Essential Cell Biology 5th edition

A:4 Answers
D. Carbon-14 has two additional neutrons in its nucleus.
Because the chemical properties of an atom are
determined by its electrons, carbon-14 is chemically
identical to carbon-12.
ANSWER 2–3 The statement is correct. Both ionic
and covalent bonds are based on the same principles:
an exchange of electrons. In polar covalent bonds, the
electrons are shared unequally between the interacting
atoms. In ionic bonds, the electrons are completely lost by
one atom and gained by the other. And at the other end of
the spectrum, electrons can be shared equally between two
interacting atoms to form a nonpolar covalent bond. There
are bonds of every conceivable intermediate state, and for
borderline cases it becomes arbitrary whether a bond is
described as a very polar covalent bond or an ionic bond.
ANSWER 2–4 The statement is correct. The hydrogen–
oxygen bond in water molecules is polar, so the oxygen
atom carries a more negative charge than the hydrogen
atoms. These partial negative charges are attracted to the
positively charged sodium ions, but are repelled from the
negatively charged chloride ions.
ANSWER 2–5
A. Hydronium (H 3 O + ) ions result from water dissociating
into protons and hydroxyl ions, each proton binding to a
water molecule to form a hydronium ion
(2H 2 O → H 2 O + H + + OH – → H 3 O + + OH – ). At neutral
pH—that is, in the absence of an acid providing more
H 3 O + ions or a base providing more OH – ions—the
concentrations of H 3 O + ions and OH – ions are equal. We
know that at neutrality the pH = 7.0, and therefore the
H + concentration is 10 –7 M. The H + concentration equals
the H 3 O + concentration.
B. To calculate the ratio of H 3 O + ions to H 2 O molecules, we
need to know the concentration of water molecules. The
molecular weight of water is 18 (i.e., 18 g/mole), and
1 liter of water weighs 1 kg. Therefore, the concentration
of water is 55.6 M (= 1000 [g/L]/[18 g/mole]),
and the ratio of H 3 O + ions to H 2 O molecules is
1.8 × 10 –9 (= 10 –7 /55.6); that is, only two water molecules
in a billion are dissociated at neutral pH.
ANSWER 2–6 The synthesis of a macromolecule with
a unique structure requires that in each position only one
stereoisomer is used. Changing one amino acid from its
l- to its d-form would result in a different protein. Thus, if
for each amino acid a random mixture of the d- and l-forms
were used to build a protein, its amino acid sequence could
not specify a single structure, but many different structures
(2 N different structures) would be formed (where N is the
number of amino acids in the protein).
Why l-amino acids were selected in evolution as the
exclusive building blocks of proteins is a mystery; we could
easily imagine a cell in which certain (or even all) amino acids
were used in the d-forms to build proteins, as long as these
particular stereoisomers were used exclusively.
ANSWER 2–7 The term “polarity” has two different
meanings. In one meaning, polarity refers to a directional
asymmetry—for example, in linear polymers such as
polypeptides, which have an N-terminus and a C-terminus;
or nucleic acids, which have a 3ʹ and a 5ʹ end. Because
bonds form only between the amino and the carboxyl
groups of the amino acids in a polypeptide, and between
the 3ʹ and the 5ʹ ends of nucleotides, nucleic acids and
polypeptides always have two different ends, which give the
chains a defined chemical polarity.
In the other meaning, polarity refers to a separation of
electric charge in a bond or molecule. This kind of polarity
promotes hydrogen-bonding to water molecules, and
because the water solubility, or hydrophilicity, of a molecule
depends upon its being polar in this sense, the term “polar”
also indicates water solubility.
ANSWER 2–8 A major advantage of condensation
reactions is that they are readily reversible by hydrolysis
(and water is readily available in the cell). This allows cells
to break down their macromolecules (or macromolecules of
other organisms that were ingested as food) and to recover
the subunits intact so that they can be “recycled;” that is,
used to build new macromolecules.
ANSWER 2–9 Many of the functions that macromolecules
perform rely on their ability to associate and dissociate
readily. This chemical flexibility allows cells, for example,
to remodel their interior when they move or divide, and
to transport components from one organelle to another.
Covalent bonds would be too strong and too permanent for
such a purpose, requiring a specific enzyme to break each
kind of bond.
ANSWER 2–10
A. True. All nuclei are made of positively charged protons
and uncharged neutrons; the only exception is the
hydrogen nucleus, which consists of only one proton.
B. False. Atoms are electrically neutral. The number of
positively charged protons is always balanced by an
equal number of negatively charged electrons.
C. True—but only for the cell nucleus (see Chapter 1), not
for the atomic nucleus discussed in this chapter.
D. False. Elements can have different isotopes, which differ
only in their number of neutrons.
E. True. In certain isotopes, the large number of neutrons
destabilizes the nucleus, which decomposes in a process
called radioactive decay.
F. True. Examples include granules of glycogen, a polymer
of glucose, found in liver cells; and fat droplets, made of
aggregated triacylglycerols, found in fat cells.
G. True. Individually, these bonds are weak and readily
broken by thermal motion, but because interactions
between two macromolecules involve a large number of
such bonds, the overall binding can be quite strong; and
because hydrogen bonds form only between correctly
positioned groups on the interacting macromolecules,
they are very specific.
ANSWER 2–11
A. One cellulose molecule has a molecular weight of
n × (12[C] + 2 × 1[H] + 16[O]). We do not know n, but
we can determine the ratio with which the individual
elements contribute to the weight of cellulose. The
contribution of carbon atoms is 40%
[= 12/(12 + 2 + 16) × 100%]. Therefore, 2 g (40%
of 5 g) of carbon atoms are contained in the cellulose
that makes up this page. The atomic weight of carbon
is 12 g/mole, and there are 6 × 10 23 atoms or
molecules in a mole. Therefore, 10 23 carbon atoms
[= (2 g/12 [g/mole]) × 6 × 10 23 (molecules/mole)] make
up this page.