Chapter 3 Review, pages 160–165
Knowledge
1. c
2. a
3. b
4. d
5. d
6. b
7. a
8. a
9. d
10. b
11. False. In an endothermic reaction, the products contain more potential energy than the
reactants.
12. True
13. True
14. True
15. False. ATP hydrolysis is an example of an exergonic reaction.
16. False. Approximately 10 million ATP molecules are hydrolyzed and re-synthesized every
second in a typical cell.
17.True
18. False. The shape of an enzyme is what mainly determines the function of the enzyme
19.True
20. (a) iv
(b) iii
(c) ii
(d) i
21. (a) iv
(b) i
(c) ii
(d) iii
22. (a) iv
(b) i
(c) iii
(d) ii
23. (a) The O–H bond requires more energy to break because the oxygen atom is very
electronegative as compared to carbon atom.
(b) The C–O bond releases more energy. The bond C–C bond has a lower average bond energy.
24. Yes, it is possible for a catabolic pathway to have steps in which ΔG is positive. The overall
effect of a catabolic pathway is the release of energy, but some steps may require the addition of
energy.
25. Answers may vary. Sample answer: The first law of thermodynamics states that energy can
never be created or destroyed but only changed from one form to another; the second law of
thermodynamics states that disorder always increases during energy transformations, as some
free energy is always lost during transformations.
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Chapter 3: An Introduction to Metabolism 3-2
26. Energy coupling is an important process in which the energy released by an exergonic
reaction is used to provide a source of energy for an endergonic reaction. Without this energy no
endergonic reactions could proceed. The cell could not perform any processes requiring energy.
27. The use of ATP as an energy source in cells is so widespread because ATP provides
generous amounts of energy while being easy to recycle and reuse without the production of
extra wastes. ATP is the primary carrier of free energy used to drive almost all endergonic
processes in all living things. Since all organisms use it as a means of transferring energy within
the cell, this indicates that the process was developed early in the history of life.
28. When scientists name enzymes, the root of their name is often based upon the substrate, and
the names of many enzymes end in the suffix -ase. This makes it easy to recognize an enzyme’s
function based on its substrate. Sample example: Protease breaks down proteins and lipase
breaks down lipids.
29. The three ways an enzyme can help a reactant molecule reach the transition state are:
1) enzymes can bring reactants closer together; 2) enzymes can expose a reactant molecule to
charged environments to promote catalysis; and 3) enzymes can change the shape of a substrate.
30. (a) A represents the energy of the reactants; B represents the energy of the transition state;
C represents the energy of the products; D represents the activation energy; and E represents the
free energy that is released by the transition from reactants to products.
(b) The reaction is exothermic as there is less energy in the products than the reactants.
Understanding
31. The process shown is photosynthesis. During photosynthesis, plants capture light energy
from the Sun. This energy is then converted by the plants into chemical potential energy. The
potential energy is stored in plant cells in the form of glucose and other molecules. During the
process, some energy is released as thermal energy, and some is stored as potential energy, but
the total amount of energy does not change.
32. Organisms must constantly consume energy-rich food molecules to survive because for each
activity in the cell some energy is converted into thermal energy, most of which is released as
waste. The overall amount of energy is always decreasing. Therefore, energy must be replaced
using food molecules.
33. (a) The transition state of a reaction is a temporary, usually higher-energy, state in which
bonds between atoms of the reactants are breaking as new bonds between atoms of the products
are able to form.
(b) Reactants must overcome the activation energy barrier to achieve the transition state.
34. (a) Answers may vary. Sample answer: To demonstrate a spontaneous endothermic process
you could melt an ice cube at a high temperature.
(b) Answers may vary. Sample answer: Melting an ice cube at a high temperature obeys the
second law of thermodynamics. The thermal energy moving from the source of thermal energy
to the melting ice can be seen as a movement from order to disorder.
35. Although the ordered structure of an energy pyramid may appear to contradict the second law
of thermodynamics, the waste energy given off by biological reactions increases the entropy of
the environment, so that the result for the entire biosystem is an increase in entropy.
