1. Phototrophic Energy Metabolism: Photosynthesis
Chapter 11
Phototrophic Energy Metabolism: Photosynthesis

2. Phototrophic Energy Metabolism: Photosynthesis
Most chemotrophs depend on an external source of organic substrates for survival
Photosynthetic organisms produce the chemical energy and organic carbon required by chemotrophs
They use solar energy to reduce CO2 to produce carbohydrates, fats, and proteins 2

3. Important terminology
Photosynthesis: the conversion of light energy to chemical energy and its subsequent use in synthesis of organic molecules
Phototrophs: organisms that convert solar energy into chemical energy as ATP and reduced coenzymes 3

4. Types of phototrophs
Photoheterotrophs: organisms that acquire energy from sunlight but depend on organic sources of reduced carbon
Photoautotrophs: organisms that use solar energy to synthesize energy-rich organic molecules using starting materials such as CO2 and H2O 4

5. An Overview of Photosynthesis
Photosynthesis involves two major biochemical processes
Energy transduction reactions: light energy is captured and converted into chemical energy
Carbon assimilation reactions: (carbon fixation reactions) carbohydrates are formed from CO2 and H2O

6. Figure 11-1

7. BioFlix: Photosynthesis
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8. The Energy Transduction Reactions Convert Solar Energy to Chemical Energy
Light energy is captured by green pigment molecules called chlorophylls, present in the green leaves of plants and the cells of algae and photosynthetic bacteria
Light absorption by a chlorophyll molecule excites one of its electrons, which is then ejected from the molecule and enters an electron transport system

9. Unidirectional proton pumping
The photosynthetic ETS is coupled to unidirectional proton pumping
The electrochemical gradient produced is used to generate ATP through photophosphorylation
This is similar to oxidative phosphorylation in mitochondria

10. Reduction of carbon
Photoautotrophs use NADPH2 to reduce carbon for incorporation into organic molecules
Oxygenic phototrophs (plants, algae, cyanobacteria) use water as the donor of two electrons
Anoxygenic phototrophs (green and purple photosynthetic baceria) use compounds such as sulfide, thiosulfate, or succinate as donors

11. Photoreduction
Oxygenic phototrophs release oxygen as water is oxidized
Anoxygenic phototrophs release oxidized forms of the electron donors
In both types of organisms the light dependent generation of NADPH is called photoreduction

12. The Carbon Assimilation Reactions Fix Carbon by Reducing Carbon Dioxide
Most of the energy accumulated by the generation of ATP and NADPH is used for carbon dioxide fixation and reduction
“H2A” is a suitable electron donor, and “A” is the oxidized form of the donor

13. Oxygenic phototrophs
For oxygenic phototrophs, in which water is the electron donor, we can summarize the reaction as follows:
The intermediate product of carbon fixation is a triose (3-carbon sugar) rather than the hexose shown in the equation

14. Production of sugars
The intermediates of photosynthesis are used for biosynthesis of a variety of products, including glucose, sucrose, and starch
Sucrose is the major transport carbohydrate in most plants
Starch is the major storage carbohydrate in most plants

15. The Chloroplast Is the Photosynthetic Organelle in Eukaryotic Cells
The most familiar oxygenic phototrophs are the green plants, in which the photosynthetic organelle is the chloroplast
Chloroplasts are large and a mature leaf may contain
The shape varies from simple flattened spheres to ribbon-shaped

16. Figure 11-2A

17. Figure 11-2B

18. Not all plant cells contain chloroplasts
Newly differentiated plant cells have smaller organelles called proplastids, which may develop into any of several types of plastids depending on the function of the cell
Amyloplasts are specialized for storing starch
Chromoplasts give flowers and fruits their distinctive colors

19. Chloroplasts Are Composed of Three Membrane Systems
A chloroplast has both an outer membrane and an inner membrane
These are usually separated by an intermembrane space
The inner membrane encloses the stroma, a gel-like matrix full of enzymes for C, N, and S reduction and assimilation

20. Figure 11-3A,B

21. Figure 11-3C

22. The outer membrane is freely permeable
The outer membrane contains porins similar to those in the mitochondrial outer membrane
These allow passage of solutes with molecular weights up to 5000
In the inner membrane transport proteins control the flow of metabolites between the stroma and intermembrane space

