Chapter 5 Review, pages 246–251
Knowledge
1. a
2. b
3. b
4. d
5. d
6. b
7. b
8. b
9. b
10. a
11. True
12. False. Chlorophyll a, when excited, becomes oxidized as it passes an electron to a primary
electron acceptor.
13. False. One molecule required for photosynthesis is carbon dioxide (or water, or chlorophyll).
14. False. ATP synthesis takes place in the stroma of a chloroplast.
15. True
16. False. In photosynthesis, light is responsible for providing the energy to remove electrons
from H2O.
17. False. Melvin Calvin determined that atmospheric CO2 was incorporated indirectly into
glucose during the Calvin cycle.
18. True
19. False. Most autotrophs and heterotrophs carry out cellular respiration.
20. False. ATP synthase generates ATP by oxidative phosphorylation.
21. (a) v
(b) iv
(c) i
(d) iii
(e) ii
22. (a) ii
(b) iv
(c) v
(d) iii
(e) i
23. An analogy to explain the role of accessory pigments in photosynthesis is accessory pigments
are like radio antenna in that they receive many varied frequencies, but only react to specific
ones by sending them to the receiver (chlorophyll a).
24. The net reaction of the Calvin cycle in redox terms: high-energy electrons and protons (H+)
reduce fixed CO2 in order to manufacture carbohydrates and other metabolites.
25. The significance of the products of the light reactions of photosynthesis is that the light
reactions produce oxygen, high-energy ATP, and NADPH. The Calvin cycle uses these ATP and
NADPH molecules from the light reactions, along with carbon dioxide, to produce sugars.
26. No, the light-dependent reactions and the Calvin cycle do not occur in the same part of the
chloroplasts. The light-dependent reactions occur in the thylakoid membranes, while the lightindependent reactions (Calvin cycle) occur in the stroma of the chloroplasts.
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Chapter 5: Photosynthesis: The Energy of Life 5-2
27. The role of the non-protein organic groups in the light-dependent reactions of photosynthesis
is to act as electron carriers and redox agents.
28. The sequence of the redox reactions that take place in the electron transport system of the
light reactions of photosynthesis is as follows:
• Absorption of photon and oxidation of P680 by the primary electron acceptor of PSII
• P680+ oxidizes water
• Oxidation of the primary acceptor by plastoquinone
• Oxidation of plastoquinone by the cytochrome complex
• Oxidation of cytochrome complex by plastocyanin
• Oxidation of plastocyanin by P700+ (after it has been energized by a photon and oxidized by
the primary electron acceptor of PSI)
• Oxidation of the PSI primary electron acceptor by ferredoxin
• Oxidation of ferredoxin by NADP+
29. Ferredoxin is an important molecule for the Calvin cycle and other energy-dependent
reactions in the chloroplasts because it transfers electrons from PSI to either NADP+ or to PQ.
The transfer to PQ begins cyclic electron transport, which allows for the production of the
additional ATP from photosystem I that is needed by the Calvin cycle and other energydependent reactions in the chloroplast.
30. The role of ATP synthase is exactly the same in mitochondria and in chloroplasts. ATP
synthase in the thylakoid membrane or in the mitochondrial membrane captures the energy of
protons moving through the channel formed by the ATP synthase to phosphorylate ADP,
producing ATP.
31. (a) P680+ oxidizes water, producing oxygen and causing it to be released into the
atmosphere.
(b) When P680+ pulls the electron from the water, it releases the H+ into the thylakoid lumen,
helping to create a proton gradient.
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Chapter 5: Photosynthesis: The Energy of Life 5-3
32. Answers may vary. Sample answer:
33. When the CO2:O2 ratio drops, rubisco takes an O2 into the Calvin cycle rather than a CO2
molecule. This results in photorespiration.
34. Two ways photorespiration is detrimental to a plant are that photorespiration uses ATP and it
results in the release of fixed carbon as CO2 from the cell.
35. Photorespiration is a greater problem in warmer climates versus cooler climates because the
solubility of carbon dioxide decreases more rapidly than that of oxygen as the temperature
increases. This increases the availability of oxygen at the rubisco active site. In addition, plants
may need to close their stomata in hot and dry weather to conserve water, but this also prevents
gas exchange. As carbon dioxide is consumed during the Calvin cycle, its concentration drops
and photorespiration increases. In high temperatures, as much as 50 % of the plant’s energy can
be wasted by photorespiration.