36. An exothermic reaction and increases entropy are similar in that they both occur
spontaneously.
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Chapter 3: An Introduction to Metabolism 3-3
37. Table 1
Free energy
(released or absorbed)
Change in
free energy
(+ or – )
absorbed
large
+
Anabolic
released
small
–
Catabolic
38. Free energy is the energy available to do work. If there is a release of free energy during a
reaction (it is exergonic) then that energy could be used to do work. If a reaction must gain free
energy to proceed (it is endergonic) then work must have been done to make the reaction happen.
39. Photosynthesis is an example of an endergonic reaction, as it requires a constant input of
electromagnetic energy to take place. It is classified as endothermic because the products have
more free energy than the reactants.
40. Any type of biological reaction that depends on ATP for energy results in products with
higher levels of order or size than reactants. In general, synthesis reactions would be more likely
to require added energy from ATP.
41. If ATP is in close contact with a reactant molecule, it can help to drive an endergonic
reaction by transferring a phosphate group to the reactant. This transfer causes a release in
energy that can drive the reaction forward.
42. ATP is considered the energy currency of all living cells because ATP is the primary carrier
of free energy used to drive almost all endergonic processes in all living things. Like currency,
the ATP is constantly recirculated to release, and then store, energy for reactions.
43. Answers may vary. Sample answer: The ATP cycle is the continued hydrolysis and
resynthesis of ATP. ADP uses energy from a source (such as respiration) to undergo a synthesis
reaction with Pi, forming ATP. Then the ATP again reacts to form ADP and Pi, releasing energy
to do work, and then the process repeats.
44. In most cases, even in a spontaneous reaction, only a few molecules reach the transition state
of the reaction in a given time without any energy input. Enzymes lower the activation energy
and increase the rate at which reactants reach the transition state. Since time is important in a
classroom, adding an enzyme can increase the number of successful collisions in the reaction,
thereby increasing the rate of the reaction, helping it to proceed in a reasonable timeframe.
45. In order for any cellular reactions to proceed, they need to overcome an activation barrier.
This barrier is an amount of energy that needs to be added before the reaction can proceed.
Therefore cells need an initial source of energy to overcome these barriers, usually in the form of
thermal energy.
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Relative size of
product
Chapter 3: An Introduction to Metabolism 3-4
46. A substrate fits into an active site on an enzyme that is specific for only that substrate. In
some cases the substrate alters the enzyme so that it fits more readily into the site, or the enzyme
“bends” around the substrate for the best fit.
47. Answers may vary. Sample answer: Once a substrate binds to an enzyme it reaches its
transition state. The substrate may also change shape or be exposed to an environment where it
comes into contact with other reactants, allowing them to react as well.
48. Answers may vary. Answers should include the following concepts from the chapter: An
increase in enzyme concentration or temperature will speed up the reaction rate because the
temperature increase will add extra energy to the reaction, and an increase in enzyme
concentration will make more active sites available to the reactants. An increase in substrate
concentration may increase the reaction rate to a point, but if the substrate concentration
becomes too great it may not affect the rate because there will not be enough active sites for all
of the substrate molecules.
49. The electron and the atomic nucleus exist in a dynamic relationship. As the electron moves
toward the nucleus it decreases its potential energy and it increases its potential energy as it
moves away from the nucleus. Work must be done to move the electron away from the
positively-charged nucleus, causing an increase in potential energy.
50. Fat molecules provide the highest source of energy of all types of food because fats contain
the highest ratio of C–H bonds, when compared to other macromolecules. C–H bonds are easily
broken, and therefore when these same atoms form new, much stronger bonds with oxygen, large
net amounts of energy are released.
51. You would find the chemical potential energy that is stored in food in the electrons held in
the chemical bonds of the food molecules.
