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MOLECULAR BIOLOGY OF THE CELL, SIXTH EDITION
CHAPTER 2: CELL CHEMISTRY AND BIOENERGETICS
ยฉ Garland Science 2015
1.
Which of the following elements is not normally found in cells?
A. Copper
B. Iron
C. Silver
D. Cobalt
E. Zinc
2.
A hydrophobic molecule is typically โฆ
A. able to form hydrogen bonds with itself but not with water.
B. able to form hydrogen bonds with water.
C. charged.
D. hard to dissolve in a solvent.
E. incapable of interacting favorably with water.
3.
For each of the following pairs, indicate whether they interact via hydrogen bonds (H) or
ionic bonds (I), or do not favorably interact (N). Your answer would be a four-letter string
composed of letters H, I, and N only, e.g. HNNI.
( ) ATP and Mg2+
( ) Urea and water
( ) Glucose and the enzyme hexokinase (which uses glucose as a substrate)
( ) A phospholipid tail and inorganic phosphate
4.
Which of the following chemicals do you NOT expect to be readily dissolved in water?
A. Uric acid
B. Hexane
C. Glycerol
D. Ethanol
E. Potassium chloride
5.
Weak noncovalent attractions in the cell can be very strong in a nonaqueous environment.
Some of these attractions are as strong as covalent interactions in a vacuum (their bond energy is
approximately 340 kJ/mole), but become more than twenty-five times weaker (their bond energy
becomes approximately 13 kJ/mole) in water. What type of attraction shows this phenomenon?
A. Electrostatic attractions
B. Hydrogen bonds
C. van der Waals attractions
D. Hydrophobic force
E. All of the above
6.
The bond energies associated with noncovalent attractions in the cell are too weak to
resist disruption by thermal motion. However, cellular macromolecules can interact specifically
AND strongly with each other (or fold by themselves) merely via such interactions. How is this
possible?
A. The bond energies increase radically when two interacting molecules approach each
other.
B. The interacting molecules also fortify their binding via covalent bonds to keep them
from dissociation.
C. Many weak bonds together in a complementary geometry can afford a strong binding.
D. The cell lowers its internal temperature to reduce thermal motion of molecules and
enhance the weak attractions.
7.
What is the pH of a 10โ8 M solution of hydrochloric acid? Round the pH value to the
nearest integer, e.g. 10.
A. 8
B. 7
C. 6
D. 5
E. 4
8.
The cell can change the pH of its internal compartments using membrane transport
proteins that pump protons into or out of a compartment. How many protons should be pumped
into an endocytic vesicle that is 10โ15 liters in volume and has a neutral pH in order to change the
pH to 5? Avogadroโs number is 6 ร 1023. Omit complications such as the membrane potential,
buffers, and other cellular components.
A. 6000
B. 60,000
C. 120,000
D. 600,000
E. 6,000,000
9.
Which of the following is true regarding a fatty acid molecule in water?
A. It is positively charged at physiological pH, but can become neutral when the pH is
high enough.
B. It is positively charged at physiological pH, but can become neutral when the pH is
low enough.
C. It is negatively charged at physiological pH, but can become neutral when the pH is
high enough.
D. It is negatively charged at physiological pH, but can become neutral when the pH is
low enough.
E. It is not charged at physiological pH.
10.
The amino acid serine has an amino group, a carboxyl group, and a hydroxyl group.
Which of the following better represents the structure of this amino acid at neutral pH?
A
+
H3N
H
O
C
C
B
Oโ
+
H3N
CH3
O
C
C
C
+
H3N
NH2
OH
D
H
O
H3N+
C
C
OH
H
H
H
O
C
C
Oโ
CH2
OH
E
H
O
H2N
C
C
OH
H
O
N
C
C
H
OH
H
11.
The three families of cellular macromolecules are polymerized and depolymerized by a
general mechanism involving water. Each of them has a set of monomers whose polymerization
changes the total free energy of the system. Which of the following statements is true regarding
these macromolecules?
A. Each polymerization step requires free-energy input and proceeds by the consumption
of one water molecule.
B. Each depolymerization step requires free-energy input and proceeds by the
consumption of one water molecule.
C. Each polymerization step requires free-energy input and proceeds by the release of
one water molecule.
D. Each depolymerization step requires free-energy input and proceeds by the release of
one water molecule.
12.
Sort the following from a low to a high contribution to the total mass of an E. coli
bacterium. Your answer would be a four-letter string composed of letters A to D only, e.g.
DCBA.
(A) Water
(B) Sugars
(C) Proteins
(D) Nucleic acids
13.
Which of the following statements is true regarding cellular metabolism?
A. A living organism decreases the entropy in its surroundings.
B. During catabolism, heat is generated, and the cell uses this heat to perform work
during anabolism.
C. The heat released by an animal cell as part of its metabolic processes comes from the
bond energies in the foodstuffs that are consumed by the animal.
D. Living organisms defy the second law of thermodynamics, but still obey the first law.
14.
The folding of proteins can be considered a simple conversion from the unfolded to the
natively folded state. At about 27ยฐC (or 300 K), the free-energy change of folding for a particular
protein is measured to be โ40 kJ/mole. If the enthalpy change (ฮH) of folding is โ640 kJ/mole,
what is the entropy change (ฮS) of folding for this protein? Write down your answer with the
appropriate sign (+ or โ) and in kJ/mole/K, e.g. โ1000 kJ/mole/K.