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Chapter 5: Photosynthesis: The Energy of Life 5-4
36. The C4 cycle and spatial separation of the Calvin cycle help plants avoid photorespiration by
reducing the exposure of rubisco to oxygen. The C4 cycle allows plants to sequester carbon
dioxide in the form of malate and make it available to rubisco in high concentration in the bundle
sheaths away from high concentrations of oxygen.
37. The steps in the C4 cycle are: CO2 first combines with three-carbon PEP to produce fourcarbon oxaloacetate, which is reduced to malate by electrons transferred from NADPH. Once it
reaches the site of the Calvin cycle, the malate gets oxidized to pyruvate, releasing CO2, and
finally pyruvate is converted back to PEP in a reaction that uses ATP.
38. The advantage that C4 plants have in an arid climate is that C4 plants prevent photorespiration
by sequestering carbon dioxide more efficiently. They do not need to keep their stomata open for
as long as a C3 plants, which conserves water.
39. Table 1
Mitochondria
Chloroplasts
• ovoid shape
• ovoid shape
• have multiple membranes
• have multiple membranes
• have own DNA
• have own DNA
Structure • have intermembrane space
• have intermembrane space
(matrix)
(stroma)
• cristae continuous with
• thylakoids discontinuous
inner membrane
with inner membrane
• generate ATP from glucose • generate ATP from light
during cellular respiration
energy (via photosynthesis)
to produce glucose for
• consume O2 and release
storage
CO2
Function
• consume CO2 and release O2
• use electron transport
chains
• use electron transport chains
• generate ATP by
• generate ATP by
chemiosmosis
chemiosmosis
Understanding
40. Although van Helmont’s conclusions to his willow tree experiment were partially wrong
(most of the tree’s dry mass came from CO2 and only a small portion came from water), his work
laid the foundation of quantitative plant research and focused later experiments on the gases and
liquids that the plant interacts with as a potential source of nutrients and, thereby, expanded our
understanding of plant physiology.
41. A green plant would grow better under a red light because a green plant would reflect most
of the green light it receives, and not be able to absorb and use the green light for photosynthesis.
42. Photosynthesis is an anabolic process that is the primary source of energy for most food webs
on the planet. According to the second law of thermodynamics, some usable energy is lost with
every energy conversion, and entropy increases. Therefore energy must be input (absorbed) into
this system because this process causes entropy to decrease. The Sun supplies this energy, but it
must be converted into chemical energy that organisms can use through the process of
photosynthesis.
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Chapter 5: Photosynthesis: The Energy of Life 5-5
43. Light energy is absorbed by the antenna complex and transferred to an electron in the
chlorophyll a molecule, causing the electron to move progressively farther away from the
nucleus of the original atom. Eventually, the electron is promoted to a high enough energy level
that the attractive force of the nucleus is no longer able to hold the electron on to its original
atom, and the electrons can now be sequestered by a nearby acceptor molecule. The electron is
then further transported to other carriers in the chain.
44. The standard equation makes it appear that the formation of glucose is the result of water
bonding directly to carbon dioxide and directly producing glucose and oxygen gas. This reaction
does not account for the separate reactions of the electron transport chain and the Calvin cycle. It
also does not account for the role of light energy, chemiosmosis, enzymes, other molecules, and
intermediate reaction steps.
45. Photons drive the electron transport chain in photosystem I and photosystem II by exciting
electrons in P680 and P700.
46. Chemiosmosis is the process of protons moving through the ATP synthase channel across the
thylakoid membrane, and down their electrochemical concentration gradient, which produces
ATP via the ATP synthase complex.
47. The term “dark reaction” suggests that the Calvin cycle occurs in the dark, when, in reality, it
never occurs in the dark. The Calvin cycle can only occur during the day or in the presence of an
adequate light source. Although it does not use light directly, the cycle requires the ATP and
NADPH that are being continuously produced/regenerated by the light-dependent reactions. In
the absence of light, these molecules would be depleted and the Calvin cycle would stop.
48. Plants in arid climates have three adaptations that make it more difficult to obtain enough
carbon dioxide to keep the Calvin cycle running:
• They have low surface-to-volume ratio (small leaf surface area).