23. Thylakoids
Chloroplasts have a third membrane system, called thylakoids
These are flat, saclike structures in the stroma, arranged in stacks called grana
Grana are interconnected by stroma thylakoids

24. Thylakoid lumen
Grana and stroma thylakoids enclose a single continuous compartment called the thylakoid lumen
The semipermeable barrier of the thylakoid membrane allows for creation of an electrochemical proton gradient between the lumen and stroma

25. Figure 11-3C,D

26. Video: Chloroplast structure
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27. Organisms without chloroplasts
Photosynthetic bacteria have no chloroplasts
In some of them, such as the cyanobacteria, the plasma membrane folds inward to form photosynthetic membranes
To some extent, cyanobacteria appear to be free-living chloroplasts, a resemblance that has contributed to the endosymbiont theory

28. Figure 11-4

29. Figure 11A-1

30. Photosynthetic Energy Transduction I: Light Harvesting
The first stage of photosynthetic energy transduction is the capture of solar energy
Light behaves as a stream of particles called photons, each carrying a quantum (indivisible packet) of energy
When a photon is absorbed by a pigment such as chlorophyll, the energy of the photon is transferred to an electron

31. Photoexcitation
The energy transferred from a photon energizes the electron from its ground state in a low-energy orbital, to an excited state in a high-energy orbital
This first step of photosynthesis is called photoexcitation
Different pigments have different absorption spectra, to describe the wavelengths absorbed

32. Figure 11-5

33. Photoexcited electrons are unstable
A photoexcited electron in a pigment molecule is unstable and must either return to the ground state or transfer to a stable high-energy orbital
If it returns to the ground state, the energy is lost as heat or light
The energy can also be transferred to an electron in an adjacent molecule, in a process called resonance energy transfer

34. Transfer of the photoexcited electron
If the excited electron is transferred to another molecule, it is called photochemical reduction
Photochemical reduction is essential for converting light energy into chemical energy

35. Chlorophyll Is Life’s Primary Link to Sunlight
Chlorophyll is found in nearly all photosynthetic organisms
Chlorophyll a and b each have a central porphyrin ring, which absorbs visible light
Their strongly hydrophobic phytol side chains anchor the chlorophylls in the thylakoid membranes

36. Chlorophyll a and b
The Mg2+ in chlorophyll a and b affects the electron distribution in the porphyrin ring and ensures that high-energy orbitals are available
Chlorophyll a has a broad absorption spectrum with maxima at about 420 and 660 nm
Chlorophyll b has a formyl group in place of a methyl group, which shifts the maxima toward the center of the spectrum

37. Figure 11-6

38. 3-D Structure: Chlorophylla
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39. Other types of chlorophyll
All plants and green algae contain both chlorophyll a and b
Brown algae, diatoms, and dinoflagellates supplement chlorophyll a with chlorophyll c
Red algae have chlorophyll d
Red algae and cyanobacteria have phycobilin

40. Bacteriochlorophyll
Bacteriochlorophyll is a subfamily restricted to anoxygenic phototrophs
It is characterized by a saturated site not found in other chlorophylls
The absorption maxima of bacteriochlorophylls are shifted toward the near-ultraviolet and the far-red regions

41. Accessory Pigments Further Expand Access to Solar Energy
Most photosynthetic organisms also contain accessory pigments, which absorb photons that cannot be captured by chlorophyll
The energy is transferred to a chlorophyll molecule by resonance energy transfer
Two types of accessory pigments are carotenoids and phycobilins

42. Carotenoids
Two carotenoids that are abundant in the thylakoid membranes of most plants and green algae are -carotene and lutein
When not masked by chlorophyll these pigments confer an orange or yellow tint to leaves
They absorb photons from a broad range of the blue region of the spectrum

43. Phycobilins
Phycobilins are found only in red algae and cyanobacteria; two common examples are
Phycoerythrin, which allows absorption of light that penetrates the ocean’s surface water
Phycocyanin, which is characteristic of cyanobacteria near the surface of a lake, or on land

44. Light-Gathering Molecules Are Organized into Photosystems and Light-Harvesting Complexes
Functional units, photosystems, contain
Chlorophyll
Accessory proteins
Chlorophyll-binding proteins that stabilize the chlorophyll in a photosystem
Other proteins that bind components of the electron transport system

