Biology in Focus - Chapter 8

Biology in Focus - Chapter 8 - Photosynthesis

1. CAMPBELL BIOLOGY IN FOCUS
© 2014 Pearson Education, Inc.
Urry • Cain • Wasserman • Minorsky • Jackson • Reece
Lecture Presentations by
Kathleen Fitzpatrick and Nicole Tunbridge
8
Photosynthesis

2. Overview: The Process That Feeds the Biosphere
Photosynthesis is the process that converts solar
energy into chemical energy
Directly or indirectly, photosynthesis nourishes
almost the entire living world
© 2014 Pearson Education, Inc.

3. Autotrophs sustain themselves without eating
anything derived from other organisms
Autotrophs are the producers of the biosphere,
producing organic molecules from CO
2
and other
inorganic molecules
Almost all plants are photoautotrophs, using the
energy of sunlight to make organic molecules
© 2014 Pearson Education, Inc.

4. © 2014 Pearson Education, Inc.
Figure 8.1

5. Heterotrophs obtain their organic material from
other organisms
Heterotrophs are the consumers of the biosphere
Almost all heterotrophs, including humans, depend
on photoautotrophs for food and O
2
© 2014 Pearson Education, Inc.

6. Photosynthesis occurs in plants, algae, certain other
protists, and some prokaryotes
These organisms feed not only themselves but also
most of the living world
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7. © 2014 Pearson Education, Inc.
Figure 8.2
(a) Plants
(d) Cyanobacteria
(e) Purple sulfur
bacteria
(b) Multicellular
alga
(c) Unicellular eukaryotes
1
0

m
m
1

m
m
4
0

m
m

8. © 2014 Pearson Education, Inc.
Figure 8.2a
(a) Plants

9. © 2014 Pearson Education, Inc.
Figure 8.2b
(b) Multicellular alga

10. © 2014 Pearson Education, Inc.
Figure 8.2c
(c) Unicellular eukaryotes
1
0

m
m

11. © 2014 Pearson Education, Inc.
Figure 8.2d
(d) Cyanobacteria
4
0

m
m

12. © 2014 Pearson Education, Inc.
Figure 8.2e
(e) Purple sulfur
bacteria
1

m
m

13. Concept 8.1: Photosynthesis converts light
energy to the chemical energy of food
The structural organization of photosynthetic cells
includes enzymes and other molecules grouped
together in a membrane
This organization allows for the chemical reactions
of photosynthesis to proceed efficiently
Chloroplasts are structurally similar to and likely
evolved from photosynthetic bacteria
© 2014 Pearson Education, Inc.

14. Chloroplasts: The Sites of Photosynthesis in
Plants
Leaves are the major locations of photosynthesis
Their green color is from chlorophyll, the green
pigment within chloroplasts
Chloroplasts are found mainly in cells of the
mesophyll, the interior tissue of the leaf
Each mesophyll cell contains 30–40 chloroplasts
© 2014 Pearson Education, Inc.

15. CO
2
enters and O
2
exits the leaf through microscopic
pores called stomata
The chlorophyll is in the membranes of thylakoids
(connected sacs in the chloroplast); thylakoids may
be stacked in columns called grana
Chloroplasts also contain stroma, a dense interior
fluid
© 2014 Pearson Education, Inc.

16. © 2014 Pearson Education, Inc.
Figure 8.3
Leaf cross section
20 mm
Mesophyll
Stomata
ChloroplastsVein
CO
2O
2
Mesophyll cell
Chloroplast
Stroma
Thylakoid
Thylakoid
space
Outer
membrane
Intermembrane
space
Inner membrane
Granum
1 mm

17. © 2014 Pearson Education, Inc.
Figure 8.3a
Leaf cross section
Mesophyll
Stomata
ChloroplastsVein
CO
2O
2