• They have fewer leaf stomata to reduce water loss.
• They keep their stomata closed during the day to conserve water, which blocks CO2 from
entering.
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Chapter 5: Photosynthesis: The Energy of Life 5-6
49. The Earth’s atmosphere is 21 % oxygen and approximately 0.04 % carbon dioxide. Since the
amount of O2 in the atmosphere is so much greater than the amount of CO2, rubisco will bind
with CO2 only about 75 % of the time. The other 25% of the time, rubisco will bind with O2,
resulting in photorespiration. Therefore if the supply of CO2 becomes reduced, the rate of
photorespiration increases, which drains the resources of the cell.
50. You would expect to find the highest number of C4 plants in southern Ontario versus
northern Ontario. At higher temperatures, C3 plants are much less efficient than C4 plants
because of increased photorespiration. If they are able to survive, they will grow and reproduce
more slowly than C4 plants in these conditions. Therefore, they are more likely to be
“outcompeted” in southern Ontario. In cooler northern climates, the opposite is true.
Photorespiration is not a problem, so the C3 plants that don’t need to use ATP for the extra C4
cycle are more efficient than the C4 plants; therefore, C3 plants would be expected to grow and
reproduce faster and outcompete the C4 plants.
51. The reason that C4 plants and CAM plants are not more successful than C3 plants is because
they are adapted for different environments. The C3 plants are more efficient than C4 and CAM
plants under cool, moist conditions and under normal light. Both C4 and CAM plants have
adapted to drier environments, but require specialized cell structures and processes to be
successful in these environments. Since most of the Earth’s terrestrial environments are
relatively moist, C3 plants are most abundant because they have adapted to be the most efficient
plant for that type of environment.
52. The advantage of PEP carboxylase in carbon dioxide fixation compared with rubisco is that it
functions well regardless of the oxygen concentration in the environment. PEP carboxylase does
not bind with oxygen like rubisco, so there is not a risk of photorespiration damage with PEP
carboxylase. In addition, since PEP carboxylase allows C4 plants to be more efficient at carbon
dioxide fixation, they can still perform adequate photosynthesis even when their stomata cannot
be opened for very long.
53. Artificial photosynthesis would use technological devices to model the actions of the
photosystems in living cells. These would be used to split water molecules and use further
reactions to form hydrogen gas—a fuel that stores chemical energy. Solar panels generate
electrical energy directly. They transform light energy into electrical energy.
54. Two ways in which the electron transport systems in photosynthesis and cellular respiration
are different are 1) photosynthesis uses two continuous electron transport chains while
respiration uses only one, and 2) both of the electron transport chains in photosynthesis require
the input of additional energy (light) while the electron transport system in respiration does not
require any additional energy input.
55. The key feature that scientists are trying to harness in their development of artificial
photosynthesis is the ability of photosystems to oxidize water. They would like to be able to
produce free energized electrons from water using light energy and transfer them to protons to
produce hydrogen gas. Hydrogen is a clean-burning fuel and using light energy would be very
environmentally friendly.
56. The light-dependent and light-independent reactions (Calvin cycle) are dependent on each
other. The NADPH molecules produced during the light-dependent reaction deliver hydrogen
and high-energy electrons to the Calvin cycle for use in carbohydrate synthesis. The Calvin cycle
regenerates ADP and NADP+, which are used in the light-dependent reactions.
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Chapter 5: Photosynthesis: The Energy of Life 5-7
57. The statement “The development of photosynthetic organisms on Earth had a profound effect
on Earth’s atmosphere” is true because photosynthetic organisms released large amounts of
oxygen into Earth’s early atmosphere, dramatically changing the composition of Earth’s
atmosphere.
Analysis and Application
58. The fact that a green grass stain on clothing will eventually “fade away” even if not washed
suggests that the chlorophyll pigment molecule is more chemically and physically stable when it
is within a living cell and/or it degrades slowly within living cells (but is being continuously
replaced).
59. (a) Answers may vary. Sample answers:
• Measure and document all the water added to plant.
• Dry out the tree before measuring mass (like the earth) to compare dry masses.
• Grow the tree indoors under controlled conditions.