15.
Which of the following correctly summarizes the overall process of photosynthesis?
A. CO2 + O2 โ H2O + sugars
B. CH2O + CO2 + O2 โ H2O + sugars
C. CO2 + H2O โ H2 + CO2
D. CO2 + H2O โ O2 + sugars
16.
Which of the following statements is true regarding reactions involving oxidation and
reduction?
A. The carbon atom is more oxidized in formaldehyde (CH2O) than in methanol
(CH3OH).
B. Oxidation of food in all organisms requires oxygen.
C. A molecule is oxidized if it gains an electron (plus a proton) in a reaction.
D. A dehydrogenation reaction is a reduction.
E. In an organic molecule, the number of CโH bonds increases as a result of oxidation.
17.
Enzymes are the cellโs catalyst crew. They make the life of the cell possible by carrying
out various reactions with astounding performance. Which of the following is NOT true
regarding cellular enzymes?
A. Enzymes lower the activation energy of the reactions that they catalyze.
B. Enzymes can specifically drive substrate along certain reaction pathways.
C. Enzymes can push energetically unfavorable reactions forward by coupling them to
energetically favorable reactions.
D. Enzymes are proteins, but RNA catalysts, called ribozymes, also exist.
E. Enzymes can change the equilibrium point for reactions that they catalyze.
18.
In the following diagram showing the reaction pathway for a simple single-substrate
enzymatic reaction, which of the quantities corresponds to the activation energy of the forward
reaction?
Total energy
a
b
c
Reaction pathway
A. (a โ b)
B. (a + b)
C. (a โ c)
D. (a + c)
E. (b โ c)
Number of molecules
19.
In the following diagram showing the distribution of thermal energy in a population of
substrate molecules, the energy thresholds indicated by numbers represent …
1
2
Energy per molecule
A. the activation energy at high and low temperature.
B. the reaction rate at high and low pH.
C. the activation energy with and without an enzyme.
D. the reaction rate at high and low substrate concentrations.
E. the activation energy at high and low substrate concentrations.
20.
A cellular enzyme catalyzes the catabolic reaction shown below. Its coenzyme is shown
in the box. Which of the following is correct regarding this reaction?
State 1
State 2
A. The substrate is reduced in this reaction and the coenzyme is converted from state 1 to
state 2.
B. The substrate is oxidized in this reaction and the coenzyme is converted from state 1
to state 2.
C. The substrate is reduced in this reaction and the coenzyme is converted from state 2 to
state 1.
D. The substrate is oxidized in this reaction and the coenzyme is converted from state 2
to state 1.
21.
The molecules inside the cell constantly collide with other molecules and diffuse through
the cytoplasm in a random walk. The average net distance traveled by such a molecule after a
certain time period t is proportional to the square root of t, i.e. (t)0.5, as well as to its diffusion
coefficient. If, on average, it takes a molecule 100 milliseconds to travel a net distance of 0.5 ยตm
from its starting point, how long would it normally take for the same molecule to travel a net
distance of 5 ยตm from the same starting point?
A. 0.2 second
B. 0.3 second
C. 1 second
D. 10 seconds
E. 0.32 seconds
22.
Sort the following molecules from a low to high rate of diffusion inside the cytosol. Your
answer would be a four-letter string composed of letters A to D only, e.g. ADCB.
(A) Myoglobin (a protein)
(B) Glycine (an amino acid)
(C) Ribosome (a proteinโRNA complex)
(D) CO2
23.
The equilibrium constant for the reaction that breaks down each molecule of substrate A
to one molecule of B and one molecule of C is equal to 0.5. Starting with a mixture containing
only molecules A at 1 M concentration, what will be the concentration of molecule A after
reaching equilibrium under these conditions?
A. 0.5 M
B. 0.25 M
C. 0.125 M
D. 0.333 M
E. 0.667 M
24.
The free-energy change (ฮG) for a simple reaction, A โ B, is 0 kJ/mole at 37ยฐC when
the concentrations of A and B are 10 M and 0.1 M, respectively. What is the free-energy change
for the reaction when the concentrations of A and B are instead 0.01 M and 1 M, respectively?
Recall that ฮGยฐ = โ5.9 ร log(Keq). Write down your answer as a number with the appropriate
sign (+ or โ) and in kJ/mole, e.g. +11.8 kJ/mole.
25.
Imagine the reaction A โ B with a negative ฮG value under experimental conditions.
Which of the following statements is true about this reaction?
A. The reaction is energetically unfavorable.
B. The reaction proceeds spontaneously and rapidly under these conditions.
C. Increasing the concentration of B molecules would increase the ฮG value (toward
more positive values).
D. The reaction would result in a net decrease in the entropy (disorder) of the universe.
E. The reaction cannot proceed unless it is coupled to another reaction with a positive
value of ฮG.
26.
In the first reaction of the glycolytic pathway, the enzyme hexokinase uses ATP to
catalyze the phosphorylation of glucose, yielding glucose 6-phosphate and ADP. The ฮGยฐ value
for this reaction is โ17 kJ/mole. The enzyme glucose 6-phosphatase catalyzes a โreverseโ
reaction, in which glucose 6-phosphate is converted back to glucose, and a phosphate is released.
The ฮGยฐ value for this reaction is โ14 kJ/mole. What is the ฮGยฐ value for the following
reaction?