18. © 2014 Pearson Education, Inc.
Figure 8.3b
20 mm
Mesophyll cell
Chloroplast
Stroma
Thylakoid
Thylakoid
space
Outer
membrane
Intermembrane
space
Inner membrane
Granum
1 mm

19. © 2014 Pearson Education, Inc.
Figure 8.3c
20 mm
Mesophyll cell

20. © 2014 Pearson Education, Inc.
Figure 8.3d
Stroma
Granum
1 mm

21. Tracking Atoms Through Photosynthesis:
Scientific Inquiry
Photosynthesis is a complex series of reactions that
can be summarized as the following equation
6 CO
2
+ 12 H
2
O + Light energy ® C
6
H
12
O
6
+ 6 O
2
+ 6 H
2
O
© 2014 Pearson Education, Inc.

22. The Splitting of Water
Chloroplasts split H
2
O into hydrogen and oxygen,
incorporating the electrons of hydrogen into sugar
molecules and releasing oxygen as a by-product
© 2014 Pearson Education, Inc.

23. © 2014 Pearson Education, Inc.
Figure 8.4
Products:
Reactants: 6 CO
2
6 O
2
C
6
H
12
O
6
6 H
2
O
12 H
2
O

24. Photosynthesis as a Redox Process
Photosynthesis reverses the direction of electron
flow compared to respiration
Photosynthesis is a redox process in which H
2
O is
oxidized and CO
2
is reduced
Photosynthesis is an endergonic process; the energy
boost is provided by light
© 2014 Pearson Education, Inc.

25. © 2014 Pearson Education, Inc.
Figure 8.UN01
becomes reduced
becomes oxidized

26. The Two Stages of Photosynthesis: A Preview
Photosynthesis consists of the light reactions (the
photo part) and Calvin cycle (the synthesis part)
The light reactions (in the thylakoids)
Split H
2
O
Release O
2
Reduce the electron acceptor, NADP
+
, to NADPH
Generate ATP from ADP by adding a phosphate
group, photophosphorylation
© 2014 Pearson Education, Inc.

27. The Calvin cycle (in the stroma) forms sugar from
CO
2
, using ATP and NADPH
The Calvin cycle begins with carbon fixation,
incorporating CO
2
into organic molecules
© 2014 Pearson Education, Inc.
Animation: Photosynthesis

28. © 2014 Pearson Education, Inc.
Figure 8.5
Light
CO
2H
2
O
P
i
Chloroplast
Light
Reactions
Calvin
Cycle
[CH
2
O]
(sugar)
O
2
ADP
ATP
NADP
+
+
NADPH

29. © 2014 Pearson Education, Inc.
Figure 8.5-1
Light
H
2
O
Chloroplast
Light
Reactions
P
i
ADP
NADP
+
+

30. © 2014 Pearson Education, Inc.
Figure 8.5-2
Light
H
2
O
P
i
Chloroplast
Light
Reactions
O
2
ADP
ATP
NADP
+
+
NADPH

31. © 2014 Pearson Education, Inc.
Figure 8.5-3
Light
CO
2
H
2
O
P
i
Chloroplast
Light
Reactions
Calvin
Cycle
O
2
ADP
ATP
NADP
+
+
NADPH

32. © 2014 Pearson Education, Inc.
Figure 8.5-4
Light
CO
2
H
2
O
P
i
Chloroplast
Light
Reactions
Calvin
Cycle
[CH
2
O]
(sugar)
O
2
ADP
ATP
NADP
+
+
NADPH

33. Concept 8.2: The light reactions convert solar
energy to the chemical energy of ATP and NADPH
Chloroplasts are solar-powered chemical factories
Their thylakoids transform light energy into the
chemical energy of ATP and NADPH
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34. The Nature of Sunlight
Light is a form of electromagnetic energy, also called
electromagnetic radiation
Like other electromagnetic energy, light travels in
rhythmic waves
Wavelength is the distance between crests of waves
Wavelength determines the type of electromagnetic
energy
© 2014 Pearson Education, Inc.