(b) Answers may vary. Sample answers:
• Grow the tree in water so water is the only variable.
• Measure and record all water added to plant.
• Use only distilled water (no unknown variables in water).
60. Answers may vary. Press briefings should include suggestions that the plants may have
evolved to contain large amounts of chlorophyll b, which uses blue/green light. Since no red
light reaches the planet’s surface, plants that could use blue or green light for photosynthesis
would have a competitive advantage. The plants would likely appear very dark in colour on their
home planet, because red light does not reach them and they absorb the blue and green light.
They would likely appear red if they were brought to Earth, because they do not contain
chlorophyll a, which absorbs red light, so they would reflect it.
61. Answers may vary. Experiments should include:
• Grow several identical plants under identical conditions other than the colour/wavelength of the
light they are exposed to.
• Take careful measurements of the growth of each plant for several months to determine which
colour/wavelength results in the best growth.
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Chapter 5: Photosynthesis: The Energy of Life 5-8
62. (a)
(b) Violet-blue light would be the least useful light for viewing spinach because it absorbs those
colours. Green-yellow light would be the most useful because it is reflected back to the eye.
(c)
(d) There are no peaks in the 500 to 620 nm range because spinach leaves do not absorb light in
this range (green-yellow).
(e) Chlorophyll a and chlorophyll b are primarily responsible for the absorption in the 400 to
480 nm range.
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Chapter 5: Photosynthesis: The Energy of Life 5-9
63. Since diuron (DCMU) is a photosystem II inhibitor, it prevents electrons from being
transferred from photosystem II through plastoquinone and on to the rest of the electron transport
carriers. The non-cyclic electron transport chain is blocked. This means that diuron essentially
stops photosynthesis, killing the plant.
64. Answers may vary. Sample answer: The high concentration of ATP and little NADPH may
be due to a problem/defect with photosystem II (i.e., the cyclic pathway is operating properly,
but the non-cyclic pathway is not).
65. (a) The plants are kept in the dark to keep the sugar content at a common baseline.
(b) Bean plants grown under high light conditions produce more sugar than plants grown under
low light conditions at different temperatures. The higher light provides more light energy for
photosynthesis. The positive effect on photosynthesis with increasing temperature is only seen at
high light levels. At low light, increasing the temperature has little effect. The lower the light, the
less energy is available for carbon dioxide fixation.
(c) The plants in low light stopped manufacturing sugar before the plants in high light because
the plants in low light had less carbon dioxide fixation occurring. The high temperature may
have also caused the plants to close their stomata in an effort to conserve water, which would
also decrease their available CO2.
66. Answers may vary. Sample answers:
• I could use grow lights in a greenhouse so that I can alter day length, which increases the time
for photosynthesis.
• I could increase the intensity of the light by installing more powerful lights.
• I could increase the temperature of the environment by using a heater, which has a positive
effect on photosystem II.
• I could increase the CO2 concentration, which reduces the amount of O2 that binds to rubisco
and increases the efficiency of photosynthesis, by filtering out excess oxygen and/or introducing
extra CO2 using tanks of CO2. The lower O2 concentration may also help minimize respiration.
67. Answers may vary. Answers should include:
Apparatus
Tests performed
C3
C4
CAM
Use a light and dark
equipment for
regime to determine no malic acid
malic acid
malic acid
Calvin’s
the presence of fixed
in dark
in dark
in dark
experiment
malic acid.
Vary temperature,
best growth: best growth: best growth:
humidity, and CO2
low
high
high
levels and look for
temperature, temperature, temperature,
greenhouse
best growth
high
normal CO2,
normal
compared to the other
humidity,
high
CO2, low
plants under the same
high CO2
humidity
humidity
conditions.
Paint nail polish on
underside of leaf;
magnifying
peel off polish when
numerous
fewer
very few
glass and nail
dry; use magnifying
stomata
stomata
stomata
polish
glass to count number
of stomata.
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Chapter 5: Photosynthesis: The Energy of Life 5-10
68. (a) The knocking out of the gene responsible for NADP+ reductase enzyme would have a
negative effect on non-cyclic ATP synthesis. Non-cyclic ATP synthesis would cease since the
electron normally donated from ferredoxin to NADP+ via NADP+ reductase would not occur;
therefore, it would force the plant to undergo only cyclic ATP synthesis.