ATP + H2O โ ADP + Pi
A. โ3 kJ/mole
B. +3 kJ/mole
C. โ31 kJ/mole
D. +31 kJ/mole
E. โ15.5 kJ/mole
27.
The enzyme phosphoglucose isomerase converts glucose 6-phosphate to its isomer
fructose 6-phosphate in the second step of glycolysis. The equilibrium constant for the reaction is
0.36. Evaluating the ฮGยฐ of the reaction (ฮGยฐ = โ5.9 ร log Keq), decide which of the following
conclusions is true.
A. The ฮGยฐ is negative, therefore the reaction proceeds in the forward direction.
B. The ฮGยฐ is negative, but whether or not the reaction proceeds would depend on ฮG,
not ฮGยฐ.
C. The ฮGยฐ is positive, but in a cell that is active in glycolysis, the reaction can still
proceed in the forward direction.
D. The ฮGยฐ is positive, therefore the reaction proceeds in the reverse direction.
28.
Which of the following represents an โactivatedโ carrier molecule?
A. AMP
B. NADH
C. NAD+
D. NADP+
E. CoA
29.
ATP is the main energy currency in cells, and it can especially be used to drive
condensation reactions that produce macromolecular polymers. How does ATP normally catalyze
the condensation reaction, which by itself is energetically unfavorable?
A. It transfers its terminal phosphate to an enzyme and is released as ADP.
B. It transfers its two terminal phosphates to an enzyme, and is released as AMP.
C. It covalently attaches to both of the substrates.
D. It transfers either one or two terminal phosphate(s) to one of the substrates and is
released as either ADP or AMP.
E. It covalently attaches to the enzyme, forming an enzymeโAMP adduct.
30.
Despite their overall similarity, NADH and NADPH are not used indiscriminately by the
cell. What are the distinctive features of these two carrier molecules?
A. NADPH has an extra phosphate near its nicotinamide ring, giving it distinct electrontransfer properties.
B. In the cell, NADH is usually in excess over NAD+, but NADP+ is usually in excess
over NADPH.
C. NADH is normally involved in anabolic reactions, whereas NADPH is normally
involved in catabolism.
D. Both NADPH and NADH are recognized by the same enzymes with similar affinities,
since the extra phosphate group in NADPH is not involved in such recognition.
E. In the cell, NADH is found mostly in the form that acts as an oxidizing agent,
whereas NADPH is found mostly in the form that acts as a reducing agent.
31.
In an enzymatic reaction involving NADH or NADPH, reduction of a substrate
accompanies the oxidation of these carrier molecules to NAD+ or NADP+, respectively. What
else typically happens in such a reaction?
A. A molecule of water is released to the solution upon completion of the reaction.
B. A proton is released during the oxidation of the carriers.
C. A proton is taken up by the substrate that is being reduced.
D. A proton is taken up by the carrier molecule that is being oxidized.
E. A phosphate group is transferred to the substrate.
32.
What is the reaction performed on the molecule labeled as substrate in the following
diagram? What is the name of the activated carrier?
Substrate
Product
A. This is a methylation reaction and the activated carrier is ATP.
B. This is a methylation reaction and the activated carrier is S-adenosylmethionine.
C. This is a carboxylation reaction and the activated carrier is ATP.
D. This is a carboxylation reaction and the activated carrier is carboxylated biotin.
E. This is an acetylation reaction and the activated carrier is acetyl CoA.
33.
Under anaerobic conditions, glycolysis provides most of the ATP that the cell needs. In
animal cells, pyruvate, the end product of glycolysis, is converted to lactic acid by lactate
dehydrogenase, as shown below:
CH3(CO)COOโ + X โCH3(CHOH)COOโ + Y
What is the correct carrier pair (in place of X and Y) in this reaction?
A. X is (ADP + Pi), and Y is (ATP)
B. X is (NADP+), and Y is (NADPH + H+)
C. X is (NAD+), and Y is (NADH + H+)
D. X is (NADH + H+), and Y is (NAD+)
E. X is (NADP++ H+), and Y is (NADPH)
34.
Macromolecules in the cell can be made from their monomers using one of two
polymerization schemes. One is called head polymerization, in which the reactive bond required
for polymerization is carried on the end of the growing polymer. In contrast, in tail
polymerization, the reactive bond is carried by each monomer for its own incorporation. In the
figure, indicate the polymerization scheme and the type of macromolecule.
A. Head polymerization of a protein
B. Tail polymerization of a protein
C. Head polymerization of a polysaccharide
D. Head polymerization of a nucleic acid
E. Tail polymerization of a nucleic acid
35.
What is the end product of glycolysis in the cytoplasm of eukaryotic cells? How many
carbon atoms does the molecule have?
A. Acetyl CoA; it has two carbon atoms attached to coenzyme A
B. Phosphoenolpyruvate; it has three carbon atoms
C. Glucose 6-phosphate; it has six carbon atoms
D. Pyruvate; it has three carbon atoms
E. Glyceraldehyde 3-phosphate; it has three carbon atoms
36.
The substrate for the glycolytic enzyme glyceraldehyde 3-phosphate dehydrogenase is
glyceraldehyde 3-phosphate (with one phosphate group) while its product is 1,3bisphosphoglycerate (with two phosphate groups). Where does the extra phosphate group come
from?
A. From combining two molecules of the substrate
B. ATP
C. Fructose 1,6-bisphosphate
D. Pi
E. NADH
37.
