Chapter 13

Question 1

energy transductions

Answer 1

Bioenergetics is the quantitative study of energy transductions —
changes of one form of energy into another — that occur in living
cells, and of the nature and function of the chemical processes
underlying these transductions. A

Question 2

Bioenergetics is t

Answer 2

the quantitative study of energy relationships
and energy conversions in biological systems. Biological energy
transformations obey the laws of thermodynamics.

Question 3

Living cells constantly perform work. They require

Answer 3

energy for
maintaining their highly organized structures, synthesizing
cellular components, transporting small molecules and ions
across membranes, and generating electric currents.

Question 4

All chemical reactions are influenced by two forces:

Answer 4

the
tendency to achieve the most stable bonding state (for which
enthalpy, H, is a useful expression) and the tendency to achieve
1726
the highest degree of randomness, expressed as entropy, S. The
driving force in a reaction is ΔG, the free-energy change, which
represents the net effect of these two factors: ΔG = ΔH − T ΔS.

Question 5

The standard transformed free-energy change, ΔG′°, is

Answer 5

a
physical constant that is characteristic for a given reaction and
can be calculated from the equilibrium constant for the reaction:
ΔG′° = −RT ln K′
eq.

Question 6

The actual free-energy change, ΔG, is

Answer 6

a variable that depends
on ΔG′° and on the concentrations of reactants and products:
ΔG = ΔG′° + RT ln([products]/[reactants]). When ΔG is large
and negative, the reaction tends to go in the forward direction;
when ΔG is large and positive, the reaction tends to go in the
reverse direction; and when ΔG = 0, the system is at
equilibrium.

Question 7

The free-energy change for a reaction is independent of t

Answer 7

the
pathway by which the reaction occurs. Free-energy changes are
additive; the net chemical reaction that results from successive
reactions sharing a common intermediate has an overall freeenergy change that is the sum of the ΔG values for the individual
reactions

Question 8

Living systems make use of a large number of chemical
reactions that can be classified into five general types:

Answer 8

reactions
that make or break carbon–carbon bonds; internal
rearrangements and eliminations; free-radical reactions; group
transfers; and oxidation-reduction reactions. Heterolytic
cleavages occur o

Question 9

Carbonyl groups play a special role in reactions that form or
cleave C—C bonds. Carbanion intermediates are

Answer 9

common and
are stabilized by adjacent carbonyl groups or, less o

Question 10

A redistribution of electrons can produce

Answer 10

internal
rearrangements, isomerizations, and eliminations. Such reactions
include intramolecular oxidation-reduction, change in cis-trans
arrangement at a double bond, and transposition of double
bonds.

Question 11

Homolytic cleavage of covalent bonds to generate

Answer 11

free radicals
occurs in some pathways

Question 12

Phosphoryl transfer reactions are

Answer 12

an especially important type
of group transfer in cells, required for the activation of molecules
for reactions that would otherwise be highly unfavorable.

Question 13

Oxidation-reduction reactions involve

Answer 13

the loss or gain of
electrons: one reactant gains electrons and is reduced, while the
other loses electrons and is oxidized. Oxidation reactions
generally release energy and are important in catabolism.

Question 14

Biochemists often write reactions that are not

Answer 14

balanced for H
+
and don’t attempt to describe the state of
phosphate ionization.

Question 15

ATP then donates some of its chemical energy to

Answer 15

endergonic processes such as the synthesis of metabolic
intermediates and macromolecules from smaller precursors, the
transport of substances across membranes against concentration
gradients, and mechanical motion

Question 16

good pic on p 1771

Answer 16

kk

Question 17

Nucleophilic attack by an alcohol on the γ phosphate (Fig. 13-
20a) displaces

Answer 17

displaces ADP and produces a new phosphate ester. Studies
with
18Ö -labeled reactants have shown that the bridge oxygen in
the new compound is derived from the alcohol, not from ATP; the
group transferred from ATP is therefore a phosphoryl (−PO
2−
3
),
not a phosphate (−OPO
2−
3
).