(b) The effect on cyclic ATP synthesis would be positive as the only route for high-energy
electrons from ferredoxin would now be plastoquinone. This would cause protons to be shuttled
across the membrane without photosystem II; therefore, only ATP would be produced and no
NADP+ could be reduced.
(c) The Calvin cycle would be shut off as it requires the NADPH produced using the NADP+
reductase during non-cyclic electron transport to reduce 3-phosphoglycerate to G3P, two
molecules of which combine to form glucose.
69. The arid environment of the lowland areas of the Galapagos is an environment that CAM
plants would thrive in. That is why there are more cacti and other succulents. These plants open
their stomata to release O2 and sequester CO2 only at night. During the day, CAM plants close
their stomata to conserve water and the sequestered CO2 is released, reducing photorespiration.
The highlands are cool and moist so C3 plants can grow without risk of water loss, so they can
keep their stomata open to allow gas exchange, which reduces photorespiration.
70. Since the control plants show the presence of malic acid but not G3P, you can infer that these
plants must undergo crassulacean acid metabolism (CAM) since the leaves incorporated CO2
into malate in the dark (stomata opened in the dark).
71. (a) The CO2 compensation point for the C3 plant is higher than the CO2 compensation point
for the C4 plant because C3 plants require higher CO2 concentrations to outcompete O2 for the
active site of rubisco.
(b) The rate of C4 plant photosynthesis rises faster than the rate of C3 photosynthesis as the CO2
concentration increases because the PEP carboxylase enzyme in C4 plants is more efficient at
fixing CO2 than rubisco.
(c) In an environment where the CO2 concentration is increasing, C4 plants will gain an
immediate advantage, but at the higher concentration levels, C3 plants have higher rates of CO2
uptake because they are more efficient (require no intermediate molecule to fix CO2). That is, at
higher CO2 concentrations photorespiration is reduced so it is more efficient for rubisco to fix
CO2 directly.
72. Assuming that the herbicide was able to get into the plants, the weeds should all be dead or
dying in the field sprayed with rubisco inhibitor, since all these plants have rubisco, which is
needed in photosynthesis. In the field sprayed with PEP carboxylase inhibitor, the C4 and CAM
plants should be dead or dying while the C3 plants should be healthy since only the first two
types of plants have PEP carboxylase.
73. Even under ideal growing conditions, CAM plants grow much more slowly than C3 plants
because, although they can grow in higher temperatures and with less water than C3 plants, they
can only store a limited amount of CO2 during the night. During the day when this is “used up”
they can’t obtain any more because if they opened their stomata they would desiccate. C3 plants
under ideal settings can continuously take in CO2 and perform the reactions of the Calvin cycle
whenever there is adequate light.
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Chapter 5: Photosynthesis: The Energy of Life 5-11
74. (a) Table 3
Type of plant
C3
C4
Store
healthier
no change
Greenhouse
no change
no change
(but appear
healthier than
C3 plants)
(b) As temperatures increase, C3 plants begin to photorespire as carbon dioxide solubility
decreases. They also transpire more water in response to the high temperature. Reducing the
temperature in the store created better conditions for the C3 plants by reducing photorespiration
and water loss, so they appeared healthier. C4 plants reduce transpiration by closing their stomata
during hot, dry days to conserve water. The high affinity of PEP carboxylase for carbon dioxide,
even at low CO2:O2 concentration ratios, allows C4 plants to be more efficient at photosynthesis
when temperatures are higher.
Evaluation
75.
76. Plants evolved to contain chlorophyll b and accessory pigments to increase the plants’ ability
to absorb the energy of wavelengths of light beyond that which chlorophyll a can absorb. This
increases the plants’ action spectrum beyond the absorption spectrum of chlorophyll a.
77. Answers may vary. Sample answer: Rubisco can be considered the most important enzyme in
the biosphere because it is the enzyme that catalyzes the first reaction of the Calvin cycle.
Without rubisco, plants could not make carbohydrates. Without plants, consumers would not
survive. Therefore, rubisco is the primary method by which solar energy enters into the food
chain on which many organisms depend on for survival.
78. (a) Photosystem II does not function during cyclic electron transport, and cyclic electron
transport generates ATP by proton pumping, but does not produce NADPH. In addition, noncyclic electron transport consumes water and produces oxygen, while cyclic electron transport
does not.