45. Photosystems and Light-Harvesting Complexes
Most pigments of a photosystem serve as light-gathering antenna pigments
These absorb photons and pass the energy to a neighboring chlorophyll or accessory protein by resonance energy transfer
Some antenna pigments are “wired” together by quantum mechanical probability effects

46. The reaction center
The events that drive electron flow and proton pumping do not begin until the energy reaches the reaction center of a photosystem
Here, two chlorophyll a molecules called the special pair are found
These molecules catalyze the conversion of solar energy into chemical energy

47. Figure 11-7

48. Figure 11B-1

49. The light-harvesting complex
Each photosystem is associated with a light-harvesting complex (LHC), which collects light energy and can move in response to changing light conditions
The LHC does not contain a reaction center
Instead it passes the collected energy to a nearby photosystem by resonance energy transfer

50. The light-harvesting complex (continued)
Plants and green algae have LHCs composed of chlorophyll a and b molecules along with carotenoids and pigment-binding proteins
Red algae have a phycobilisome, which contains phycobilins
Together a photosystem and the associated LHCs are referred to as a photosystem complex

51. Oxygenic Phototrophs Have Two Types of Photosystems
In the 1940s Emerson and colleague discovered that two separate photosystems are involved in oxygenic photosynthesis
They found that photosynthesis driven by a combination of wavelengths exceeded the sum of activities with either wavelength alone
This synergistic phenomenon was called the Emerson enhancement effect

52. Photosystems I and II
Photosystem I (PSI) has an absorption maximum of 700nm, whereas photosystem II (PSII) has an absorption maximum of 680 nm
Each electron that passes from water to NADP+ must be photoexcited once for each photosystem
With illumination of 690nm and above, photosynthesis is severely impaired

53. Photosystems I and II (continued)
Each electron is first excited by PSII and then by PSI
The special pair of chlorophyll a molecules in the reaction center of each photosystem are designated P680 for PSII and P700 for PSI
The granal and stromal thylakoid membranes have differing amounts of the two photosystems, which can move to respond to changing light conditions

54. Photosynthetic Energy Transduction II: NADPH Synthesis
The second stage of photosynthesis uses a series of electron carriers to transport electrons from chlorophyll to the coenzyme nicotine adenine dinucleotide phosphate (NADP+)
It forms NADPH when reduced
This is called photoreduction and involves a chloroplast electron transport system (ETS)

55. Similarity to mitochondrial electron transport
The chloroplast electron transport system is similar to that of mitochondria
The complete pathway includes several components
Many of the molecules are similar to those of the mitochondrial ETS—cytochromes, iron-sulfur proteins, etc.

56. Go and Eo
Recall that Go (standard free energy) and Eo (standard reduction potential) are opposite in sign
This means that electrons will spontaneously flow toward a compound with a higher reduction potential
Absorption of light by each photosystem boosts electrons to the top of an ETS
NOTE: there is a typo on page 302- Eo’ is missing in front of the bracketed phrase (standard reduction potential)

57. Electron flow
As electrons flow from PSII to PSI, a portion of their energy is conserved in a proton gradient across the thylakoid membrane
From PSI the electrons flow to ferredoxin and then to NADP+
NADP+ is the coenzyme mainly used for anabolic pathways, whereas NAD+ is usually involved in catabolic pathways

58. Figure 11-8

59. Figure 11-8A

60. Figure 11-8B

61. Figure 11-8C

62. Figure 11-8D

63. Photosystem II Transfers Electrons from Water to a Plastoquinone
Photosystem II uses electrons from water to reduce a plastoquinone (QB) to plastoquinol (QBH2)
PSII is associated with light-harvesting complex II (LHCII), which contains about 250 chlorophyll and many carotenoid molecules
Energy captured by antenna pigments of PSII or LHCII is funneled to the reaction center

64. Figure 11-9

65. Figure 11-9A

66. Figure 11-9B

67. Figure 11-9C

68. Figure 11-9D

69. Photosystem II (continued)
Captured energy in the reaction center lowers the reduction potential of a P680 molecule making it a better electron donor
A photoexcited electron is passed to pheophytin (Ph), a chlorophyll a molecule with two protons in place of the Mg2+
The charge separation between P680+ and Ph– prevents the electron from returning to its ground state