52. The potential energy that is stored within a molecule is released when the atoms are
rearranged and new bonds are formed
53. (a) In the process of glycolysis, ∆G increases for steps 1 through 6, with most of this energy
being supplied by two ATPs. Then, the ∆G decreases dramatically from step 6 to the end of
glycolysis. Much of this free energy is captured during the synthesis of four ATP and two highenergy NADH.
(b) Overall, the process has a negative ∆G and is catabolic.
54. During a combustion reaction, a large amount energy is rapidly given off as thermal energy.
In a controlled oxidation process there are many steps, energy release occurs in small amounts in
each step, and it is easier for the cell to use this energy; as a result there is not as much wasted
thermal energy.
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Chapter 3: An Introduction to Metabolism 3-5
Analysis and Application
55. Photosynthesis in plants is a good example of the first law of thermodynamics in action.
Electromagnetic energy from the Sun is transferred to plants, which transform it into chemical
potential energy through photosynthesis. This energy is stored in plant cells in the form of
glucose and other complex molecules. The energy in these sugars and other molecules can then
be transformed into other forms of energy to do work in the plant or transferred to other
organisms that eat the plant. These organisms will use the energy in many ways, including
transforming it into kinetic energy to do work. The original electromagnetic energy has been
transferred and transformed, but not lost.
56. (a) (i) 2 C2H6 + 7 O2 → 4 CO2 + 6 H2O
(ii)
Reactants
Bond
Average bond
Number of
energy (kJ/mol) bonds in ethane
C–H
411
12
O–H
459
C–C
346
2
C–O
359
C=O
799
O=O
494
Since the energies above are for 2 moles of ethane,
Number of
bonds in oxygen
Total bond
energy
4932
692
7
3458
total bond energy =
= 4541 kJ/mol
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Chapter 3: An Introduction to Metabolism 3-6
Products
Bond
Average bond
energy (kJ/mol)
Number of
bonds in water
C–H
411
O–H
459
C–C
346
C–O
359
C=O
799
O=O
494
Since the energies above are for 2 moles,
Number of
bonds in carbon
dioxide
12
Total bond
energy
5508
8
6392
total bond energy =
= 5950 kJ/mol
(iii) net energy change = (total bond energy of products − total bond energy of reactants)
= (5950 kJ/mol − 4541 kJ/mol)
= 1409 kJ/mol
1409 kJ of energy is released by the combustion of 1 mol of ethane gas.
b)
(i) C6H12O6 + 6 O2  6 CO2 + 6 H2O
(ii)
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Chapter 3: An Introduction to Metabolism 3-7
Reactants
Bond
Average Bond
energy (kJ/mol)
Number of
bonds in
fructose
7
5
5
5
1
C–H
411
O–H
459
C–C
346
C–O
359
C=O
799
O=O
494
total bond energy = 9496 kJ/mol + 2964 kJ/mol
= 12 460 kJ/mol
Products
Bond
Average bond
Number of
energy (kJ/mol) bonds in water
Number of
bonds in oxygen
Total bond
energy
6
2877
2295
1730
1795
799
2964
Number of
bonds in carbon
dioxide
Total bond
energy
C–H
411
O–H
459
12
5508
C–C
346
C–O
359
C=O
799
12
9588
O=O
494
total bond energy = 5508 kJ/mol + 9588 kJ/mol
= 15 096 kJ/mol
(iii) net energy change = total bond energy of products − total bond energy of reactants
= 15 096 kJ/mol − 12 460 kJ/mol
= 2636 kJ/mol
Therefore the net energy released from the combustion of one mole of fructose is 2636 kJ.
Since fructose has a mass of 180 g/mol, the energy released from the combustion of one gram of
fructose is
, or 14 kJ/g.
57. (a) Answers may vary. Sample answer: When glucose is oxidized, a large amount of the
energy is released as thermal energy, reducing the efficiency of the reaction. This energy is
unusable by the body and is given off.
(b) My answer follows the second law of thermodynamics, which states that some of the energy
released by a reaction is unusable for work by the system and is lost as thermal energy.