35. The electromagnetic spectrum is the entire range
of electromagnetic energy, or radiation
Visible light consists of wavelengths (including
those that drive photosynthesis) that produce colors
we can see
Light also behaves as though it consists of discrete
particles, called photons
© 2014 Pearson Education, Inc.

36. © 2014 Pearson Education, Inc.
Figure 8.6
Gamma
rays
10
−5
nm10
−3
nm1 nm 10
3
nm 10
6
nm
1 m
(10
9
nm)10
3
m
Radio
waves
Micro-
waves
X-rays InfraredUV
Visible light
Shorter wavelength Longer wavelength
Lower energyHigher energy
380 450 500 550 650600 700 750 nm

37. Photosynthetic Pigments: The Light Receptors
Pigments are substances that absorb visible light
Different pigments absorb different wavelengths
Wavelengths that are not absorbed are reflected or
transmitted
Leaves appear green because chlorophyll reflects
and transmits green light
© 2014 Pearson Education, Inc.
Animation: Light and Pigments

38. © 2014 Pearson Education, Inc.
Figure 8.7
Reflected
light
Light
Absorbed
light
Chloroplast
Granum
Transmitted
light

39. A spectrophotometer measures a pigment’s ability
to absorb various wavelengths
This machine sends light through pigments and
measures the fraction of light transmitted at each
wavelength
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40. © 2014 Pearson Education, Inc.
Figure 8.8
Refracting
prism
White
light
Green
light
Blue
light
Chlorophyll
solution
Photoelectric
tube
Galvanometer
Slit moves to pass
light of selected
wavelength.
The low transmittance (high
absorption) reading indicates
that chlorophyll absorbs most
blue light.
The high transmittance (low
absorption) reading indicates
that chlorophyll absorbs very
little green light.
Technique
1
2
4
3

41. An absorption spectrum is a graph plotting a
pigment’s light absorption versus wavelength
The absorption spectrum of chlorophyll a suggests
that violet-blue and red light work best for
photosynthesis
Accessory pigments include chlorophyll b and a
group of pigments called carotenoids
An action spectrum profiles the relative
effectiveness of different wavelengths of radiation in
driving a process
© 2014 Pearson Education, Inc.

42. © 2014 Pearson Education, Inc.
Figure 8.9
Chloro-
phyll a
R
a
t
e

o
f
p
h
o
t
o
s
y
n
t
h
e
s
i
s
(
m
e
a
s
u
r
e
d

b
y

O
2

r
e
l
e
a
s
e
)
Results
A
b
s
o
r
p
t
i
o
n

o
f

l
i
g
h
t
b
y

c
h
l
o
r
o
p
l
a
s
t
p
i
g
m
e
n
t
s
Chlorophyll b
Carotenoids
Filament
of alga
Aerobic bacteria
(a) Absorption spectra
(b) Action spectrum
(c) Engelmann’s experiment
400 700600500
400 700600500
400 700600500
Wavelength of light (nm)

43. © 2014 Pearson Education, Inc.
Figure 8.9a
Chloro-
phyll a
A
b
s
o
r
p
t
i
o
n

o
f

l
i
g
h
t
b
y

c
h
l
o
r
o
p
l
a
s
t
p
i
g
m
e
n
t
s
Chlorophyll b
Carotenoids
(a) Absorption spectra
400 700600500
Wavelength of light (nm)

44. © 2014 Pearson Education, Inc.
Figure 8.9b
(b) Action spectrum
400 700600500
R
a
t
e

o
f
p
h
o
t
o
s
y
n
t
h
e
s
i
s
(
m
e
a
s
u
r
e
d

b
y

O
2

r
e
l
e
a
s
e
)

45. © 2014 Pearson Education, Inc.
Figure 8.9c
Filament
of alga
Aerobic bacteria
(c) Engelmann’s experiment
400 700600500

46. The action spectrum of photosynthesis was first
demonstrated in 1883 by Theodor W. Engelmann
In his experiment, he exposed different segments of
a filamentous alga to different wavelengths
Areas receiving wavelengths favorable to
photosynthesis produced excess O
2
He used the growth of aerobic bacteria clustered
along the alga as a measure of O
2
production
© 2014 Pearson Education, Inc.