70. Photosystem II (continued)
Solar energy has been harvested and converted into electrochemical potential energy in the form of the charge separation
The electron is passed to QA, a plastoquinone (similar to coenzyme Q) tightly bound to protein D2
QB receives two electrons from QA and picks up two protons from the stroma to form QBH2

71. Photosystem II (continued)
QBH2 enters a mobile pool of QBH2 inside the photosynthetic membrane, where it passes two electrons and two protons to the cytochrome b6 /f complex
Formation of one mobile plastoquinone molecule depends on two sequential photoreactions

72. Photosystem II (continued)
To replace the electron lost to plastoquinone, oxidized P680+ is reduced by an electron from water
PSII includes an oxygen-evolving complex that catalyzes the splitting and oxidation of water, producing O2, electrons, and protons
Two water molecules donate four electrons one at a time to four molecules of P680+

73. Photosystem II (continued).
The protons accumulating in the lumen contribute to an electrochemical proton gradient across the thylakoid membrane, and the O2 diffuses out of the chloroplast

74. Photosystem II summarized
The net reaction catalyzed by four photoexcitations at PSII can be summarized as
The light-dependent oxidation of water is called water photolysis

75. The Cytochrome b6 /f Complex Transfers Electrons from a Plastoquinone to Plastocyanin
Electrons carried by QBH2 flow through an ETS coupled to unidirectional proton pumping into the lumen
This happens by way of the cytochrome b6 /f complex, which is composed of seven different integral transmembrane proteins including two cytochromes and an iron-sulfur protein

76. The Cytochrome b6 /f complex
QBH2 donates two electrons via cytochrome b6 and the iron-sulfur protein to cytochrome f
Each oxidation of QBH2 releases two protons into the thylakoid lumen
Additional protons can be pumped into the lumen by the Q cycle

77. The Q cycle .
Because half the electrons are recycled back to QB, the Q cycle would double the number of protons translocated to the lumen

78. Electrons are passed to plastocyanin
Reduced cytochrome f donates electrons to a copper-containing protein called plastocyanin (PC), which is a mobile electron carrier
PC, a peripheral membrane on the lumenal side of the thylakoid membrane, carries electrons one at a time to PSI .

79. Photosystem I Transfers Electrons from Plastocyanin to Ferredoxin
PSI transfers photoexcited electrons from reduced plastocyanin to the protein ferredoxin, the immediate electron donor to NADP+
The PSI reaction center includes a chlorophyll a molecule called Ao (instead of pheophytin), as well as phylloquinone and three Fe-Su centers that form an ETS from Ao to ferredoxin

80. Light-harvesting complex I
PSI in plants and green algae is associated with light-harvesting complex I (LHCI), with fewer antennae molecules than LHCII
Energy is funneled to a reaction center with a special pair of chlorophyll a molecules, P700
The energy absorbed by PSI lowers the reduction potential of the P700s so that a photoexcited electron is rapidly passed to Ao

81. Charge separation
The charge separation between P700+ and reduced Ao prevents the electron from returning to the ground state
The electron lost by P700 is replaced by an incoming electron from reduced plastocyanin
From Ao electrons flow exergonically through the ETS to ferredoxin, the final electron acceptor for PSI

82. Ferredoxin
Ferredoxin (Fd) is a mobile iron-sulfur protein found in the stroma
Overall the net reaction catalyzed by PSI can be summarized as follows

83. Ferredoxin-NADP+ Reductase Catalyzes the Reduction of NADP+
The final step in photoreduction is the transfer of electrons from ferredoxin to NADP+, producing the NADPH needed for carbon reduction and assimilation
The enzyme responsible is ferredoxin-NADP+ reductase (FNR) .