Question 18

ATP is the chemical link between

Answer 18

catabolism and anabolism. It
is the energy currency of the living cell. The exergonic conversion
of ATP to ADP and Pi, or to AMP and PPi, is coupled to many
endergonic reactions and processes.

Question 19

The free-energy change for ATP hydrolysis under

Answer 19

r cellular
conditions is its phosphorylation potential, ΔGp.

Question 20

Direct hydrolysis of ATP is the source of energy in

Answer 20

some
processes driven by conformational changes. In general,
however, it is not ATP hydrolysis but the transfer of a phosphoryl
group from ATP to a substrate or an enzyme that couples the
energy of ATP breakdown to endergonic transformations of
substrates.

Question 21

Phosphate compounds with high free energies of hydrolysis
can donate

Answer 21

e their phosphoryl group to form another phosphate
compound with a smaller free energy of hydrolysis.

Question 22

ATP can also donate a pyrophosphoryl (PPi) or adenylyl (AMP)
group to a variety of metabolic intermediates, activating them for

Answer 22

nucleophilic displacement reactions.

Question 23

Through these group transfer reactions, ATP provides the
energy for

Answer 23

r a large number of anabolic reactions, including the
synthesis of informational macromolecules, and for the transport
of molecules and ions across membranes against concentration
gradients and electrical potential gradients. Muscle contraction is
also powered by ATP.

Question 24

To maintain its high group transfer potential, ATP
concentration must be

Answer 24

held far above the equilibrium
concentration by energy-yielding reactions of catabolism.

Question 25

ATP can donate a phosphoryl group to

Answer 25

nucleoside
diphosphates by transphosphorylation to keep the levels of GTP,
UTP, CTP, and the deoxynucleotides far above their equilibrium
concentrations.

Question 26

In many organisms, a central energy-conserving process is the

Answer 26

stepwise oxidation of glucose to CO2, in which some of the
energy of oxidation is conserved in ATP as electrons are passed to
O2.

Question 27

Biological oxidation-reduction reactions can be described in
terms of

Answer 27

f two half-reactions, each with a characteristic standard
reduction potential, E
′°.

Question 28

Many biological oxidation reactions are dehydrogenations in
which

Answer 28

one or two hydrogen atoms (H
+ + e
−) are transferred from
a substrate to a hydrogen acceptor. In some biological redox
reactions, the substrate loses both electrons and protons, the
equivalent of losing hydrogen. The many enzymes that catalyze
such reactions are called dehydrogenases.

Question 29

When two electrochemical half-cells, each containing the
components of a half-reaction, are connected, electrons tend to

Answer 29

flow to the half-cell with the higher reduction potential. The
strength of this tendency is proportional to the difference
between the two reduction potentials (∆E) and is a function of the
concentrations of oxidized and reduced species.

Question 30

The standard free-energy change for an oxidation-reduction
reaction is directly proportional to

Answer 30

the difference in standard
reduction potentials of the two half-cells: ΔG′° = −nFΔE′°.

Question 31

Oxidation-reduction reactions in living cells involve

Answer 31

specialized
electron carriers. NAD and NADP are the freely diffusible
coenzymes of many dehydrogenases. Both NAD
+
and NADP
+
accept two electrons and one proton. In addition to its role in
oxidation-reduction reactions, NAD
+
is the source of AMP in the
bacterial DNA ligase reaction and of ADP-ribose in the cholera
toxin reaction, and it is hydrolyzed in the deacetylation of
proteins by some sirtuins.

Question 32

Lack of the vitamin niacin prevents NAD synthesis and leads to

Answer 32

pellagra

Question 33

FAD and FMN, the flavin nucleotides, serve as

Answer 33

tightly bound
prosthetic groups of flavoproteins. They can accept either one or
two electrons and one or two protons. Their reduction potentials
depend on the flavoprotein with which they are associated.