Chapter 26: Lipids
Fatty Acids Are Long-Chain Carboxylic Acids
• Lipids: organic compounds that are soluble in nonpolar solvents, due to their
significant hydrocarbon component.
• Fatty Acids: carboxylic acids with long hydrocarbon chains and the most
naturally occurring fatty acids contain an even number of carbon atoms that
o Polyunsaturated Fatty Acids: fatty acids with more than one double
o The melting points of saturated fatty acids increase with increasing
molecular weight because of increased van der Waals interactions
between the molecules.
o The melting points of unsaturated fatty acids (alkenes) also increase
with increasing molecular weight, but are lower than those of saturated
fatty acids (alkanes) with comparable molecular weights.
o Double bonds are in cis configuration and are always separated by one
o The melting points of the unsaturated fatty acids decrease as the
number of double bonds increases.
Waxes Are High-Molecular Weight Esters
• Waxes: esters formed from long-chain carboxylic acids and long-chain alcohols.
• Waxes have a large even number of carbons on both sides of the ester.
Fats and Oils
• Triacylglycerols or Triglycerides: compounds in which each of the three OH
groups of glycerol has formed an ester with a fatty acid
• Simple Triacylglycerol: if the three fatty acid components of a triacylglycerol are
• Mixed Triacylglycerol: contains two or more different fatty acid components and
are more common than simple triacylglycerols.
• Fats: triacylglycerols that are solids or semisolids at room temperature.
o They are either saturated or have only one double bond.
o They are packed closely together, giving them high melting points so
they are solids at room temperature.
o Typically comes from animals.
• Oils: liquid triacylglycerols.
o Typically comes from plant products.
o They are composed of unsaturated components and cannot be packed
o They have low melting points, causing them to be liquid at room
• The fat content of the average man is about 21% whereas the fat content of the
average woman is about 25%.
• Polyunsaturated fats and oils are easily oxidized by O 2y means of a radical
o A radical removes a hydrogen from methylene group that is flanked by
two double bonds and reacted with O , f2rming a peroxy radical with
conjugated double bonds.
o The peroxy radical removes a hydrogen from a methylene group of
another amolecule of fatty acid, forming an alkyl hydroperoxide. Phospholipids and Sphingolipids Are Components of Membranes
• Phospholipids: lipids that contain a phosphate group.
o Phosphoacylglycerols: major component of cell membranes and belongs
to the phospholipids group.
o They are similar to triacylglycerols except that a terminal OH group of
glycerol is esterified with phosphoric rather than with a fatty acid,
forming a phosphatidic acid.
• The most common phosphoacylglycerols are phosphodiesters.
• Phosphoacylglycerols from membranes by arranging themselves in a lipid
bilayer, where the polar heads are on both surfaces of the bilayer and the fatty
acid chains form the interior of the bilayer.
• Saturated fatty acids decrease membrane fluidity because their hydrocarbon
chains pack closely together while unsaturated fatty acids increase fluidity
because they pack less closely together.
• Sphingolipids: major lipid components in the myelin sheaths of nerve fibers and
contain an amino alcohol called sphingosine instead of glycerol.
o The amino group of sphingosine is bonded to the acyl group of a fatty
o Both asymmetric centers in sphingosine have the S configuration.
Prostaglandins Regulate Physiological Responses
• Prostaglandins: found in all body tissues and are responsible for regulating a
variety of physiological responses like inflammation, blood pressure, fever and
o All have five-membered rings with a seven-carbon carboxylic acid
substituent and an eight-carbon hydrocarbon substituent.
o The two substituents are trans to each other.
• Prostaglandins are classified using the formula PGX, where X designates the
functional groups of the compound’s five-membered ring.
o PGAs, PGBs and PGCs all contain a carbonyl group and a double bond in
the five-membered ring.
o PGDs and PGEs are β-hydroxy ketones.
o PGFs are 1,3-diols, where α indicates a cis diol and β indicates a trans
• Biosynthesis of Prostaglandins, Thromboxanes and Prostacyclins
o In the first step, a hydrogen atom is removed from a carbon flanked by
two double bonds. This hydrogen is removed easily because the
resulting radical is stabilized by electron delocalization.
o The radical reacts with oxygen to form a peroxy radical.
o The peroxy radical rearranges and reacts with a second molecule of
o The enzyme then uses its hydroperoxidase activity to concert the OOH
group into an OH group, forming PGH ,2which rearranges to form PGE , 2
• PGH 2s a precursor for the synthesis of prostaglandins, thromboxanes and
• Aspirin inhibits the cyclooxygenase activity of prostaglandin endoperoxide
synthase; therefore, it inhibits the synthesis of prostaglandins and decreases
the inflammation produced by these compounds.
Terpenes Contain Carbon Atoms in Multiples of Five
• Terpenes: they can be hydrocarbons, contain oxygen or be alcohols, ketones or
aldehydes. o Terpenoids: oxygen-containin