84. Noncyclic electron flow
The components of the chloroplast ETS provide a continuous unidirectional flow of electrons from water to NADP+
This is called noncyclic electron flow, and the net result is

85. Photosynthetic Energy Transduction III: ATP Synthesis
In the final stage of photosynthetic energy transduction the potential energy stored in a proton gradient is used to synthesize ATP
This process is called photophosphorylation
The thylakoid membrane is virtually impermeable to protons, so a substantial proton gradient can develop

86. Pmf in chloroplasts
In chloroplasts, the DpH is more important than DVm and contributes about 80% of the proton motive force
Light-induced proton pumping causes the pH in the lumen to drop to 6, while the stromal pH rises to about 8 due to proton depletion

87. The ATP Synthase Complex Couples Transport of Protons Across the Thylakoid Membrane to ATP Synthesis
The movement of protons back across the membrane to regions of lower concentration drives the synthesis of ATP by an ATP synthase
The ATP synthase complex found in chloroplasts is called the CF0CF1 complex, very similar to the F0F1 complex of mitochondria

88. The CF0CF1 complex
CF1 is a hydrophylic group of polypeptides protruding from the stromal side of the thylakoid membrane, and containing three catalytic sites for ATP synthesis
CF0 is a hydrophobic assembly of polypeptides anchored to the thylakoid membrane

89. Components of CF0
Subunits I and II form a stalk that connects CF0 and CF1
Subunit IV is the proton translocator, through which protons flow back to the stroma
Subunit III is a ring of polypeptides next to subunit IV, the rotation of which is coupled to ATP synthesis, similar to mitochondria

90. Four protons per ATP
Recent evidence suggests that four protons are translocated for every ATP generated
There are 14 copies of subunit III, and 3 ATP are generated by one complete rotation, leading to estimates of more than four protons per ATP .

91. Cyclic Photophosphorylation Allows a Photosynthetic Cell to Balance NADPH and ATP Synthesis
When NADPH consumption is low, or more ATP is needed, cyclic electron flow can divert the reducing power of PSI into ATP synthesis rather than NADP+ reduction
This is called cyclic photophosphorylation
No water is oxidized nor O2 released, because PSII is not involved

92. Figure 11-10

93. A Summary of the Complete Energy Transduction System
The component parts of the complete system can be summarized as follows:
1. Photosystem II complex
Assembly of chlorophyll, pigments, proteins
Oxidization of water, evolving O2
P680 becomes photoexcited, enabling it to reduce plastoquinone

94. The complete energy transduction system (continued)
2. Cytochrome b6 /f complex
Accepts electrons from plastoquinone (noncyclic) or ferredoxin (cyclic)
Pumps protons unidirectionally into thylakoid lumen
3. Photosystem I complex
Assembly of chlorophyll, pigments, proteins
P700 becomes photoexcited, enabling it to reduce ferredoxin (stromal protein)

95. The complete energy transduction system (continued)
4. Ferredoxin-NADP+reductase
Enzyme on stromal side of thylakoid membrane
Catalyzes transfer of electrons from two reduced ferredoxins to NADP+
NADPH product essential reducing agent in many anabolic pathways

96. The complete energy transduction system (continued)
5. ATP synthase complex (CF0CF1)
CFoCF1 proton channel and ATP synthase
Uses energy from exergonic flow of protons to synthesize ATP in the stroma
ATP essential for carbon fixation/assimilation

97. Photosynthetic Carbon Assimilation I: The Calvin Cycle
The main pathway for movement of inorganic carbon into the biosphere is the Calvin cycle
In plants and algae, the cycle is confined to the chloroplast stroma, where ATP and NADPH accumulate

98. Entry of CO2 into plants
In plants, CO2 enters the leaves through special pores called stomata
Once inside a leaf, CO2 diffuses into mesophyll cells and usually travels into the stroma
The stroma is the site of carbon fixation

99. Three stages of the Calvin cycle
1. The carboxylation of ribulose-1,5-bisphosphate, and generation of two 3-phosphoglycerate molecules
2. Reduction of 3-phosphoglycerate into glyceraldehyde-3-phosphate
3. Regeneration of the original acceptor to allow continued carbon assimilation

100. Figure 11-11

101. Activity: The Calvin cycle – Part 1
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102. Activity: The Calvin cycle – Part 2
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103. Activity: The Calvin cycle – Part 3
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104. Activity: The Calvin cycle – Part 4
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105. Carbon Dioxide Enters the Calvin Cycle by Carboxylation of Ribulose-1,5-Bisphosphate
The first stage begins with the covalent attachment of CO2 to ribulose-1,5-bisphosphate (CC-1)
This leads to production of two 3-carbon molecules, 3-phosphoglycerate
Ribulose-1,5-bisphosphate carboxylase/oxygenase(“rubisco”) is the most abundant protein on the planet

106. The overall reaction

107. Figure 11-12

108. 3-Phosphoglycerate Is Reduced to Form Glyceraldehyde-3-Phosphate
The reduction of 3-phosphoglycerate to form glyceraldehyde-3-phosphate is essentially the reverse of the oxidative sequence of glycolysis
The coenzyme is NADPH instead of NADH
Phosphoglycerokinase catalyzes reaction CC-2 and glyceraldehyde-3-phosphate dehydrogenase catalyzes CC-3

109. Energy is consumed in the first stages of carbon fixation
For every CO2 molecule fixed by rubisco two ATP molecules must be hydrolyzed and two NADPH molecules are oxidized .