58. A reaction is spontaneous if its reactants have more free energy than its products, that is, if
ΔG is negative. In this case, the reactants have 5672 kJ of free energy and the products have
2347 kJ of free energy. 5672 is greater than 2347, so the reaction is spontaneous.
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Chapter 3: An Introduction to Metabolism 3-8
59. (a) (i) 2 NH2CH2COOH + 3 O2  3 CO2 + 3 H2O + CO(NH2)2
(ii)
(iii) Reactants
Bond
Average bond
energy (kJ/mol)
C–H
411
O–H
459
C–C
346
C–O
359
C=O
799
O=O
494
C–N
305
N–H
391
Since energies above are for 2 moles,
Number of
bonds in glycine
4
2
2
2
2
Number of
bonds in oxygen
3
2
4
Total bond
energy
1644
918
692
718
1598
1482
610
1564
total bond energy =
= 4613 kJ/mol
Products
Bond
C–H
O–H
C–C
C–O
C=O
O=O
C–N
N–H
Average bond Number of
energy
bonds in
(kJ/mol)
urea
411
459
346
359
799
494
305
391
Number of
bonds in
water
Number of
bonds in
carbon
dioxide
6
1
2
4
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Total bond
energy
2754
6
5593
610
1564
Chapter 3: An Introduction to Metabolism 3-9
Since energies above are for 2 moles,
total bond energy =
= 5260.5 kJ/mol
(iii) net energy change = total bond energy of products − total bond energy of reactants
= (5260.5 kJ/mol – 4613 kJ/mol) = 647.5 kJ/mol
(iv) determine the molar mass of glycine = NH2CH2COOH
= 75.08 g/mol
(v) determine energy = (energy per mol)/ (molar mass)
=( 647.5 kJ/mol)/ (75.8 g/mol)
= 8.54 kJ/g
(vi) Therefore the total energy content per gram of glycine is 8.54 kJ/g.
b) (i) C6H12O6 + 6 O2  6 CO2 + 6 H2O
(ii)
Reactants
Bond
Average bond
Number of
energy (kJ/mol) bonds in glucose
C–H
411
7
O–H
459
5
C–C
346
5
C–O
359
7
C=O
799
O=O
494
total bond energy = 9415 kJ + 2964 kJ
= 12 379 kJ
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Number of
bonds in oxygen
Total bond
energy
2877
2295
1730
2513
6
2964
Chapter 3: An Introduction to Metabolism 3-10
Products
Bond
Average bond
energy (kJ/mol)
Number of
bonds in water
C–H
411
O–H
459
12
C–C
346
C–O
359
C=O
799
O=O
494
total bond energy = 5508 kJ/mol + 9588 kJ/mol
= 15 096 kJ/mol
Number of
bonds in carbon
dioxide
Total bond
energy
5508
12
9588
= total bond energy of products − total bond energy of reactants
= 15 096 kJ/mol − 12 379 kJ/mol = 2717 kJ/mol
(iv) determine the molar mass of glucose = C6H12O6 = 180.18 g/mol
(v) determine energy = (number of mol)(energy per mol)
= (1/180.18 g/mol )( 2717/mol)
= 15.1 kJ/g
(vi) Therefore the amount of energy released from the combustion of glucose with oxygen is
15.1 kJ/g and this reaction is exothermic.
c) Both have about the same energy content per gram with the protein being slightly higher:
17.07 kJ/g > 15.01 kJ/g.
d) This comparison might suggest that carbohydrates and proteins, or at least amino acids, are
comparable as energy sources.
60. (a) Since the free energy of the reactants in cellular respiration is greater than the free energy
of the products, cellular respiration is an exergonic process.
(b) It is a spontaneous reaction.
(c) Some of the energy released by the oxidation of glucose is transferred to other reactions, such
as the synthesis of ATP. Some energy is released as thermal energy.
61. (a) Answers may vary. Any part of a living thing, such as a leaf, a twig, a piece of fruit, or a
honeycomb, would show a highly ordered structure.
(b) Answers may vary. Sample answer: Even though the structure being examined is highly
ordered, the release of waste energy as it grew and developed and as it maintains its structure
results in an increase of entropy in the environment.