Steps 6 and 7 of glycolysis are catalyzed by the enzymes glyceraldehyde 3-phosphate
dehydrogenase and phosphoglycerate kinase, respectively. Together, they …
A. result in the oxidation of an aldehyde to a carboxylic acid.
B. produce both ATP and NADH.
C. couple the oxidation of a CโH bond to the activation of carrier molecules.
D. catalyze the only glycolytic reactions that create a high-energy phosphate linkage
directly from inorganic phosphate.
E. All of the above.
38.
Arsenate is a toxic ion that can interfere with both glycolysis and oxidative
phosphorylation. Arsenate resembles Pi (inorganic phosphate) and can replace it in many
enzymatic reactions. One such reaction is catalyzed by glyceraldehyde 3-phosphate
dehydrogenase in step 6 of glycolysis. Upon completion of the reaction, instead of the normal
product, 1,3-bisphosphoglycerate, the mixed anhydride 1-arsenato-3-phosphoglycerate is formed;
this undergoes rapid spontaneous hydrolysis into arsenate plus 3-phosphoglycerate, the latter
being a normal product of step 7 in glycolysis. What would be the effect of arsenate poisoning in
glycolysis?
A. It results in more ATP and NADH molecules generated for every glucose molecule.
B. It results in fewer ATP molecules generated per glucose molecule, but NADH
generation is not directly affected.
C. It brings glycolysis to an abrupt stop.
D. It results in fewer ATP and NADH molecules generated per glucose molecule.
E. It does not affect the number of ATP or NADH molecules generated per glucose
molecule.
39.
Which of the following is true regarding energy production and storage in plants and
animals?
A. Plant and animal cells make starch for long-term energy storage.
B. Most of the ATP in a plant cell has been generated in the chloroplast and transported
to other parts of the cell.
C. Oxidation of one gram of starch releases more energy than oxidation of fat, but since
starch absorbs a lot of water, it is not as efficient as fat in energy storage.
D. Animals, but not plants, can store fats in the form of triacylglycerol (triglyceride).
E. Plant seeds often contain large amounts of fats and starch.
40.
What are the molecules that normally supply carbon and oxygen atoms, respectively, for
the citric acid cycle?
A. Oxaloacetate, oxaloacetate
B. Acetyl CoA, O2
C. Oxaloacetate, O2
D. Acetyl CoA, H2O
E. Pyruvate, pyruvate
41.
Indicate if each of the following descriptions matches lipids (1), nucleic acids (2),
polysaccharides (3), or proteins (4). Your answer would be a four-digit number composed of
digits 1 to 4 only, e.g. 1332.
( ) Their monomers contain phosphorus and nitrogen.
( ) They constitute almost half of the cellโs dry mass.
( ) They are the main constituent of all cellular membranes.
( ) They are largely hydrophobic and can store energy.
42.
Sort the following molecules (A to E) based on the oxidation of the carbon atom, from
higher to lower oxidation states. Your answer would be a five-letter string composed of letters A
to E only, e.g. ADCBE. Put the letter corresponding to the highest oxidation level on the left.
A
B
C
D
E
43.
Indicate true (T) and false (F) statements below regarding glycolysis. Your answer would
be a four-letter string composed of letters T and F only, e.g. TTTT.
( ) Molecular oxygen is used in glycolysis to oxidize glucose.
( ) Along the glycolytic pathway, ATP is both consumed and generated.
( ) In the course of glycolysis, one molecule of NADH is formed per molecule of
glucose.
( ) Following the production of one molecule of fructose 1,6-bisphosphate, the rest of the
glycolytic pathway generates four molecules of ATP.
44.
Fill in the blank in the following paragraph.
โDuring intense โanaerobicโ physical exercise, the high energy
demand in the muscle cells leads to an accumulation of lactic acid
in these cells and their surrounding tissues. Similarly, the yeast
Saccharomyces cerevisiae can produce ethanol when grown
anaerobically. The lactate or ethanol production takes place in a
process called …โ
45.
Sort the following molecules based on the amount of energy that is released when their
phosphate bond is hydrolyzed as indicated. Your answer would be a four-letter string composed
of letters A to D only, e.g. ADCB. Put the molecule with the highest amount of hydrolysis energy
on the left.
(A) ATP when hydrolyzed to ADP
(B) Glucose 6-phosphate when hydrolyzed to glucose
(C) 1,3-bisphosphoglycerate when hydrolyzed to 3-phosphoglycerate
(D) Phosphoenolpyruvate when hydrolyzed to pyruvate
46.
Indicate true (T) and false (F) statements below regarding fatty acid metabolism. Your
answer would be a four-letter string composed of letters T and F only, e.g. TTTT.
( ) Most animals derive their energy from fatty acids between meals.
( ) Fatty acids are converted to acetyl CoA in the cytosol, which is then transported into
mitochondria for further oxidation.
( ) Fatty acids are stored in fat droplets in the form of triacylglycerols.
( ) The breakdown of fatty acids into each acetyl CoA unit requires the hydrolysis of two
ATP molecules.
47.
Indicate whether each of the following descriptions matches glycolysis (G) or the Krebs
cycle (K). Your answer would be a four-letter string composed of letters G and K only, e.g.
GGGK.
( ) It oxidizes acetyl CoA to CO2.
( ) In eukaryotic cells, it is carried out in the cytosol.
( ) It produces FADH2.
( ) ฮฑ-Ketoglutarate, one of its intermediates, is used to synthesize the amino acid
glutamic acid.