110. Regeneration of Ribulose-1,5-Bisphosphate Allows Continuous Carbon Assimilation
One of six triose phosphate molecules generated is used for biosynthesis of organic molecules
The remaining five are used to regenerate three molecules of the (five-carbon) acceptor ribulose-1,5-bisphosphate (CC-4)
The reactions are catalyzed by aldolases, transketolases, phosphatases, and isomerases

111. Regeneration of ribulose-1,5-bisphosphate requires energy
Three molecules of ribulose-5-phosphate are converted to ribulose-1,5-bisphosphate by phosphoribulokinase (PRK)
Regeneration of the three ribulose-1,5-bisphosphate consumes three more ATPs .

112. The Complete Calvin Cycle and Its Relation to Photosynthetic Energy Transduction.
The Calvin cycle uses 9 ATP molecules and 6 NADPH for every 3-carbon carbohydrate synthesized
Activity of the cyclic pathway of PSI and/or the Q cycle are needed to produce the ATP
112

113. Overall reaction including energy transduction and carbon assimilation.
The absorption of 26 photons is accounted for by
12 photoexcitation events at PSI and 12 at PSII during noncyclic electron flow
2 photoexcitation events at PSI during cyclic flow
If the Q cycle is operating, fewer photons will be needed
113

114. Combining photosynthetic energy transduction with the Calvin cycle.
This reaction is almost identical to the net photosynthetic reaction

115. Maximum efficiency of photosynthetic energy transduction and carbon assimilation
Assuming wavelength = 670nm, 26 moles of photons are 1118 kcal of energy
Glyceraldehyde differs in free energy from CO2 and H2O by 343 kcal/mol
So the efficiency of energy transduction is about 31%, greater than most man-made energy transducing machinery
115

116. Regulation of the Calvin Cycle
In the dark, phototrophs must meet a steady demand for energy and carbon using the surplus accumulated when light is available
Several regulatory systems are used to ensure that the Calvin cycle does not operate unless light is available

117. The Calvin Cycle Is Highly Regulated to Ensure Maximum Efficiency
The first level of control is regulation of key enzymes in the Calvin cycle
These enzymes are not synthesized in tissues that are not exposed to light
Also, reduced ferredoxin, ATP, and NADPH, act as signals to activate Cavin cycle enzymes

118. Rubisco and other enzymes are points for metabolic control
Rubisco is an obvious control point as it catalyzes the carboxylation reaction of the Calvin cycle
Sedoheptulose bisphosphatase and PRK, with roles in regenerating the acceptor molecule, are also regulated
All are stimulated by high pH and high [Mg2+]

119. Rubisco and other enzymes are points for metabolic control (continued)
All three of the regulated enzymes
Are unique to the Calvin cycle
Catalyze reactions that are essentially irreversible

120. Regulation based on ferredoxin
In the light, electrons donated by water are used to reduce ferredoxin, then transferred to thioredoxin (enzyme: ferredoxin-thioredoxin reductase)
Glyceraldehyde-3-phosphate dehydrogenase, sedoheptulose bisphosphatase, and PRK are activated by the conformational change caused by thioredoxin, which is not available in the dark

121. Figure 11-13

122. Early glycolysis is inhibited in the light
The same mechanisms that activate enzymes of the Calvin cycle, inactivate enzymes of degradative pathways
When the Calvin cycle is operating in the light, phosphofructokinase, an important control point in glycolysis, is inhibited

123. Rubisco Activase Regulates Carbon Fixation by Rubisco
Rubisco activase removes inhibitory sugar-phosphate compounds from the rubisco active site
Rubisco activase has ATPase activity, which is sensitive to the ADP/ATP ratio
In the dark, accumulated ADP inhibits rubisco activase, leaving rubisco inactive
Note: This heading on page 313, upper right side, has a typo in the heading “FIxation” instead of Fixation
123