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Chapter 5: Photosynthesis: The Energy of Life 5-12
(b)
Cyclic electron transport
Non-cyclic electron transport
• in both photosystem I and
• only photosystem I
photosystem II
• source of electron is water
• electron comes from chlorophyll
(NADP+ is the final acceptor) and
and returns to chlorophyll
electron goes out of system
• oxygen not produced
• oxygen is produced as by-product
• generates ATP
• generates ATP and NADPH
79. (a) Fructose and glucose (6-carbon sugars) and sucrose (12-carbon sugar) are seen on the
chromatogram after 30 s (Figure 5 (b)) but not on the chromatogram after 5 s, (Figure 5 (a))
which suggests that they are not immediately produced.
(b) Evidence suggesting PGA may be one of the first intermediate compounds in the Calvin
cycle is that PGA was in abundance after only a few seconds of exposure to light and continued
to increase in abundance as the experiment progressed.
(c) There is a small amount of labeled G3P (triose phosphate) on the 5-s chromatogram but none
labeled on the 30-s chromatogram, which suggests that it is produced rapidly during
photosynthesis but quickly converted to larger carbohydrates.
80. Autotrophs also need heterotrophs to survive because when the autotrophs die, their carbon is
trapped. Autotrophs need heterotrophic decomposers to recycle the carbon needed for
photosynthesis.
81. Increasing temperatures and CO2 levels due to climate change could have different effects on
C3, C4, and CAM plants. As CO2 and temperatures increase, C3 plants would benefit by the
increased concentration of CO2, which would tend to reduce photorespiration. However, the
increase in temperature would increase water loss and, therefore, might be a detriment to these
same plants that are not well suited to dry environments. The increase in temperature also
reduces the solubility of CO2, so the increase in atmospheric concentrations might not actually
result in an increase in available CO2. CAM plants are the plants best suited for this type of
environment, and would likely thrive. Water will play an important role. If water is in short
supply, then C4 and CAM plants will be best adapted for the new conditions. In regions where
water is abundant, C3 plants may be best adapted.
Reflect on Your Learning
82. Alternative pathways for carbon fixation likely evolved over time because the C3 pathway is
not very efficient, and plants growing in hot and arid climates cannot afford to keep their stomata
open all day. Each pathway has certain advantages in addition to its disadvantages.
83. Answers may vary. Sample answer: Artificial photosynthesis should be explored further and
implemented, if possible, because this form of energy production would not pollute the
environment, it is renewable, the raw materials needed for this type of energy production are
readily available, and artificial photosynthesis would not produce much waste.
84. Answers may vary. Sample answer: I learned that carbon dioxide fixation and the splitting of
water to produce oxygen occur in different reactions, so the process of gas consumption and
production in photosynthesis is more complicated than I thought.
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Chapter 5: Photosynthesis: The Energy of Life 5-13
85. Answers may vary. Sample answer: I learned about the important role plants play in
providing humans and other heterotrophs with food. Without plants and other photoautotrophs
acting as primary producers to convert light energy into chemical energy, we would not survive.
To help promote plants, I could recycle paper to reduce the need for cutting down trees.
Research
86. (a) Answers may vary. Students should research how bioscrubber technology is used to
control CO2 emissions, and consider how it can be implemented on a small scale.
(b) Presentations may vary but should include key points from Section 5.4 along with
appropriate graphics.
87. Answers may vary. Answers should include growing conditions and plant type.
88. (a) Answers may vary. Sample answers:
• Broadening the wavelengths absorbed over a wider range of the solar spectrum
• Storing unused solar energy more efficiently
• Decreasing oxygen affinity (or increasing carbon dioxide affinity) of rubisco
(b) Answers may vary. Sample answers:
• Plants could play a larger role as an alternative renewable fuel source.
• Less land will be needed to grow the same amount of food.
(c) Answers may vary. Sample answers:
• Plants could be genetically engineered to contain pigments that absorb light at different
wavelengths.
• Rubisco could be genetically engineered to improve its CO2 binding efficiency.
• Plants could be genetically engineered to become biological voltaic cells.
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Chapter 5: Photosynthesis: The Energy of Life 5-14