48.
Indicate whether each of the following molecules is an intermediate in glycolysis (G) or
in the tricarboxylic acid cycle (T). Your answer would be a four-letter string composed of letters
G and T only, e.g. GGTT.
( ) Fumarate
( ) Malate
( ) Phosphoenolpyruvate
( ) Succinate
The Citric Acid Cycle: Questions 49-52
The citric acid cycle is summarized in the following figure. Answer the following question(s)
about this cycle.
?
49.
In step 1 of the citric acid cycle drawn above, what is the molecule indicated with a
question mark?
A. O2
B. ATP
C. H2O
D. H+
E. Pyruvate
50.
In the citric acid cycle shown above, which steps produce CO2 as a by-product? List all
such steps by their number, from the smallest number to the largest. Your answer would be a
number composed of digits 1 to 8 only, e.g. 258.
51.
In the citric acid cycle shown above, which steps produce either NADH or FADH2? List
all such steps by their number, from the smallest number to the largest. Your answer would be a
number composed of digits 1 to 8 only, e.g. 258.
52.
Aconitase catalyzes an isomerization reaction in the citric acid cycle shown above, in
which H2O is first removed and then added back to the substrate. Which step is catalyzed by this
enzyme? Write down the step number as your answer, e.g. 5.
53.
The electron carriers NADH and FADH2 donate their electrons to the electron-transport
chain in the inner mitochondrial membrane, leading to ATP synthesis powered by an H+ gradient
across the membrane. If, on average, the oxidation of each NADH or FADH2 molecule in this
pathway results in the production of 2.5 and 1.5 molecules of ATP, respectively, how many ATP
(and GTP) molecules are produced on average as a result of the complete oxidation of one
molecule of acetyl CoA in the mitochondrion? Consider only the citric acid cycle and oxidative
phosphorylation.
A. 10
B. 12
C. 13.5
D. 14.5
E. 15
54.
Indicate true (T) and false (F) statements below regarding the cellular metabolism of
nucleotides and amino acids. Your answer would be a four-letter string composed of letters T and
F only, e.g. TTTT.
( ) Nitrogen fixation occurs in the mitochondria in most animal cells to generate amino
acids.
( ) All 20 natural amino acids must be provided in our diet and are therefore โessential.โ
( ) There are NO essential nucleotides that must be provided in the diet.
( ) Catabolism of amino acids in our body leads to the production of urea which is
excreted.
Answers
1. Answer: C
Difficulty: 1
Section: The Chemical Components of a Cell
Feedback: Metal ions such as copper, iron, cobalt, and zinc are used as cofactors that are
necessary for the function of some enzymes.
2. Answer: E
Difficulty: 1
Section: The Chemical Components of a Cell
Feedback: Hydrophobic molecules usually have no charge and form no or few hydrogen
bonds, and are therefore not favored by the network of hydrogen bonds in liquid water.
They do dissolve in nonpolar organic solvents.
3. Answer: IHHN
Difficulty: 2
Section: The Chemical Components of a Cell
Feedback: ATP is negatively charged and can form ionic bonds with magnesium ions.
Urea is highly soluble in water due to its hydrogen-bonding capacity. Similarly,
interaction of a polar molecule like glucose with the active site of an enzyme can be
mediated by hydrogen bonds and other noncovalent (or even covalent) bonds. In contrast,
the fatty acid tails in phospholipids are hydrophobic and do not favorably interact with
negatively charged phosphate molecules.
4. Answer: B
Difficulty: 3
Section: The Chemical Components of a Cell
Feedback: Hexane is an alkane hydrocarbon incapable of hydrogen-bonding with water
molecules, which results in an entropically unfavorable state when the two interact. All
the other mentioned chemicals can be readily dissolved in water because they have polar
bonds or can dissociate into ions.
5. Answer: A
Difficulty: 2
Section: The Chemical Components of a Cell
Feedback: The probing of the charged ions by water molecules greatly reduces the bond
energy of ionic bonds (electrostatic interactions) in aqueous solutions. Hydrogen bonds
are also weakened in water, but they are not that strong in a vacuum to begin with.
6. Answer: C
Difficulty: 1
Section: The Chemical Components of a Cell
Feedback: Although each noncovalent bond is weak, when many of them are formed
simultaneously (in a complementary interface), their energies can sum to produce a tight
binding.
7. Answer: B
Difficulty: 3
Section: The Chemical Components of a Cell
Feedback: The concentration of hydronium ions would be the sum of those obtained from
the dissociation of water and the acid: [H3O+] = 10โ7 + 10โ8 = 1.1 ร 10โ7. The pH value
will then be calculated as: pH = โlog [H3O+] = โlog [1.1 ร 10โ7] = 7 โ log (1.1) = 6.96.
This is very close to neutral pH.
8. Answer: A
Difficulty: 3
Section: The Chemical Components of a CellFeedback: The initial number of hydronium
ions would be: [H3O+]1 = 10โ15 L ร 10โ7 mole/L = 10โ22 mole. The final number at pH 5
would be: [H3O+]2 = 10โ15 L ร 10โ5 mole/L = 10โ20 mole. The difference is: [H3O+]2 โ
[H3O+]1 = 9.9 ร 10โ21 mole. This is equivalent to approximately 6000 protons that need to
be pumped in.