47. Chlorophyll a is the main photosynthetic pigment
Accessory pigments, such as chlorophyll b, broaden
the spectrum used for photosynthesis
A slight structural difference between chlorophyll a
and chlorophyll b causes them to absorb slightly
different wavelengths
Accessory pigments called carotenoids absorb
excessive light that would damage chlorophyll
© 2014 Pearson Education, Inc.
Video: Chlorophyll Model

48. © 2014 Pearson Education, Inc.
Figure 8.10
Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts; H atoms not
shown
Porphyrin ring:
light-absorbing
“head” of molecule;
note magnesium
atom at center
CH
3 in chlorophyll a
CHO in chlorophyll b
CH
3

49. Excitation of Chlorophyll by Light
When a pigment absorbs light, it goes from a ground
state to an excited state, which is unstable
When excited electrons fall back to the ground state,
photons are given off, an afterglow called
fluorescence
If illuminated, an isolated solution of chlorophyll will
fluoresce, giving off light and heat
© 2014 Pearson Education, Inc.

50. © 2014 Pearson Education, Inc.
Figure 8.11
Photon
(fluorescence)
Ground
state
(b) Fluorescence
Excited
state
Chlorophyll
molecule
Photon
Heat
e
−
(a) Excitation of isolated chlorophyll molecule
E
n
e
r
g
y

o
f

e
l
e
c
t
r
o
n

51. © 2014 Pearson Education, Inc.
Figure 8.11a
(b) Fluorescence

52. A Photosystem: A Reaction-Center Complex
Associated with Light-Harvesting Complexes
A photosystem consists of a reaction-center
complex (a type of protein complex) surrounded by
light-harvesting complexes
The light-harvesting complexes (pigment
molecules bound to proteins) transfer the energy of
photons to the reaction center
© 2014 Pearson Education, Inc.

53. © 2014 Pearson Education, Inc.
Figure 8.12
(b) Structure of a photosystem(a) How a photosystem harvests light
Chlorophyll STROMA
THYLAKOID
SPACE
Protein
subunits
STROMA
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
Photosystem
Photon
Light-
harvesting
complexes
Reaction-
center
complex
Primary
electron
acceptor
Special pair of
chlorophyll a
molecules
Transfer
of energy
Pigment
molecules
T
h
y
l
a
k
o
i
d

m
e
m
b
r
a
n
e
T
h
y
l
a
k
o
i
d

m
e
m
b
r
a
n
e
e
-

54. © 2014 Pearson Education, Inc.
Figure 8.12a
(a) How a photosystem harvests light
STROMA
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
Photosystem
Photon
Light-
harvesting
complexes
Reaction-
center
complex
Primary
electron
acceptor
Special pair of
chlorophyll a
molecules
Transfer
of energy
Pigment
molecules
T
h
y
l
a
k
o
i
d

m
e
m
b
r
a
n
e
e
−

55. © 2014 Pearson Education, Inc.
Figure 8.12b
(b) Structure of a photosystem
Chlorophyll STROMA
THYLAKOID
SPACE
Protein
subunits
T
h
y
l
a
k
o
i
d

m
e
m
b
r
a
n
e

56. A primary electron acceptor in the reaction center
accepts excited electrons and is reduced as a result
Solar-powered transfer of an electron from a
chlorophyll a molecule to the primary electron
acceptor is the first step of the light reactions
© 2014 Pearson Education, Inc.

