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24
Answers and Solutions to Text Problems
24.1
Other names for the citric acid cycle are the Krebs cycle and tricarboxlyic acid cycle.
24.2
The citric acid cycle begins by combining acetyl CoA and oxaloacetate.
24.3
One turn of the citric acid cycle converts 1 acetyl CoA to 2CO2, 3NADH + 3H+, FADH2, GTP
(ATP), and HSCoA.
24.4
Oxaloacetate is regenerated at the end of the citric acid cycle?
24.5
The reactions in steps 3 and 4 involve oxidative decarboxylation, which reduces the length of the
carbon chain by one carbon in each reaction.
24.6
There is a dehydration in reaction 2,
24.7
NAD+ is reduced by the oxidation reactions 3, 4, and 8 of the citric acid cycle.
24.8
FAD is reduced in reaction 6 when succinate is converted to fumarate.
24.9
In reaction 5, GDP undergoes a direct substrate phosphorylation to yield GTP, which converts
ADP to ATP and regenerates GDP for the citric acid cycle.
24.10
3 NADH and 1 FADH2 are produced in one turn of the citric acid cycle.
24.11
a. The six-carbon compounds in the citric acid cycle are citrate and isocitrate.
b. Decarboxylation reactions remove carbon atoms as CO2, which reduces the number
of carbon atoms in a chain (reactions 3 and 4).
c. The one five-carbon compound is _-ketoglutarate.
d. Several reactions are oxidation reactions; isocitrate → _-ketoglutarate; _-ketoglutarate
→ succinyl CoA; succinate→ fumarate; malate → oxaloacetate
e. Secondary alcohols are oxidized in reactions 3 and 8.
24.12
a. Two CO2 per citric acid cycle
b. Succinyl CoA, succinate, fumarate, malate, oxaloacetate
c. One GTP
d. Oxidations of isocitrate (reaction 3) and α-ketoglutarate (reaction 4)
e. Hydration of aconitate (reaction 1) and fumarate (reaction 6)
24.13
a. Citrate synthase combines oxaloacetate with acetyl CoA.
b. Succinate dehydrogenase and aconitase converts a carbon-carbon single bond to a double
bond.
c. Fumarase adds water to the double bond in fumarate; aconitase adds water to the double bond
in iconitate from citrate.
24.14
a. aconitase
24.15
a. NAD+ accepts 2H from the oxidative decarboxylation of isocitrate.
b. GDP is phosphorylated in the formation of succinate.
c. FAD accepts 2H when the carbon-carbon single bond in succinate is oxidized to a carboncarbon double bond in fumarate.
24.16
a. NAD+
b. α-ketoglutarate dehydrogenase
b. NAD+
c. NAD+
c. nucleotide phosphokinase
Chapter 24 Answers and Solutions
24.17
Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase are allosteric enzymes, which
increase or decrease the flow of materials through the citric acid cycle.
24.18
Low levels of NADH activate the citric acid cycle and high levels of NADH inhibit the cycle.
24.19
High levels of ADP means there are low levels of ATP. To provide more ATP for the cell,
the reaction rate of the citric acid cycle increases.
24.20
When NADH and ATP levels are high, there is a decrease in the production of acetyl CoA from
pyruvate, which slows the rate of the citric acid cycle. Low levels of ATP stimulate the
conversion of pyruvate to acetyl CoA providing fuel for the citric acid cycle and increasing the
rate.
24.21
The Fe3+ is the oxidized form of the iron in cytochrome c.
22.22
FMNH2 is the reduced form.
24.23
a. The loss of 2H+ and 2e- is oxidation.
24.24
a. reduction
24.25
NADH and FADH2 produced in glycolysis, oxidation of pyruvate, and the citric acid cycle
provide the electrons for electron transport.
24.26
As electrons move along the electron transport chain, the energy level drops.
24.27
FAD is reduced to FADH2, which provides 2H+ and 2e- for coenzyme Q, then cytochrome b,
and then cytochrome c.
24.28
NAD+, FMN, cytochrome a3, O2
24.29
The mobile carrier coenzyme Q (or Q) transfers electrons from complex I to III. It also transfers
electrons from complex II to complex III.
24.30
Cytochrome c is the mobile carrier that carries an electron from complex III to complex IV>
24.31
When NADH transfers electrons to FMN in complex I, NAD+ is produced.
24.32
The electrons from FADH2 are transferred to coenzyme Q to give QH2 and oxidized FAD.
24.33
a. NADH + H+ + FMN → NAD+ + FMNH2
b. QH2 + 2 Fe3+ cyt b → Q + 2 Fe2+ cyt b + 2H+
24.34
a. Q + FADH2 → QH2 + FAD
b. 2 cyt a (Fe3+) + 2 cyt a3 (Fe2+) → 2 cyt a (Fe2+) + 2 cyt a3 (Fe3+)
24.35
In oxidative phosphorylation, the energy from the oxidation reactions in the electron transport
chain is used to drive ATP synthesis.
