Essential Cell Biology 5th edition

Answers A:5
B. The volume of the page is 4 × 10 –6 m 3 (= 21.2 cm ×
27.6 cm × 0.07 mm), which is the same as the volume
of a cube with a side length of 1.6 cm (= 3 √4 × 10 –6 m 3 ).
Because we know from part A that the page contains
10 23 carbon atoms, geometry tells us that there could be
about 4.6 × 10 7 carbon atoms (= 3 √10 23 ) lined up along
each side of this cube. Therefore, in cellulose, about
200,000 carbon atoms [= (4.6 × 10 7 ) × (0.07 × 10 –3 m)/
1.6 × 10 –2 m] span the thickness of the page.
C. If tightly stacked, 350,000 carbon atoms with a 0.2-nm
diameter would span the 0.07-mm thickness of the page.
D. The 1.7-fold difference in the two calculations reflects
(1) that carbon is not the only atom in cellulose and
(2) that paper is not an atomic lattice of precisely
arranged molecules (as a diamond would be for precisely
arranged carbon atoms), but a random meshwork of
fibers containing many voids.
ANSWER 2–12
A. The occupancies of the three innermost electron levels
are 2, 8, 8.
B. helium already has full outer electron shell
oxygen gain 2
carbon gain 4 or lose 4
sodium lose 1
chlorine gain 1
C. Helium, with its fully occupied outer electron shell, is
chemically unreactive. Sodium and chlorine, on the other
hand, are extremely reactive and readily form stable Na +
and Cl – ions that form ionic bonds, as in table salt.
ANSWER 2–13 Whether a substance is a liquid or a gas
at a given temperature depends on the attractive forces
between its molecules. H 2 S is a gas at room temperature
and H 2 O is a liquid because the hydrogen bonds that
hold H 2 O molecules together do not form between H 2 S
molecules. A sulfur atom is much larger than an oxygen
atom, and because of its larger size, the outermost
electrons are not as strongly attracted to the nucleus of the
sulfur atom as they are in an oxygen atom. Consequently,
the hydrogen–sulfur bond is much less polar than the
hydrogen–oxygen bond. Because of the reduced polarity,
the sulfur in an H 2 S molecule is not strongly attracted to
the hydrogen atoms in an adjacent H 2 S molecule, and the
hydrogen bonds that are so predominant in water do not
form.
ANSWER 2–14 The reactions are diagrammed in Figure
A2–14, where R 1 and R 2 are amino acid side chains.
H 2 N
Figure A2–14
R 1
C
H
COOH
H 2 O
HYDROLYSIS
H 2 N
R 1
C
H
O
C
+
N
H
H 2 N
H 2 O
R 2
C
H
R 2
C
H
COOH
COOH
CONDENSATION
ANSWER 2–15
A. False. The properties of a protein depend on both the
amino acids it contains and the order in which they are
linked together. The diversity of proteins is due to the
almost unlimited number of ways in which 20 different
amino acids can be combined in a linear sequence.
B. False. Lipids assemble into bilayers by noncovalent
bonds. A membrane is therefore not a macromolecule.
C. True. The backbone of nucleic acids is made up
of alternating ribose (or deoxyribose in DNA) and
phosphate groups. Ribose and deoxyribose are sugars.
D. True. About half of the 20 naturally occurring amino
acids have hydrophobic side chains. In folded proteins,
many of these side chains face toward the inside of
the folded-up proteins, because they are repelled from
water.
E. True. Hydrophobic hydrocarbon tails contain only
nonpolar covalent bonds. Thus, they cannot participate
in hydrogen-bonding and are repelled from water. We
consider the underlying principles in more detail in
Chapter 11.
F. False. RNA contains the four listed bases, but DNA
contains T instead of U. T and U are very much alike,
however, and differ only by a single methyl group.
ANSWER 2–16
A. (a) 400 (= 20 2 ); (b) 8000 (= 20 3 ); (c) 160,000 (= 20 4 ).
B. A protein with a molecular mass of 4800 daltons is
made of about 40 amino acids; thus there are 1.1 × 10 52
(= 20 40 ) different ways to make such a protein. Each
individual protein molecule weighs 8 × 10 –21 g
(= 4800/6 × 10 23 ); thus a mixture of one molecule each
weighs 9 × 10 31 g (= 8 × 10 –21 g × 1.1 × 10 52 ), which
is 15,000 times the total weight of the planet Earth,
weighing 6 × 10 24 kg. You would need a very large
container indeed.
C. Given that most cellular proteins are even larger than the
one used in this example, it is clear that only a minuscule
fraction of the total possible amino acid sequences is
used in living cells.
ANSWER 2–17 Because all living cells are made up of
chemicals, and because all chemical reactions (whether
in living cells or in test tubes) follow the same rules, an
understanding of basic chemical principles is fundamentally
important to the understanding of biology. In the course of
this book, we will frequently refer back to these principles,
on which all of the more complicated pathways and
reactions that occur in cells are based.
ANSWER 2–18
A. Hydrogen bonds require specific groups to interact; one
is always a hydrogen atom linked by a polar covalent
bond to an oxygen or a nitrogen, and the other is usually
a nitrogen or an oxygen atom. Van der Waals attractions
are weaker and occur between any two atoms that are
in close enough proximity. Both hydrogen bonds and
van der Waals attractions are short-range interactions
that come into play only when two molecules are already
close. Both types of bonds can therefore be thought
of as a means of “fine-tuning” an interaction; that is,
helping to position two molecules correctly with respect
to each other once they have been brought together by
diffusion.
B. Van der Waals attractions would form in all three
examples. Hydrogen bonds would form in (c) only.