Selected Solutions to End of Chapter 10 Problems
1. The operational definition of not being soluble in water has flaws: many non-lipid biological
molecules are not very soluble in water. What is the threshold? Or, is there one. Take into account
the amino acids tyrosine and phenylalanine, two of the aromatic amino acids which have substantial
parts of the molecule being nonpolar. There was an EOC problem in Chapter 3 on getting tyrosine
into solution: the trick was to dissolve it into a moderately alkaline (NaOH) solution, why did that
work? Would it also work for phenylalanine? Tryptophan? (Try to figure this out before going to the
answers for these at the end of this file)
2. a. All these fatty acids are 18 carbons. Data: They vary in the physical property of melting temp,
and also in one chemical property:
Melting Temp
Chemistry
Stearic acid:
69.6oC
18:0
Oleic acid:
13.4oC
18:1 Δ9
Linoleic acid:
-5oC
18:2 Δ9, 12
Linolenic acid:
-11oC
18:3 Δ9, 12, 15
c. This part asks about branched chain fatty acids even though they are not significantly covered
in the text or “lecture”. They occur as a minor component in bacteria, but are the very heart of
archaeal lipids. Then tend to give them lower melting points because the molecules with branches
can’t stick together.
3. Béarnaise Sauce: egg yolk wisked into melted buffer which is mainly fat (triacylglycerol). While
egg yolk has some protein, it is mainly “lectithin which is the old word for phophatidylcholine. This
works as an emulsifier because the phospholipids have a strong nonpolar part (fatty acids in fat) and
polar part (phospho-choline in egg yolk). Emulsifiers in cooking work to give sauces a thicker texture
making the sauce not runny like water. And in this case, it makes them taste good as well !
7. We can express this as a table:
Lipid
Phosphatidylethanolamid
Sphingomyelin
Galactosylcereberoside
Gangloside
Cholesterol
Hydrophobic Unit
2 Fatty acids
Sphingosine + Fatty acid
Sphingosine + Fatty acid
Sphingosine + Fatty acid
Steroid nucleus + alkyl side
chain
Hydrophilic Unit
Phospho-ethanolamine
Phospho-choline
Galactose
Oligosaccharide
OH group on steroid nucleus
8. Structure of an omega-6 fatty acid: 16:1. So which carbon is unsaturated in a 16 carbon fatty acid
(palmitic acid)?
This is so easy…between the 6th and 7th carbon in from the omega carbon (last in the aliphatic chain).
Using the regular system it would be 16:1 Δ10 which means it is unsaturated between carbons 10 and
11.
9. Hey, this is sort of the reverse of problem 1. Going from unsaturated to saturated fatty acids. This
process was used initially to conserve butter (from cows) by inventing margarine (making vegetable
oils more solid like butter) by saturating (reducing) the points of unsaturation thereby changing the
physical properties (melting temp) of the lipid. This worked, but unfortunately the chemical process
was partly reversible making both cis and trans points of unsaturation. The trans fat no longer had a
bend at that point but resembled a saturated fatty acid. The cis conformation is so biologically
important.
10. Using alkali (usually NaOH) causes the alkaline hydrolysis of the ester link between the fatty
acids and glycerol. The fatty acid becomes a bit more soluble (ester => carboxylic acid), and the
glycerol carbons (all alcohols). This along with warm water gets these solid fat-clogging plumbing
problems resolved, or flushed down the drain.
11. Figuring out lipid structure: what is it? A lipid that has one phosphate per fatty acid. Could this
be a:
Glycerolphospholipid: no, there are two fatty acids per phosphate (and glycerol, and “X”).
Gangliioside: no, they don’t have phosphate (sugar(s) attached to ceramide).
Sphingomyelin: yes, one fatty acid per phosphate (attached to ceramide, with choline attached
to the phosphate as well).
12. Another molar ratio problem: complete hydrolysis yields one glycerol, one palmitic acid, one
palmitoelic acid, one phosphate, and one serine. This is pretty easy, it is phosphatidylserine. When
you draw it (please do without looking at the structures in the text) you have to make sure the
unsaturaded fatty acid is esterified to glycerol carbon 2, with the saturated fatty acid on glycerol
carbon 1.
13. Waxes: esters between a fatty acid and a fatty alcohol…both with very long alkyl chains These
are the very most nonpolar biological lipids known, they easily get to over 40 carbons long. The
example in the text is beeswax, an ester between palmitic acid (16 carbons) and triacontanol (30
carbons) which is 46 nonpolar carbons long and thereby a good relatively solid structure with
malleable properties. (A note on earwax: earwax is not a wax, but dead sloughed off cells from the
middle ear).
14. Snake venoms are different combinations of hydrolases. Here the Eastern Diamonback
rattlesnake and the Indian cobra venom contains phospholipase-A2 which cleaves off the fatty acid at
the 2 carbon of glycerol. This leaves a monoacyl-glycerolphospholipid called lyso-lecithin which acts
as a detergent in breaking up cell membrane organization  cell lysis.
a. lyso-lecithin has lost a nonpolar part (one fatty acid down from two) and more polar, the 2
carbon of glycerol is no longer part of an ester, but an alcohol adding to the polarity of the
phosphate and polar head group).
b. Pain an inflammation can be treated with certain steroids. These steroids are antiinflammatory, and some insert into membranes to re-establish membrane structure.
c. Phosophlipase A2 is part of forming signaling molecules from unsaturated fatty acids, which
occur mostly at the second carbon atom of glycerol. This activity is highly regulated and part
of a suite of endoplasmic reticulum enzymes.
Answers to Question 1: tyrosine (Y) phenol group has a pKa of 10.7. So if the pH gets higher than
10.7, the phenol group ionizes releasing an H, and resulting in a R-O- which, of course is polar and
helps dissolve Y into an alkanilzed water. This won’t work for phenylalanie (F) which differs only
from Y in that it’s benzene ring has no ionizable groups. The same goes for the indole R group of
tryptophan (W). This makes all these amino acids just sparingly soluble at pH 7.