24.36
As energy is released from oxidations along the transport chain, it is used to move protons into
the intermembrane space. The accumulation of high-energy protons creates a proton gradient.
b. The gain of 2H+ + 2e- is reduction.
b. oxidation
Metabolism and Energy Production
24.37
Protons must pass through F0 channel of ATP synthase to return to the matrix. During the
process, energy is released to drive the synthesis of ATP in F1.
24.38
When high-energy protons move through ATP synthetase, energy is provided to combine a
phosphate with ADP, a process called oxidative phosphorylation.
24.39
The oxidation of the reduced coenzymes NADH and FADH2 by the electron transport chain
generates energy to drive the synthesis of ATP.
24.40
The electrons from NADH enter the electron chain at a higher energy level than the electrons
from FADH2. Thus, the greater energy difference for electrons from NADH provides energy
that drives the synthesis of three ATP.
ATP synthase consists of two protein complexes known as F0 and F1.
24.41
24.42
Electrons flowing through F0 provide energy that turns the center unit in F1. In F1, ADP and Pi
combine to form ATP, which is released as the shape of the sites in F1 change.
24.43
The loose (L) site in ATP synthase begins the synthesis of ATP by binding ADP and Pi.
24.44
Energy is required to convert the tight site of ATP synthase to an open(O) site, which releases
the ATP.
24.45
Glycolysis takes place in the cytoplasm, not in the mitochondria. Because NADH cannot cross
the mitochondrial membrane, one ATP is hydrolyzed to transport the electrons from NADH to
FAD. The resulting FADH2 produces only 2 ATP for each NADH produced in glycolysis.
24.46
Under anaerobic conditions, the maximum yield is 36 ATP per glucose molecule.
24.47
a. 3 ATP are produced by the oxidation of NADH in electron transport.
b. 2 ATP are produced in glycolysis when glucose degrades to 2 pyruvate.
c. 6 ATP are produced when 2 pyruvate are oxidized to 2 acetyl CoA and 2 CO2.
d. 12 ATP are produced in one turn of the citric acid cycle as acetyl CoA is converted to 2 CO2.
24.48
a. 2 ATP
24.49
The oxidation reactions of the citric acid cycle produce a source of reduced coenzymes for the
electron transport chain and ATP synthesis.
24.50
In the citric acid cycle, oxaloacetate is regenerated, which is available to pick up acetyl CoA and
start the cycle again.
24.51
The electron transport chain regenerates the oxidized forms of the coenzymes NAD+ and FAD
for use again by the citric acid cycle.
24.52
The reduced coenzymes NADH/H+ and FADH2 are needed for the electron transport chain.
24.53
a. Citrate and isocitrate are six-carbon compounds in the citric acid cycle.
b. α-Ketoglutarate is a five-carbon compound.
c. The compounds α-ketoglutarate, succinyl-CoA, and oxaloacetate have keto groups.
24.54
a. succinyl CoA, succinate, fumarate, malate, and oxaloacetate
b. Citrate, isocitrate, and malate have hydroxyl groups.
c. There are carbon-carbon double bonds in aconitate and fumarate. There are carbon-oxygen
double bonds in many of the compounds in the citric acid cycle.
b. 36 ATP
c. 2 ATP
d. 2 ATP
Chapter 24 Answers and Solutions
In Reaction 4, α-ketoglutarate, a five-carbon keto acid, is decarboxylated.
In Reaction 1 and 7, double bonds in aconitate and fumarate are hydrated.
NAD+ is reduced in Reactions 3, 4, and 8.
In Reactions 3 and 8, a secondary hydroxyl group in isocitrate and malate is oxidized.
24.55
a.
b.
c.
d.
24.56
a. FAD is reduced in reaction 6.
b. In reaction 3, isocitrate is decarboxylated to give _-ketoglutarate.
c. In reaction 6, a carbon-carbon double bond forms when fumarate undergoes dehydrogenation
with FAD.
d. In reaction 5, GDP undergoes direct phosphorylation to give GTP.
24.57
a. NAD+ is the coenzyme for the oxidation of a secondary hydroxyl group in isocitrate to a keto
group in α-ketoglutarate.
b. NAD+ and CoA are needed in the oxidative decarboxylation of α-ketoglutarate to succinyl
CoA.
24.58
a. succinate dehydrogenase
24.59
a. High levels of NADH inhibit isocitrate dehydrogenase and α-ketoglutarate dehydrogenase to
slow the rate of the citric acid cycle.
b. High levels of ATP inhibit the citric acid cycle.
24.60
a. increase the rate of the citric acid cycle to provide more ATP
b. increase the rate of the citric acid cycle to provide more ATP
24.61
a. A heme group is found in all the cytochromes (4).
b. FMN (1) contains a ribitol group.
24.62
a. (1) FMN
24.63
a. CoQ is a mobile carrier.
b. Fe-S clusters are found in complexes I, III, and IV.
c. cyt a3 is part of complex IV
24.64
a. cyt b is part of complex II
c. FMN is part of complex I
24.65
FADH2 + Q → FAD + QH2
a. FADH2 is oxidized in complex II:
2+
b. Cyt a (Fe ) is oxidized in complex IV:
Cyt a (Fe2+) + Cyt a3 (Fe3+) → Cyt a (Fe3+) + Cyt a3 (Fe2+)
24.66
a. cyt c (Fe3+) + cyt a (Fe2+)
b. NAD+ + FMNH2
b. malate dehydrogenase
b. (3) CoQ
b. cyt c is a mobile carrier
Metabolism and Energy Production
24.67
Complete the following by adding the substances that are missing:
a.
b.