62. Any exergonic reaction in a living thing will involve the breakdown of larger reactants into
smaller products. Conversely, in an endergonic reaction, larger products are made from smaller
reactants. Therefore, exergonic products tend to be smaller than endergonic products in living
things.
63. Answers may vary. Sample answer: Enzymes added to laundry detergent allow stains to be
removed at lower temperature and with less agitation. This means that the washing machine
would require less energy to get clothes clean if enzymes are present. This results in using less
electrical energy, and therefore benefits the environment.
(iii) net energy change
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Chapter 3: An Introduction to Metabolism 3-11
64. (a) Answers may vary. Answers should include two of the following: There may be an
enzyme required to accelerate the reaction that the scientist has not included, the enzymes that
are included may require the thermal energy generated by other reactions in the body to become
activated, there may be a cofactor involved that is not present in the lab, or the concentration of
the enzyme or substrate may not be optimal.
(b) Answers may vary. Answers should include two of the following: In order to increase the
rate of the reaction the scientist may wish to add thermal energy, increase the concentration of
either the substrate or the enzyme, and verify that there are no cofactors involved in the reaction.
65. (a) Olive oil is a fat, so it has an abundance of C–H bonds.
(b) C–H bonds are easily broken, so olive oil, like other fats, has a high-energy content. The
types of bonds present in a compound indicate a great deal about the energy content of a
molecule. This is because the amount of energy released by a bond is related to the type of bond
it is.
66. In general terms, as electrons change from being in a weak bond to being in a strong bond,
they move closer to a nucleus, or close to a larger nucleus. In general, the farther an electron is
from the nucleus, the more loosely it is held. Also in general, larger nuclei exert a stronger hold
on electrons than smaller nuclei do.
67. Answers may vary. Sample answer: Humans are more complex and generally more mobile
than snakes. In addition, we are homoetherms, and regulate our body temperature using our own
energy. Snakes regulate their body temperature mostly using energy from their environment, so
they require a lot less energy to be produced internally. Because the anabolic processes in
humans are more active, and we have higher energy requirements, the catabolic processes in
humans have to be more active as well in order to provide the energy we need.
68. Answers may vary. Sample answer: In general there is not enough ATP available in the foods
we can gather and eat to sustain us, but as long as we can synthesize an adequate initial amount
of ATP, our bodies can easily reassemble it using energy available from the many foods we eat.
69. Answers may vary. Sample answer: Coenzymes may be able to transport substrates to an
enzyme or help them bind to an enzyme.
Evaluation
70. Answers may vary. Sample answer: It would be impossible to have a perpetual motion
machine that is used to supply power to your home. Perpetual motion machines claim to use
energy without making use of an energy source. This violates the first law of thermodynamics,
which states that energy is neither created nor destroyed. If a machine can do work perpetually, it
needs to get the energy to do that work from somewhere. Perpetual motion machines violate the
second law of thermodynamics, which states that as energy is transformed from one form to
another, some becomes unstable and increases the entropy of the universe. As perpetual motion
machines transform stored energy into kinetic energy (to do work) some of this energy must
become unstable and would no longer be useful to the machine. Eventually the machine would
run out of energy and stop working, unless it received energy from a source.
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Chapter 3: An Introduction to Metabolism 3-12
71. Answers may vary. Sample answer: If enzymes could be used to convert wood into useable
energy, we could extract energy from wood using enzymes instead of by burning it. This could
result in much less air pollution. Also, enzymes could help convert recycled wood into a fuel
source in an efficient, cost-effective way. Less thermal energy would be lost in the process and
possibly more energy obtained per unit of wood.
72. (a) Answers may vary. Sample answer: Delaying the food ripening process and then
initiating it at a particular time benefits the producer, the transporter, and the seller because it
keeps the food from spoiling during transport, ensuring less waste and a good price for the fruit
when it is sold. It also benefits all of us who eat fruit because it ensures that ripe fruit is always
available. It does not benefit local food producers, because if a large company can ensure fresh
fruit year round, even if it has to be transported from far away, many stores will only buy from
them, instead of from local producers who can only provide fresh fruit for a short season.