9. Answer: D
Difficulty: 3
Section: The Chemical Components of a Cell
Feedback: Due to the presence of the carboxyl group, a fatty acid molecule carries a
negative charge at neutral pH. However, lowering the pH can reverse the ionization of
this group to the neutral (protonated) state.
10. Answer: C
Difficulty: 2
Section: The Chemical Components of a Cell
Feedback: The amino and carboxyl groups are common to all amino acids. The serine
side chain contains a hydroxyl group.
11. Answer: C
Difficulty: 2
Section: The Chemical Components of a Cell
Feedback: The polymerization reaction generally requires a free-energy input. Also, the
addition of each monomer to the growing polymer is a condensation reaction that is
accompanied by the release of one water molecule. The opposite reaction
(depolymerization) involves hydrolysis and consumes one water molecule.
12. Answer: BDCA
Difficulty: 2
Section: The Chemical Components of a Cell
Feedback: Water accounts for about 70% of the total mass in a typical cell. In the
remaining โdry mass,โ proteins constitute about half, the nucleic acids RNA and DNA are
next, and polysaccharides (and their sugar monomers) are still less abundant.
13. Answer: C
Difficulty: 2
Section: Catalysis and the Use of Energy by Cells
Feedback: For a living animal cell, heat-generating reactions from burning of foodstuffs
are โcoupledโ to other reactions that increase order inside the cell. Concomitantly, there is
an increase in the overall entropy of the universe (cell plus its environment), with no
violation of the laws of thermodynamics for a spontaneous process.
14. Answer: โ2 kJ/mole/K
Difficulty: 3
Section: Catalysis and the Use of Energy by Cells
Feedback: For the folding reaction, the free-energy change can be written as:
ฮG = ฮH โ TฮS
Therefore:
ฮS = (ฮH โ ฮG)/T = (โ640 kJ/mole + 40 kJ/mole) / (300 K) = โ2 kJ/mole/K
The negative value of ฮS means a decrease in entropy. This is not unexpected since
folding results in the formation of a single conformation (or a limited set of
conformations) out of an enormous number of possible coils.
15. Answer: D
Difficulty: 2
Section: Catalysis and the Use of Energy by Cells
Feedback: Photosynthesis consumes water and atmospheric CO2 to make simple sugars
and the by-product oxygen.
16. Answer: A
Difficulty: 3
Section: Catalysis and the Use of Energy by Cells
Feedback: Oxidation involves the full or partial removal of electrons from an atom, and
does not necessarily involve oxygen. In the cell, organic molecules usually release a
proton to their surrounding when oxidized in a dehydrogenation reaction, decreasing the
number of CโH bonds in the molecule.
17. Answer: E
Difficulty: 2
Section: Catalysis and the Use of Energy by Cells
Feedback: Enzymes catalyze most cellular reactions by lowering the activation energy,
but they cannot change the equilibrium constant of the reactions that they catalyze; that is,
both forward and reverse reactions are sped up by the same factor. However, they can
selectively drive substrates along one of various cellular metabolic pathways, and can also
couple unfavorable reactions to spontaneous heat-generating reactions.
18. Answer: A
Difficulty: 2
Section: Catalysis and the Use of Energy by Cells
Feedback: The activation energy corresponds to the height of the energy barrier between
the reactant and the product, and is the minimum amount of energy that should be
provided in order for the reaction to proceed.
19. Answer: C
Difficulty: 2
Section: Catalysis and the Use of Energy by Cells
Feedback: In the presence of an enzyme (line 1), the fraction of substrate molecules that
have enough thermal energy to proceed through the reaction is increased compared to that
in the uncatalyzed reaction (line 2).
20. Answer: B
Difficulty: 3
Section: Catalysis and the Use of Energy by Cells
Feedback: This is the reaction catalyzed by the enzyme succinate dehydrogenase in the
citric acid cycle. Succinate is oxidized to fumarate, and the FAD carrier is reduced to
FADH2. By subsequently donating its two electrons to the electron-transport chain,
FADH2 will be converted back to FAD for another round of the reaction.
21. Answer: D
Difficulty: 3
Section: Catalysis and the Use of Energy by Cells
Feedback: The net distance of 5 ยตm is 10 times higher than 0.5 ยตm, and would on
average take 10 seconds (i.e. 102 ร 100 milliseconds) to reach.
22. Answer: CABD
Difficulty: 2
Section: Catalysis and the Use of Energy by Cells
Feedback: In general, larger molecules diffuse more slowly compared to smaller
molecules. Interaction with other molecules (including the solvent) and the shape of the
molecule will also affect the diffusion coefficient.
23. Answer: A
Difficulty: 3
Section: Catalysis and the Use of Energy by Cells
Feedback: The equilibrium constant for this reaction is calculated as:
Keq = 0.5 = [B]eq [C]eq / [A]eq
Since the initial mixture contains only molecule A, it follows that:
[B]eq = [C]eq = 1 M โ [A]eq
Combining these equations and solving for [A]eq, we will have:
[A]eq = 0.5 M
which means the molecules B and C will also be present at 0.5 M at equilibrium.