124. Photosynthetic Carbon Assimilation II: Carbohydrate Synthesis
The most abundant protein in the chloroplast is a phosphate translocator, which catalyzes the exchange of triose phosphates in the stroma for Pi in the cytosol
This antiport system only exports triose phosphates if Pi for making new triose phosphates returns to the stroma
The triose phosphates that remain in the stroma are used for starch synthesis
124

125. Glucose-1-Phosphate Is Synthesized from Triose Phosphates
Two triose phosphates undergo a condensation reaction catalyzed by aldolase, to generate fructose-1,6-bisphosphate
This is dephosphorylated by fructose-1,6-bisphosphatase to form fructose-6-phosphate (S-1)
This is catalyzed in both stroma and cytosol by distinct forms of the enzyme, called isoenzymes

126. S-2 and S-3
Fructose-6-phosphate can be converted to glucose-6-phosphate (S-2)
This is then converted to glucose-1-phosphate (S-3)
There are separate stromal and cytosolic isoenzymes for these reactions too, as the hexoses involved cannot be transported between the cytosol and stroma

127. Figure 11-14

128. The Biosynthesis of Sucrose Occurs in the Cytosol
Sucrose synthesis is located in the cytosol of a photosynthetic cell
Triose phosphates exported from the stroma that are not used in other metabolic pathways are converted to glucose-1-phosphate
Glucose is then produced by reaction with UTP (uridine triphosphate) to produce UDP-glucose (S-4c)

129. Biosynthesis of Sucrose (continued)
The glucose of UDP-glucose is transferred to fructose-6-phosphate to form sucrose-6-phosphate (S-5c)
The hydrolytic removal of the phosphate group generates free sucrose (S-6c)

130. Control of sucrose biosynthesis
Sucrose synthesis is controlled to prevent conflict with degradation pathways
Cytosolic fructose-1,6-bisphosphatase is inhibited by fructose-2,6-bisphosphate, a regulator of glycolysis and gluconeogenesis
Sucrose phosphate synthase is stimulated by glucose-6-phosphate and inhibited by sucrose-6-phosphate, UDP, and Pi

131. The Biosynthesis of Starch Occurs in the Chloroplast Stroma
Starch synthesis is confined to plastids, where triose phosphates are converted to glucose-1-phosphate, which is then used for starch synthesis
Glucose-1-phosphate reacts with ATP to generate ADP-glucose (S-4s)
The activated glucose is added to a growing starch chain by starch synthase (S-5s)

132. Control of starch biosynthesis
Starch synthesis is regulated to prevent conflict with degradation pathways
For example, ADP-glucose phosphorylase is stimulated by glyceraldehyde-3-phosphate and inhibited by Pi

133. Photosynthesis Also Produces Reduced Nitrogen and Sulfur Compounds
ATP and NADPH generated by photosynthetic energy transduction are consumed by a variety of other anabolic pathways
For example, the reduction of nitrite (NO2–) to ammonia (NH3)
For example, the reduction of sulfate (SO42–) to sulfide (S2–)

134. Rubisco’s Oxygenase Activity Decreases Photosynthetic Efficiency
The primary reaction catalyzed by rubisco is the addition of CO2 and H2O to ribulose-1,5-bisphosphate, forming two 3-phosphoglycerate
However, in addition to this function as a carboxylase, rubisco can act as an oxygenase
In this way rubisco can add molecular oxygen rather than CO2
134

135. Rubisco’s Oxygenase Activity
The result is phosphoglycolate, which cannot be used in the Calvin cycle, and thus appears wasteful
135

136. The Glycolate Pathway Returns Reduced Carbon from Phosphoglycolate to the Calvin Cycle
Phosphoglycolate is channeled into the glycolate pathway, which returns about 75% of it to the Calvin cycle as 3-phosphoglycerate
The pathway is also called photorespiration because of its light-dependent uptake of O2 and release of CO2
Several steps of the pathway take place in a leaf peroxisome