(b) Answers may vary. Sample answer: Environmentally there are both benefits and drawbacks.
As fruit degrades it releases methane, a greenhouse gas. Being able to slow the ripening process
ensures that the fruit that is picked ends up being eaten instead of being discarded. This helps
reduce the environmental impact of the fruit. However, the transport of these products around the
globe increases greenhouse emissions, increasing the global impact.
(c) Answers may vary. Sample answer: I do not think these foods need to be labeled. The
enzyme inhibitors that are used generally only affect plants, and can be quickly released into the
air or washed off. I do not believe they pose any threat to humans.
73. Answers may vary. Sample answer: The crop without the C=O bonds should be selected.
Double bonds between C and O are very strong and difficult to break, so breaking them uses
large amounts of energy. The combustion reaction for the crop with C=O bonds will have a high
activation energy. This could make the reaction to extract energy from this fuel dangerous for
widespread use.
Reflect on Your Learning
74. Answers may vary. Sample answer: I am more likely to purchase detergents that contain
enzymes because the enzymes can lower the activation energy required for the reaction that
removes spots from clothes. This means less hot water and agitation is required, which has a
positive environmental effect.
75. Answers may vary. Sample answer: I think that as the global population increases we will
become more conscious of the energy we use and the need to use less, to avoid depleting our
resources. Since enzymes lower activation energy for many reactions, their use can help us
achieve this goal, so I think we will rely more and more on enzymes. In addition, as research
discovers more enzymes and the roles they play, there will probably be more products that
contain enzymes available to us as time goes on.
76. Answers may vary.
Research
77. Answers may vary. Answers should include the following: Chemical enzymes slough off
damaged and dead tissue. Advantages of enzymatic debridement include: some enzymes are
selective, some are not, it is usually fast acting, and there is minimal damage to healthy tissue.
Disadvantages of enzymatic debridement include: it is expensive, a prescription is required and
inflammation is possible.
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Chapter 3: An Introduction to Metabolism 3-13
78. Answers may vary. Sample answer: Some teeth whiteners contain enzymes to help dissolve
stains off of tooth enamel by lowering the activation energy of the necessary reaction. Some
baby food has been treated with enzymes to partially digest it, making it easier for babies’
inexperienced digestive systems to digest.
79. (a) Answers may vary. Sample answer: Muscle and brain cells have much higher energy
demands than fat cells do.
(b) Since muscle and brain cells use more energy than fat cells do, they have a higher
consumption rate of ATP. While fat is active tissue, its main function is storage, and it does not
use a lot of energy. Brain cells are quite active and require significant amounts of ATP to send
and process nerve signals. Muscles also require large quantities of ATP to facilitate movement.
80. Answers may vary. Answers should include the following information: 1) Most meat
tenderizers are refined from plants. Both bromelain and papain are known as proteases. They
break down the connective tissue by breaking down proteins. 2) These enzymes are used in
meats (which are identified as “seasoned”), bagged vegetables, condiments, soups, dairy
products, and even seafood. They are widespread as they help give food a pleasant texture or
taste. 3) The foods are only required to include the ingredient or process on their list of
ingredients. 4) In general, enzymes are injected just prior to slaughter. In Canada these additives
must be approved even though they are natural products, as they are considered to affect the
natural character of foods in a significant way.
81. Answers may vary. Answers may include the following information:
A co-factor is a substance that influences the effect of an enzyme. Some co-factors activate an
enzyme. Others deactivate an enzyme, sometimes by acting as a competitive inhibitor.
Competitive inhibition is a process in which an inhibitor blocks a binding site on an enzyme or
on a substrate, so that the enzyme cannot bind with the substrate. The result is a slowed or
stopped reaction. Both types of co-factor can be beneficial.
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Chapter 3: An Introduction to Metabolism 3-14