The three most common phospholipids are distinguished by their
head groups. (What? Phosphatidyl choline, inositol and serine?)
Is this necessary detail?
Chapter 2: The Lipid Nature of Biological Membranes.
Lipids make up the bulk of biological membranes, and it's reasonable to suppose their large numbers greatly influence membrane structure and function. Indeed, it's not too far-fetched to say membrane lipids and their interactions "create" biological membranes. How do such small molecules exert such a large role? To answer this question we need to examine the structure of lipids themselves.
Most cellular lipids are derived chemically from
the three-carbon alcohol, glycerol, through covalent linkages
with up to three fatty acids. Specifically, a fatty acid may form
an ester with each of glycerol's three alcohol residues,
and the resulting lipids are called glycerides All membrane
glycerides have fatty acid (hydrocarbon or acyl) residues
attached to two adjacent glycerol carbons and a polar, often charged,
residue linked covalently with the third carbon, and are called
diacyl glycerides or simply diglycerides. Diglycerides
containing phosphate as part of the polar residue are called,
not surprisingly, phospholipid. Another major plasma membrane
lipid is cholesterol. Different structural representations
of these two membrane lipids are presented on the facing page.
Note in particular, that both types of molecules consist of regions
that are hydrophilic, or water-loving, and hydrophobic, or water-hating.
This schizophrenic nature of membrane lipids seems
to be no accident! Scroll through the Table of Membrane Lipids
on the next page and note all of the more common forms share this
important feature: all membrane lipids have a hydrophobic region
as well as a hydrophilic one. The long hydrocarbon chains
of the fatty acids (or the fatty-acid like residues) projecting
to the left of each lipid will not spontaneously interact with
dipolar water molecules (or readily dissolve in aqueous solutions).
Conversely, the different residues projecting to the right of
each lipid are all polar and will readily interact with water.
More technically, membrane lipids are called amphipathic
molecules, because they possess distinct regions with such different
affinities for oil and for water. Even the very hydrophobic and
insoluble cholesterol is slightly amphipathic, by virtue of its
single alcohol residue.
The amphipathic nature of membrane lipids contrasts
strikingly with the neutral <triglycerides and cholesterol
esters> [link to 2aside1] which are more abundant in our
bodies than their amphipathic relatives.
How do you think these amphipathic molecules would behave if their concentrations were increased in an aqueous environment? in an organic solvent? at the interface between an aqueous and an organic solvent? Go on to the <next chapter> [hypertext linkage] to consider the formation of lipid structures called micelles.