57. There are two types of photosystems in the thylakoid
membrane
Photosystem II (PS II) functions first (the numbers
reflect order of discovery) and is best at absorbing a
wavelength of 680 nm
The reaction-center chlorophyll a of PS II is called
P680
© 2014 Pearson Education, Inc.

58. Photosystem I (PS I) is best at absorbing a
wavelength of 700 nm
The reaction-center chlorophyll a of PS I is called
P700
© 2014 Pearson Education, Inc.

59. Linear Electron Flow
Linear electron flow involves the flow of electrons
through both photosystems to produce ATP and
NADPH using light energy
© 2014 Pearson Education, Inc.

60. Linear electron flow can be broken down into a series
of steps
1.A photon hits a pigment and its energy is passed
among pigment molecules until it excites P680
2.An excited electron from P680 is transferred to the
primary electron acceptor (we now call it P680
+
)
3.H
2
O is split by enzymes, and the electrons are
transferred from the hydrogen atoms to P680
+
, thus
reducing it to P680; O
2
is released as a by-product
© 2014 Pearson Education, Inc.

61. © 2014 Pearson Education, Inc.
Figure 8.UN02
Calvin
Cycle
NADPH
NADP
+
ATP
ADP
Light
CO
2
[CH
2
O] (sugar)
Light
Reactions
O
2
H
2
O

62. © 2014 Pearson Education, Inc.
Figure 8.13-1
Primary
acceptor
Photosystem II
(PS II)
Light
P680
Pigment
molecules
1
2
e
−

63. © 2014 Pearson Education, Inc.
Figure 8.13-2
Primary
acceptor
2 H
+
O
2
+
Photosystem II
(PS II)
H
2
O
Light
/
2
1
P680
Pigment
molecules
1
2
3
e
−
e
−
e
−

64. © 2014 Pearson Education, Inc.
Figure 8.13-3
Primary
acceptor
2 H
+
O
2
+
ATP
Photosystem II
(PS II)
H
2
O
Light
/
2
1
P680
Pq
Electron
transport
chain
Cytochrome
complex
Pc
Pigment
molecules
1
2
3
4
5
e
−
e
−
e
−

65. © 2014 Pearson Education, Inc.
Figure 8.13-4
Primary
acceptor
2 H
+
O
2
+
ATP
Photosystem II
(PS II)
H
2
O
Light
/
2
1
P680
Pq
Electron
transport
chain
Cytochrome
complex
Pc
Pigment
molecules
Primary
acceptor
Photosystem I
(PS I)
P700
Light
1
2
3
4
5
6
e
−
e
−
e
−
e
−

66. © 2014 Pearson Education, Inc.
Figure 8.13-5
Primary
acceptor
2 H
+
O
2
+
ATP
NADPH
Photosystem II
(PS II)
H
2
O
e
−
e
−
e
−
Light
/
2
1
P680
Pq
Electron
transport
chain
Cytochrome
complex
Pc
Pigment
molecules
Primary
acceptor
Photosystem I
(PS I)
e
−
P700
e
−
e
−
Fd
Light
Electron
transport
chain
H
+
+
NADP
+
NADP
+
reductase
1
2
3
4
5
6
7
8

67. 4.Each electron “falls” down an electron transport
chain from the primary electron acceptor of PS II
to PS I
5.Energy released by the fall drives the creation of a
proton gradient across the thylakoid membrane;
diffusion of H
+
(protons) across the membrane drives
ATP synthesis
© 2014 Pearson Education, Inc.

68. 6.In PS I (like PS II), transferred light energy excites
P700, causing it to lose an electron to an electron
acceptor (we now call it P700
+
)
P700
+
accepts an electron passed down from PS II
via the electron transport chain
© 2014 Pearson Education, Inc.

69. 7.Excited electrons “fall” down an electron transport
chain from the primary electron acceptor of PS I to
the protein ferredoxin (Fd)
8.The electrons are transferred to NADP
+
, reducing it
to NADPH, and become available for the reactions
of the Calvin cycle
This process also removes an H
+
from the stroma
© 2014 Pearson Education, Inc.