24. Answer: +23.6 kJ/mole
Difficulty: 4
Section: Catalysis and the Use of Energy by Cells
Feedback: The free-energy change can be written as:
ฮG = ฮGยฐ + RT ln([B]/[A])
When ฮG is equal to zero, the system is at chemical equilibrium, and
ฮGยฐ = โ RT ln([B]eq/[A]eq) = โ RT ln(10โ2) = โ5.9 ร log (10โ2) = +11.8 kJ/mole
When the concentrations are changed, we have:
ฮG = ฮGยฐ + RT ln([B]/[A]) = ฮGยฐ + RT ln(102) = ฮGยฐ โ RT ln(10โ2) = 2 ฮGยฐ = +23.6
kJ/mole
25. Answer: C
Difficulty: 3
Section: Catalysis and the Use of Energy by Cells
Feedback: The negative ฮG value indicates that the reaction is favorable under these
conditions and would increase the entropy of the universe. However, unless we know the
steps of the reaction, the ฮG value cannot predict the reaction rate, because the latter
depends on the activation-energy barrier. Finally, the ฮG value changes as the
concentrations of reactants and products change. As the products accumulate, the reaction
will eventually reach an equilibrium, where ฮG is equal to zero.
26. Answer: C
Difficulty: 3
Section: Catalysis and the Use of Energy by Cells
Feedback: The two reactions described in the question can be written as:
ATP + glucose
โ ADP + glucose 6-phosphate
ฮGยฐ= โ17 kJ/mole
glucose 6-phosphate + H2O โ glucose + Pi
ฮGยฐ= โ14 kJ/mole
Combining these reactions yields the ATP hydrolysis reaction presented in the question.
Since the free-energy changes are additive, the ฮGยฐ value for this combined reaction is
the sum of the ฮGยฐ values for the two reactions above:
(โ17 kJ/mol) + (โ14 kJ/mole) = โ31 kJ/mole.
27. Answer: C
Difficulty: 3
Section: Catalysis and the Use of Energy by Cells
Feedback: Since the equilibrium constant is less than 1, the log Keq term is negative,
making the ฮGยฐ positive, which means the reaction is unfavorable under standard
conditions. But inside a cell performing glycolysis, a lower concentration of fructose 6phosphate than of glucose 6-phosphate can drop ฮG to a negative value. The reaction
thus proceeds in the forward direction, providing a continuous supply of substrate for the
next step in the pathway. Note that due to โcouplingโ of the reactions in the glycolytic
pathway, and even though some steps can have positive ฮGยฐ values, the overall negative
ฮGยฐ of the pathway can drive the entire chain of reactions in the forward direction, even
under the standard conditions.
28. Answer: B
Difficulty: 1
Section: Catalysis and the Use of Energy by Cells
Feedback: The activated carrier molecules carry chemical groups in high-energy linkages
and can deliver the group or the energy (or both) to metabolic reactions when necessary.
They then need to be activated again. NADH is an activated carrier, while its oxidized
form NAD+ is not.
29. Answer: D
Difficulty: 1
Section: Catalysis and the Use of Energy by Cells
Feedback: By transferring either a phosphate group or a pyrophosphate group to a
hydroxyl group on one of the monomers involved in the polymerization, ATP โactivatesโ
the monomer, making the overall reaction favorable.
30. Answer: E
Difficulty: 3
Section: Catalysis and the Use of Energy by Cells
Feedback: The extra phosphate in NADPH does not affect its electron-transfer properties,
but makes it different enough to be recognized by a different set of enzymes. NADPH
operates chiefly with enzymes that catalyze anabolic reactions, which normally need
reducing power. Accordingly, NADPH is found mostly in its reduced form (i.e. in excess
over NADP+) in the cell. The opposite is true for NADH, which is normally involved in
catabolic reactions.
31. Answer: C
Difficulty: 2
Section: Catalysis and the Use of Energy by Cells
Feedback: In a reduction reaction, NADPH (or NADH) is oxidized, donating a hydride
ion to the substrate. A substrate can also capture a proton from the surroundings, creating
two CโH bonds. The carrier is converted to its oxidized form (NADP+ or NAD+).
32. Answer: D
Difficulty: 2
Section: Catalysis and the Use of Energy by Cells
Feedback: The reaction shown is catalyzed by the enzyme pyruvate carboxylase, which
uses a covalently bound carboxylated biotin to carboxylate pyruvate and produce
oxaloacetate.
33. Answer: D
Difficulty: 3
Section: Catalysis and the Use of Energy by Cells
Feedback: Under anaerobic conditions, NAD+ can be recycled in this reaction (in which
pyruvate is reduced), so that glycolysis can continue in the absence of oxidative
phosphorylation.
34. Answer: E
Difficulty: 2
Section: Catalysis and the Use of Energy by Cells
Feedback: Each nucleotide monomer is activatedโat the expense of hydrolysis of two
ATP moleculesโinto an intermediate carrying a reactive phosphoanhydride bond.
35. Answer: D
Difficulty: 1 Section: How Cells Obtain Energy from Food
Feedback: In glycolysis, two pyruvate molecules (each with three carbon atoms) are
produced from each molecule of glucose (with six carbon atoms).
36. Answer: D
Difficulty: 2
Section: How Cells Obtain Energy from Food
Feedback: In step 6 of glycolysis, this enzyme couples the oxidation of the substrate with
the production of NADH, as well as incorporation of inorganic phosphate. The Pi is then
transferred to ADP to generate ATP in step 7.
37. Answer: E
Difficulty: 2
Section: How Cells Obtain Energy from Food
Feedback: Please refer to Figure 2โ48.
38. Answer: B
Difficulty: 3
Section: How Cells Obtain Energy from Food
Feedback: Since step 7 is bypassed, the ATP molecules that are naturally generated in that
step are no longer produced; however, NADH is still made as before in the first half of
step 6. Arsenate also has other effects on cell metabolism that collectively make it a toxic
compound. Please refer to Figure 2โ48.
