15 Lipids
15.1 Chemistry and Metabolism of the Fatty Acids
15.1.1 Structure and Nomenclature of the Fatty Acids
15.1.2 Biosynthesis of Fatty Acids
15.1.3 Conversion of Fatty Acids
15.1.4 Oxidation of Fatty Acids
15.1.5 Ketone Body Formation and Degradation
15.2 Reserve Lipids and Lipid-Rich Secretions
15.2.1 Triacylglycerols (Triglycerides)
15.2.2 Energy Substrates in Muscles and Other Tissues
15.2.3 Wax Esters, Hydrocarbons and "Ether Glycerides"
15.2.4 Fat-Rich Secretions
15.3 Lipids of the Body Surface
15.3.1 Epidermal Lipids of Terrestrial Vertebrates
The term lipid is applied to a multitude of substances of differing chemical structure and biological function whose only common feature is that
they are insoluble in water but readily soluble in
organic solvents such as chloroform, benzene,
diethyl ether or hot ethanol. A large proportion
of the lipids of living organisms are bound to proteins and these are normally extracted using a
chloroform:methanol mixture; the alcohol component takes up water from the tissue, denatures
the proteins and releases the lipids, and the
chloroform solubilizes the lipids. The hydrophobic nature of the lipids is the basis for their
main biological functions as energy reserves, as a
critical part of the water-repellent surfaces of terrestrial organisms, and as a component of cellular
membranes. Some individual lipids or lipid-like
substances have special functions, e.g. as pheromones or hormones Quvenile hormone, steroid
hormones), as regulators of hormone action
(prostaglandins), as precursors of secondary messengers (phosphatidylinositol), or as recognition
signals on the cell surface (glycolipids) [76].
The water insolubility of the lipids presents the
organism with serious problems of lipid digestion
and absorption in the gut, of intra- and extracellular lipid transport, and of the mobilization of lipid
reserves. The lipases only express their hydrolytic
activity at the interphase between lipid and
water; thus, dietary fats are finely emulsified
15.3.2 Skin Fats of the Mammals
15.3.3 Lipids of the Uropygial Gland
15.3.4 Cuticular Lipids of the Insects
15.4 Membrane Lipids
15.4.1 Phospholipids
15.4.2 Biosynthesis of the Phospholipids
15.4.3 Phosphonolipids
15.4.4 Glycolipids
15.5 Lipids with Regulatory Functions
15.5.1 Prostaglandins and Other Eicosanoids
15.5.2 Insect Juvenile Hormones
References
prior to digestion, in the vertebrates by bile acids
and alcohols and in the invertebrates by emulsifiers with a rather different chemical structure
(p.646). The resulting hydrolysis products, ionized fatty acids (soaps), 2-monoacylglycerol and
lysophosphatides, amplify the emulsification. The
end products of fat digestion in the mammalian
gut are liquid-crystalline vesicles, which are
80-120 mm in diameter, and smaller micelles.
Just how these vesicles and micelles come into
contact with the microvilli of the gut mucosa and
how their components penetrate the membranes
of the gut cells is not yet completely known, even
for the vertebrates. In the mammals, a large part
of the absorbed fatty acids are incorporated back
into triacylglycerol in the gut cells, combined with
cholesterol, phospholipids and proteins into "chylomicrons", and released into the lymph [34,
142]. Lipid absorption in the insects appears to
follow a completely different scheme. The fatty
acids in the larvae of the butterfly Pieris brassicae
are incorporated into polar lipids before being
absorbed by the gut [266].
Lipid transport in vertebrate blood or lymph
and in arthropod haemolymph is carried out by
specific lipoproteins (as described in Chapter 5).
In vertebrate blood, triacyglycerol, phospholipids
and cholesterol are bound to plasma lipoproteins,
and free fatty acids are bound to serum albumin.
The lipophorins of insects carry diacylglycerols as
15.1 Chemistry and Metabolism of the Fatty Acids
15.1.1 Structure and Nomenclature of the Fatty Acids
15.1.2 Biosynthesis of Fatty Acids
15.1.3 Conversion of Fatty Acids
15.1.4 Oxidation of Fatty Acids
15.1.5 Ketone Body Formation and Degradation
15.2 Reserve Lipids and Lipid-Rich Secretions
15.2.1 Triacylglycerols (Triglycerides)
15.2.2 Energy Substrates in Muscles and Other Tissues
15.2.3 Wax Esters, Hydrocarbons and "Ether Glycerides"
15.2.4 Fat-Rich Secretions
15.3 Lipids of the Body Surface
15.3.1 Epidermal Lipids of Terrestrial Vertebrates
The term lipid is applied to a multitude of substances of differing chemical structure and biological function whose only common feature is that
they are insoluble in water but readily soluble in
organic solvents such as chloroform, benzene,
diethyl ether or hot ethanol. A large proportion
of the lipids of living organisms are bound to proteins and these are normally extracted using a
chloroform:methanol mixture; the alcohol component takes up water from the tissue, denatures
the proteins and releases the lipids, and the
chloroform solubilizes the lipids. The hydrophobic nature of the lipids is the basis for their
main biological functions as energy reserves, as a
critical part of the water-repellent surfaces of terrestrial organisms, and as a component of cellular
membranes. Some individual lipids or lipid-like
substances have special functions, e.g. as pheromones or hormones Quvenile hormone, steroid
hormones), as regulators of hormone action
(prostaglandins), as precursors of secondary messengers (phosphatidylinositol), or as recognition
signals on the cell surface (glycolipids) [76].
The water insolubility of the lipids presents the
organism with serious problems of lipid digestion
and absorption in the gut, of intra- and extracellular lipid transport, and of the mobilization of lipid
reserves. The lipases only express their hydrolytic
activity at the interphase between lipid and
water; thus, dietary fats are finely emulsified
15.3.2 Skin Fats of the Mammals
15.3.3 Lipids of the Uropygial Gland
15.3.4 Cuticular Lipids of the Insects
15.4 Membrane Lipids
15.4.1 Phospholipids
15.4.2 Biosynthesis of the Phospholipids
15.4.3 Phosphonolipids
15.4.4 Glycolipids
15.5 Lipids with Regulatory Functions
15.5.1 Prostaglandins and Other Eicosanoids
15.5.2 Insect Juvenile Hormones
References
prior to digestion, in the vertebrates by bile acids
and alcohols and in the invertebrates by emulsifiers with a rather different chemical structure
(p.646). The resulting hydrolysis products, ionized fatty acids (soaps), 2-monoacylglycerol and
lysophosphatides, amplify the emulsification. The
end products of fat digestion in the mammalian
gut are liquid-crystalline vesicles, which are
80-120 mm in diameter, and smaller micelles.
Just how these vesicles and micelles come into
contact with the microvilli of the gut mucosa and
how their components penetrate the membranes
of the gut cells is not yet completely known, even
for the vertebrates. In the mammals, a large part
of the absorbed fatty acids are incorporated back
into triacylglycerol in the gut cells, combined with
cholesterol, phospholipids and proteins into "chylomicrons", and released into the lymph [34,
142]. Lipid absorption in the insects appears to
follow a completely different scheme. The fatty
acids in the larvae of the butterfly Pieris brassicae
are incorporated into polar lipids before being
absorbed by the gut [266].
Lipid transport in vertebrate blood or lymph
and in arthropod haemolymph is carried out by
specific lipoproteins (as described in Chapter 5).
In vertebrate blood, triacyglycerol, phospholipids
and cholesterol are bound to plasma lipoproteins,
and free fatty acids are bound to serum albumin.
The lipophorins of insects carry diacylglycerols as