70. The energy changes of electrons during linear
flow can be represented in a mechanical analogy
© 2014 Pearson Education, Inc.

71. © 2014 Pearson Education, Inc.
Figure 8.14
Photosystem II Photosystem I
NADPH
Mill
makes
ATP
P
h
o
t
o
n
P
h
o
t
o
n

72. A Comparison of Chemiosmosis in
Chloroplasts and Mitochondria
Chloroplasts and mitochondria generate ATP by
chemiosmosis but use different sources of energy
Mitochondria transfer chemical energy from food to
ATP; chloroplasts transform light energy into the
chemical energy of ATP
Spatial organization of chemiosmosis differs
between chloroplasts and mitochondria but also
shows similarities
© 2014 Pearson Education, Inc.

73. In mitochondria, protons are pumped to the
intermembrane space and drive ATP synthesis
as they diffuse back into the mitochondrial matrix
In chloroplasts, protons are pumped into the
thylakoid space and drive ATP synthesis as they
diffuse back into the stroma
© 2014 Pearson Education, Inc.

74. © 2014 Pearson Education, Inc.
Figure 8.15
Electron
transport
chain
Higher [H
+
]
P
i
H
+
CHLOROPLAST
STRUCTURE
Inter-
membrane
space
MITOCHONDRION
STRUCTURE
Thylakoid
space
Inner
membrane
Matrix
Key
Lower [H
+
]
Thylakoid
membrane
Stroma
ATP
ATP
synthase
ADP+
H
+
Diffusion

75. © 2014 Pearson Education, Inc.
Figure 8.15a
Electron
transport
chain
Higher [H
+
] H
+
CHLOROPLAST
STRUCTURE
Inter-
membrane
space
MITOCHONDRION
STRUCTURE
Thylakoid
space
Inner
membrane
Matrix
Key
Lower [H
+
]
Thylakoid
membrane
Stroma
ATP
ATP
synthase
ADP+
H
+
Diffusion
P
i

76. ATP and NADPH are produced on the side facing
the stroma, where the Calvin cycle takes place
In summary, light reactions generate ATP and
increase the potential energy of electrons by moving
them from H
2
O to NADPH
© 2014 Pearson Education, Inc.

77. © 2014 Pearson Education, Inc.
Figure 8.UN02
Calvin
Cycle
NADPH
NADP
+
ATP
ADP
Light
CO
2
[CH
2
O] (sugar)
Light
Reactions
O
2
H
2
O

78. © 2014 Pearson Education, Inc.
Figure 8.16
Photosystem II Photosystem I
To
Calvin
Cycle
H
+
THYLAKOID SPACE
(high H
+
concentration)
Thylakoid
membrane
STROMA
(low H
+
concentration)
ATP
synthase
NADPH
e
−
Light
NADP
+
ATP
ADP
+
NADP
+
reductase
Fd
H
+
+
Pq
Pc
Cytochrome
complex
4 H
+
Light
+2 H
+
O
2
H
2
O
/
2
1
4 H
+
e
−
1
2
3
P
i

79. © 2014 Pearson Education, Inc.
Figure 8.16a
Photosystem II Photosystem I
H
+
THYLAKOID SPACE
(high H
+
concentration)
Thylakoid
membrane
STROMA
(low H
+
concentration)
ATP
synthase
e
−
Light
ATP
ADP
+
Fd
Pq
Pc
Cytochrome
complex
4 H
+
Light
+2 H
+
O
2
H
2O
/
2
1
4 H
+
e
−
1
2
P
i

80. © 2014 Pearson Education, Inc.
Figure 8.16b
2
3
Photosystem I
To
Calvin
Cycle
H
+
ATP
synthase
NADPH
Light
NADP
+
ATP
ADP
+
NADP
+
reductase
Fd
H
+
+
Pc
Cytochrome
complex
4 H
+
THYLAKOID SPACE
(high H
+
concentration)
STROMA
(low H
+
concentration)
P
i