39. Answer: E
Difficulty: 1
Section: How Cells Obtain Energy from Food
Feedback: Compared to the polysaccharides glycogen (in animals) and starch (in plants),
fat is more efficient as a long-term energy storage both per gram and per volume. It can
be stored as triglycerides in both plants and animals, although the types of fatty acids
vary. In plant cells, chloroplasts generate sugars that can be oxidized by the mitochondria
to generate ATP for the cell. The ATP produced in the chloroplast by photosynthesis
cannot be transported out of that organelle.
40. Answer: D
Difficulty: 2
Section: How Cells Obtain Energy from Food
Feedback: One molecule of acetyl CoA enters the cycle by combining with oxaloacetate.
41. Answer: 2411
Difficulty: 2
Section: The Chemical Components of a Cell
Feedback: The nucleotides contain one to three phosphate groups and a nitrogencontaining base, and are polymerized to form long nucleic acid molecules such as DNA.
Proteins are made of amino acids and make up half of the dry mass of the cell, i.e.
approximately 15% of the total cell weight. Lipids have large hydrophobic fatty acid
chains and, in addition to forming bilayer membranes, can store food energy and release it
when necessary.
42. Answer: DAECB
Difficulty: 3
Section: Catalysis and the Use of Energy by Cells
Feedback: As a general rule, in organic molecules, a lower number of CโH bonds
corresponds to more oxidized carbon atoms.
43. Answer: FTFT
Difficulty: 2
Section: How Cells Obtain Energy from Food
Feedback: During the stepwise oxidation of glucose in the course of glycolysis, two
molecules of ATP are used to make fructose 1,6-bisphosphate, which is then cleaved and
eventually converted to two molecules of pyruvate, generating four molecules of ATP and
two molecules of NADH.
44. Answer: fermentation
Difficulty: 1
Section: How Cells Obtain Energy from Food
Feedback: Fermentation is an energy-yielding pathway and is often anaerobic.
45. Answer: DCAB
Difficulty: 2
Section: How Cells Obtain Energy from Food
Feedback: Hydrolysis of phosphoenolpyruvate to pyruvate is the most exergonic (releases
the highest amount of energy). ATP can be generated from ADP upon the hydrolysis of
1,3-bisphosphoglycerate to 3-phosphoglycerate. ATP hydrolysis can be used to drive the
phosphorylation of glucose.
46. Answer: TFTF
Difficulty: 2
Section: How Cells Obtain Energy from Food
Feedback: Between meals, fatty acids stored in the fat droplets in adipocytes in the form
of triacylglycerol are released by hydrolysis and enter the bloodstream. Upon entry into
other cells, they are transported to the mitochondria where they are mostly converted to
acetyl CoA in a step-by-step manner, each step producing one FADH2 and one NADH
molecule.
47. Answer: KGKK
Difficulty: 2
Section: How Cells Obtain Energy from Food
Feedback: In the citric acid cycle (Krebs cycle), which takes place in the mitochondria,
the carbon atoms of acetyl CoA are oxidized and released as CO2, while NADH, FADH2,
and GTP are generated in the process. Many intermediates of the citric acid cycle and
glycolysis are precursors for the biosynthesis of important small molecules in the cell.
48. Answer: TTGT
Difficulty: 2
Section: How Cells Obtain Energy from Food
Feedback: Phosphoenolpyruvate is converted to pyruvate in the last step of glycolysis.
Succinate, fumarate, and malate are three consecutive citric acid cycle intermediates
leading to the regeneration of oxaloacetate.
49. Answer: C
Difficulty: 3
Feedback: In the first step of the citric acid cycle, CoA is hydrolyzed by water after the
formation of a citryl CoA intermediate.
50. Answer: 34
Difficulty: 3
Feedback: Steps 3 and 4 are catalyzed by isocitrate dehydrogenase and the ฮฑ-ketoglutarate
dehydrogenase complex, respectively, and involve decarboxylation of the substrates and
the release of carbon dioxide.
51. Answer: 3468
Difficulty: 3
Refer to: The Citric Acid Cycle
Section: How Cells Obtain Energy from Food
Feedback: Steps 3, 4, and 8 produce NADH, while step 6 produces FADH2.
52. Answer: 2
Difficulty: 2
Feedback: Aconitase converts citrate to isocitrate through an aconitate intermediate
created by dehydration of the substrate.
53. Answer: A
Difficulty: 3
Section: How Cells Obtain Energy from Food
Feedback: The complete oxidation of a molecule of acetyl CoA results in the production
of three NADH molecules plus one FADH2 and one GTP (or ATP) molecule. Therefore,
as a result of oxidative phosphorylation, a total of 10 molecules are generated:
(3 ร 2.5) + (1 ร 1.5) + 1 = 10
54. Answer: FFTT
Difficulty: 2
Section: How Cells Obtain Energy from Food
Feedback: All known nitrogen-fixing cells are prokaryotic microorganisms. Animals rely
on their dietary intake of protein and nucleic acids as sources of useful nitrogen.
However, only 9 of the 20 amino acids that make up proteins and none of the nucleotides
that make up nucleic acids are essential; the remainder can be synthesized from other
ingredients in the diet. When amino acids in our body are degraded, their nitrogen atoms
eventually appear in urea molecules which are excreted.
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