important membrane structural lipids,
complex fatty acid-containing glycerolipids (GL), glycerophospholipids (GP), sphingolipids
(SP), and sterols (ST).
D. FATTY ACIDS
Fatty acids are the major building block of complex membrane lipids and so their proper-
ties are fundamental to understanding membrane structure and function. A fatty acid is
a mono-carboxylic acid often with a long un-branched aliphatic (hydrocarbon) tail which
may be either saturated or unsaturated [7] .
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