81. Concept 8.3: The Calvin cycle uses the chemical
energy of ATP and NADPH to reduce CO
2
to
sugar
The Calvin cycle, like the citric acid cycle,
regenerates its starting material after molecules
enter and leave the cycle
Unlike the citric acid cycle, the Calvin cycle is
anabolic
It builds sugar from smaller molecules by using ATP
and the reducing power of electrons carried by
NADPH
© 2014 Pearson Education, Inc.

82. Carbon enters the cycle as CO
2
and leaves as a
sugar named glyceraldehyde 3-phospate (G3P)
For net synthesis of one G3P, the cycle must take
place three times, fixing three molecules of CO
2
The Calvin cycle has three phases
Carbon fixation
Reduction
Regeneration of the CO
2
acceptor
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83. Phase 1, carbon fixation, involves the incorporation
of the CO
2
molecules into ribulose bisphosphate
(RuBP) using the enzyme rubisco
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84. © 2014 Pearson Education, Inc.
Figure 8.UN03
Calvin
Cycle
NADPH
NADP
+
ATP
ADP
Light
CO
2
[CH
2
O] (sugar)
Light
Reactions
O
2
H
2
O

85. © 2014 Pearson Education, Inc.
Figure 8.17-1
Input 3
Calvin
Cycle
as 3 CO
2
Rubisco
Phase 1: Carbon fixation
RuBP
3-Phosphoglycerate
6
3
3 P
P
P P
P

86. © 2014 Pearson Education, Inc.
Figure 8.17-2
6 P
i
NADPH
Input 3
ATP
Calvin
Cycle
as 3 CO
2
Rubisco
Phase 1: Carbon fixation
Phase 2:
Reduction
G3P
Output
Glucose and
other organic
compounds
G3P
RuBP
3-Phosphoglycerate
1,3-Bisphosphoglycerate
6 ADP
6
6
6
6
3
6 NADP
+
6
3
1
P
P P
P
P
P P
P
P

87. © 2014 Pearson Education, Inc.
Figure 8.17-3
6 P
i
NADPH
Input 3
ATP
Calvin
Cycle
as 3 CO
2
Rubisco
Phase 1: Carbon fixation
Phase 2:
Reduction
Phase 3:
Regeneration
of RuBP
G3P
Output
Glucose and
other organic
compounds
G3P
RuBP
3-Phosphoglycerate
1,3-Bisphosphoglycerate
6 ADP
6
6
6
6 P
3
P P
P
6 NADP
+
6 P
5 P
G3P
ATP
3 ADP
3
3P P
1 P
P

88. Phase 2, reduction, involves the reduction and
phosphorylation of 3-phosphoglycerate to G3P
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89. Phase 3, regeneration, involves the rearrangement
of G3P to regenerate the initial CO
2
receptor, RuBP
© 2014 Pearson Education, Inc.

90. Evolution of Alternative Mechanisms of
Carbon Fixation in Hot, Arid Climates
Adaptation to dehydration is a problem for land
plants, sometimes requiring trade-offs with other
metabolic processes, especially photosynthesis
On hot, dry days, plants close stomata, which
conserves H
2O but also limits photosynthesis
The closing of stomata reduces access to CO
2
and
causes O
2
to build up
These conditions favor an apparently wasteful
process called photorespiration
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91. In most plants (C
3
plants), initial fixation of CO
2
,
via rubisco, forms a three-carbon compound (3-
phosphoglycerate)
In photorespiration, rubisco adds O
2
instead of
CO
2
in the Calvin cycle, producing a two-carbon
compound
Photorespiration decreases photosynthetic output by
consuming ATP, O
2
, and organic fuel and releasing
CO
2
without producing any ATP or sugar
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92. Photorespiration may be an evolutionary relic
because rubisco first evolved at a time when the
atmosphere had far less O
2
and more CO
2

Photorespiration limits damaging products of light
reactions that build up in the absence of the Calvin
cycle
© 2014 Pearson Education, Inc.