137. The glycolate pathway
Phosphoglycolate is rapidly dephosphorylated by a phosphatase in the stroma (GP-1)
The resulting glycolate diffuses to a leaf peroxisome where an oxidase converts it to glyoxylate (GP-2)
This is accompanied by O2 uptake and H2O2 generation, which is immediately degraded by catalase

138. The glycolate pathway (continued)
An aminotransferase transfers an amino group to glyoxylate, forming glycine (GP-3)
Glycine diffuses from the peroxisome to a mitochondrion where a decarboxylase and a hydroxymethyl transferase convert two glycines to one serine, along with formation of NADH and release of CO2 and NH3 (GP-4)

139. The glycolate pathway (continued)
Serine diffuses back to the peroxisome, where another aminotransferase removes an amino group to form hydroxypyruvate (GP-5)
A reductase reduces it to glycerate (GP-6)
Glycerate diffuses to the chloroplast and is phosphorylated by glycerate kinase, to form 3-phosphoglycerate (GP-7)

140. The value of the glycolate pathway
75% of the phosphoglycolate molecules produced are recovered as 3-phosphoglycerate and are thus not wasted
The glycolate pathway prevents a toxic buildup of phosphoglycolate
The pathway is metabolically expensive, but still represents a net gain for the plant

141. Figure 11-15

142. C4 Plants Minimize Photorespiration by Confining Rubisco to Cells Containing High Concentrations of CO2
Plants in hot, arid environments are particularly affected by rubisco’s oxygenase activity
For example, the CO2 solubility is more affected by temperature than that of O2, creating a problem of CO2:O2 balance
For example, when stomata are closed [CO2] declines whereas photolysis continues to produce O2

143. Adaptive strategies
In some cases, problems of energy and carbon drain necessitate adaptive strategies to allow a plant to overcome the problem
One general approach is to confine rubisco to those cells with a high [CO2], to minimize the oxygenase activity
In some plants the Hatch–Slack cycle is used

144. The Hatch–Slack cycle
The Hatch–Slack cycle is a short carboxylation/ decarboxylation pathway, with oxaloacetate as the intermediate of carbon fixation
Plants that use this strategy are called C4 plants (oxaloacetate is a 4-carbon compound)
C3 plants, have the 3-carbon compound 3-phosphoglycerate as the first detectable product of carbon fixation

145. Leaf structure of C4 plants
C4 plants have two types of photosynthetic cells in their leaves
Mesophyll cells: CO2 fixation here uses an enzyme other than rubisco; these cells are exposed to CO2 and O2
Bundle sheath cells: These are relatively isolated from the atmosphere, and the entire Calvin cycle is confined to these cells

146. Figure 11-16

147. The Hatch–Slack Cycle in a C4 Leaf
The Hatch–Slack cycle begins with carboxylation of PEP to form oxaloacetate (HS-1)
Carboxylation is catalyzed by PEP carboxylase, which is abundant in mesophyll cells
Oxaloacetate is rapidly converted to malate by NADPH-dependent malate dehydrogenase (HS-2)

148. The Hatch–Slack Cycle in a C4 Leaf (continued)
Malate moves to bundle sheath cells, where decarboxylation by NADP+ malic enzyme releases CO2 (HS-3), which is refixed and reduced by the Calvin cycle
The pyruvate produced in HS-3 diffuses into mesophyll cells, where it is phosphorylated, to regenerate PEP (HS-4), at the expense of one ATP

149. Figure 11-17

150. Figure 11-17A

151. Figure 11-17B

152. Carbon assimilation in a C4 plant
Carbon assimilation in a C4 plant uses 5 ATP rather than the 3 used in C3 plants
When temperatures exceed about 30oC, the efficiency of a C4 plant may be twice that of a C3 plant
C4 plants are also less affected by conditions of low [CO2]

153. CAM Plants Minimize Photorespiration and Water Loss by Opening Stomata Only at Night
Crassulacean acid metabolism (CAM) plants open stomata only at night to minimize water loss
CO2 enters mesophyll cells and goes through the first two steps of the Hatch–Slack cycle, to produce malate
The accumulated malate is stored in vacuoles

154. CAM plants (continued)
During the day the stomata are closed and the malate diffuses to the cytosol where the Hatch–Slack cycle continues
CO2 released diffuses to chloroplast stroma, where it is refixed and reduced in the Calvin cycle
CAM plants may assimilate over 25 times as much carbon as a C3 plant does