2+
FMN
2Fe S
FMNH2
2Fe S
FAD
FADH2
3+
QH2
Q
cyt b (Fe 3+)
2+
cyt b (Fe )
24.68
a. NADH + H+ ; FMN
b. 2 cyt a (Fe2+); 2 cyt a3 (Fe2+); H2O
24.69
The transfer of electrons by complexes I, III, and IV generate energy to pump protons out of the
matrix into the inner membrane space.
24.70
The accumulation of protons decreases pH in the intermembrane space and creates a proton
gradient.
24.71
In the chemiosmotic model, energy released by the flow of protons through the ATP synthase is
utilized for the synthesis ATP.
24.72
The synthesis of ATP occurs in the ATP synthase part of the electron transport chain.
24.73
In the inner membrane space, there is a higher concentration of protons, which reduces the pH
and forms an electrochemical gradient. As a result protons flow into the matrix where the proton
concentration is lower and the pH is higher.
24.74
The protons obtained from the oxidations reactions must be pumped from the end in the matrix
to be released at the other end, which is in the intermembrane space.
24.75
Two ATP molecules are produced from the energy generated by the electrons from FADH2
moving through electron transport to oxygen.
24.76
3 ATP are produced from each NADH + H+.
24.77
a. Amytal and rotenone inhibit the transfer of electrons in NADH dehydrogenase (complex I).
b. Antimycin inhibits electron flow from cyt b to cyt c1 in complex III.
c. Cyanide and carbon monoxide inhibit the flow of electron through cytochrome c oxidase
(complex IV).
24.78
a. The coenzymes that precede the blocked site remain in their reduced forms.
b. The coenzymes that follow the blocked site remain in their oxidized forms.
Chapter 24 Answers and Solutions
24.79
The oxidation of glucose to pyruvate by glycolysis produces 6 ATP. 2 ATP are formed by direct
phosphorylation along with 2 NADH. Because the 2 NADH are produced in the cytosol, the
electrons are transferred to form 2 FADH2, which produces an additional 4 ATP. The oxidation
of glucose to CO2 and H2O produces 36 ATP.
24.80
The NADH produced from glycolysis cannot pass through the mitochondrial membrane.
Therefore, the hydrogen ions and electrons from the NADH in the cytoplasm are transferred to
dihydroxyacetone phosphate, which is reduced to glycerol-3-phosphate to regenerate NAD+.
After glycerol-3-phosphate moves into the mitochondria, the hydrogen ions and electrons
reduced FAD to give FADH2 and FADH2 produces two ATP in the electron transport chain.
24.81
a. 4 ATP x 7.3 kcal/mole = 29 kcal (actual ATP produced from glycolysis because the protons
from the NADH in the cytoplasm are shuttled to FAD in the mitochondria).
b. 6 ATP x 7.3 kcal/mole = 44 kcal (2 pyruvate to 2 acetyl CoA)
c. 24 ATP x 7.3 kcal/ mole = 175 kcal (2 acetyl CoA citric acid cycle)
d. 36 x 7.3 kcal/mol = 263 kcal (complete oxidation of glucose to CO2 and H2O)
24.82
a. 29 kcal/ 687 kcal ✕ 100 = 4.2%
b. 44 kcal/687 ✕ 100 = 6.4%
c. 175 kcal/687 ✕ 100 = 35%
24.83
In a calorimeter, the complete combustion of glucose gives 687 kcal. The efficiency of ATP
synthesis is determined by comparing the total kcal in 36 ATP (283 kcal in problem 24.81) to the
energy obtained from glucose in a calorimeter.
283 /687 x 100 = 38.0% efficient
24.84
4 moles glucose ✕283 moles ATP/1 mole glucose ✕ 7.3 kcal/1 mole ATP = 8260 kcal
When 4 moles of glucose are completely oxidized at 38% efficiency, 8260 kcal of energy would
be conserved.
24.85
The ATP synthase extends through the inner mitochondrial membrane with the F0 part in contact
with the proton gradient in the intermembrane space, while the F1 complex is in the matrix.
24.86
Protons from the matrix where oxidation reactions take place are pumped through complexes I,
IIII, and IV into the intermembrane space where they accumulate and form a proton gradient.
24.87
As protons from the proton gradient move through the ATP synthase to return to the matrix,
energy is released and used to drive ATP synthesis at F1.
24.88
ADP and Pi enter the loose (L) site on ATP synthase. When the shape of the site changes to tight
(T), ATP is formed. The site then changes to open (O), and ATP is released.
24.89
A hibernating bear has stored fat as brown fat, which can be used during the winter for heat
rather than ATP energy.
24.90
A diet medication that is an uncoupler may transport protons through the inner membrane or
block the changed in the F0 part of ATP synthase. Because ATP synthesis is bypassed, the
energy of electron transport is released as heat, which will cause an increase in body
temperature.