93. C
4
plants minimize the cost of photorespiration by
incorporating CO
2
into a four-carbon compound
An enzyme in the mesophyll cells has a high affinity
for CO
2
and can fix carbon even when CO
2

concentrations are low
These four-carbon compounds are exported to
bundle-sheath cells, where they release CO
2
that is
then used in the Calvin cycle
C
4
Plants
© 2014 Pearson Education, Inc.

94. © 2014 Pearson Education, Inc.
Figure 8.18
Bundle-
sheath
cell
Sugarcane
CO
2
Pineapple
CO
2
(a) Spatial separation of steps
C
4
CO
2
CO
2
CAM
Day
Night
Sugar
Calvin
Cycle
Calvin
Cycle
Sugar
Organic
acid
Organic
acid
Mesophyll
cell
(b) Temporal separation of steps
1
2
1
2

95. © 2014 Pearson Education, Inc.
Figure 8.18a
Sugarcane

96. © 2014 Pearson Education, Inc.
Figure 8.18b
Pineapple

97. © 2014 Pearson Education, Inc.
Figure 8.18c
Bundle-
sheath
cell
CO
2
CO
2
(a) Spatial separation of steps
C
4
CO
2
CO
2
CAM
Day
Night
Sugar
Calvin
Cycle
Calvin
Cycle
Sugar
Organic
acid
Organic
acid
Mesophyll
cell
(b) Temporal separation of steps
1
2
1
2

98. CAM Plants
Some plants, including succulents, use crassulacean
acid metabolism (CAM) to fix carbon
CAM plants open their stomata at night,
incorporating CO
2
into organic acids
Stomata close during the day, and CO
2
is released
from organic acids and used in the Calvin cycle
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99. The Importance of Photosynthesis: A Review
The energy entering chloroplasts as sunlight gets
stored as chemical energy in organic compounds
Sugar made in the chloroplasts supplies chemical
energy and carbon skeletons to synthesize the
organic molecules of cells
Plants store excess sugar as starch in the
chloroplasts and in structures such as roots, tubers,
seeds, and fruits
In addition to food production, photosynthesis
produces the O
2
in our atmosphere
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100. © 2014 Pearson Education, Inc.
Figure 8.19
Photosystem II
Electron transport chain
Calvin
Cycle
NADPH
Light
NADP
+
ATP
CO
2
H
2
O
ADP
+
3-Phosphpglycerate
G3P
RuBP
Sucrose (export)
Starch
(storage)
Chloroplast
O
2
Light
Reactions:
Photosystem I
Electron transport chain
P
i

101. © 2014 Pearson Education, Inc.
Figure 8.UN04

102. © 2014 Pearson Education, Inc.
Figure 8.UN05
Photosystem II
Photosystem I
NADP
+
ATP
Fd
H
+
+
Pq
Cytochrome
complex
O
2
H
2
O
Pc
NADP
+
reductase
NADPH
Primary
acceptor
E
l
e
c
t
r
o
n

t
r
a
n
s
p
o
r
t
c
h
a
i
n
Primary
acceptor
E
l
e
c
t
r
o
n

t
r
a
n
s
p
o
r
t
c
h
a
i
n

103. © 2014 Pearson Education, Inc.
Figure 8.UN06
Calvin
Cycle
Regeneration of
CO
2
acceptor
Carbon fixation
Reduction
1 G3P (3C)
3 CO
2
3 ´ 5C 6 ´ 3C
5 ´ 3C

104. © 2014 Pearson Education, Inc.
Figure 8.UN07
pH 7
pH 4 pH 8
pH 4