1. CHAPTER 8 Photosynthesis: Energy
from the Sun

2. Chapter 8: Photosynthesis: Energy from the Sun
Identifying Photosynthetic Reactants and Products
The Two Pathways of Photosynthesis: An Overview
Properties of Light and Pigments

3. Chapter 8: Photosynthesis: Energy from the Sun
Light Reactions: Light Absorption
Making Sugar from CO2: The Calvin–Benson Cycle
Photorespiration and Its Evolutionary Consequences
Metabolic Pathways in Plants

4. Photosynthesis
Life on Earth depends on the absorption of light energy from the sun. 4

5. Photosynthesis In plants, photosynthesis takes place in chloroplasts.5

6. Identifying Photosynthetic Reactants and Products
Photosynthesizing plants take in CO2, water, and light energy, producing O2 and carbohydrate. The overall reaction is
6 CO H2O + light  C6H12O6 + 6 O2 + 6 H2O
The oxygen atoms in O2 come from water, not from CO2. Review Figures 8.1, 8.2 6

7. figure jpg
Figure 8.1
Figure 8.1

8. figure jpg
Figure 8.2
Figure 8.2

9. The Two Pathways of Photosynthesis: An Overview
In the light reactions of photosynthesis, electron flow and photophosphorylation produce ATP and reduce NADP+ to NADPH + H+.
Review Figure 8.3 9

10. figure jpg
Figure 8.3
Figure 8.3

11. The Two Pathways of Photosynthesis: An Overview
ATP and NADPH + H+ are needed for the reactions that fix and reduce CO2 in the Calvin–Benson cycle, forming sugars.
Review Figure 8.3 11

12. figure jpg
Figure 8.3
Figure 8.3

13. Properties of Light and Pigments
Light energy comes in packets called photons, but it also has wavelike properties.
Review Figure 8.4 12

14. figure jpg
Figure 8.4
Figure 8.4

15. Properties of Light and Pigments
Pigments absorb light in the visible spectrum.
Review Figure 8.5 14

16. figure jpg
Figure 8.5
Figure 8.5

17. Properties of Light and Pigments
Absorption of a photon puts a pigment molecule in an excited state with more energy than its ground state.
Review Figure 8.6 16

18. figure jpg
Figure 8.6
Figure 8.6

19. Properties of Light and Pigments
Each compound has a characteristic absorption spectrum which reveals the biological effectiveness of different wavelengths of light.
Review Figures 8.7, 8.8 18

20. figure jpg
Figure 8.7
Figure 8.7

21. figure jpg
Figure 8.8
Figure 8.8

22. Properties of Light and Pigments
Chlorophylls and accessory pigments form antenna systems for absorption of light energy.
Review Figures 8.7, 8.9, 8.11 21

23. figure jpg
Figure 8.9
Figure 8.9

24. Light Reactions: Light Absorption
An excited pigment molecule may lose its energy by fluorescence, or by transferring it to another pigment molecule.
Review Figures 8.10, 8.11 24

25. figure jpg
Figure 8.10
Figure 8.10

26. figure jpg
Figure 8.11
Figure 8.11

27. Electron Flow, Photophos-phorylation, and Reductions
Noncyclic electron flow uses two photosystems:
Photosystem II uses P680 chlorophyll, from which light-excited electrons pass to a redox chain that drives chemiosmotic ATP production. Light-driven water oxidation releases O2, passing electrons to P680 chlorophyll.
Photosystem I passes electrons from P700 chlorophyll to another redox chain and then to NADP+, forming NADPH + H+. Review Figure 8.12 26

28. figure 08-12a.jpg
Figure 8.12 – Part 1
Figure 8.12 – Part 1

29. figure 08-12b.jpg
Figure 8.12 – Part 2
Figure 8.12 – Part 2

30. Electron Flow, Photophos-phorylation, and Reductions
Cyclic electron flow uses P700 chlorophyll producing only ATP.
Its operation maintains the proper balance of ATP and NADPH + H+ in the chloroplast.
Review Figure 8.13 29

31. figure jpg
Figure 8.13
Figure 8.13

32. Electron Flow, Photophos-phorylation, and Reductions
Chemiosmosis is the source of ATP in photophosphorylation.
Electron transport pumps protons from stroma into thylakoids, establishing a proton-motive force.
Proton diffusion to stroma via ATP synthase channels drives ATP formation from ADP and Pi.
Review Figure 8.14 31

33. figure jpg
Figure 8.14
Figure 8.14

34. Electron Flow, Photophos-phorylation, and Reductions
Photosynthesis probably originated in anaerobic bacteria that used H2S as a source of electrons instead of H2O.
Oxygen production by bacteria was important in eukaryote evolution. 33

35. Light-Dependent Reactions
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. The light-dependent reactions take place within the thylakoid membranes of chloroplasts.

36. Photosynthesis begins when pigments in photosystem II absorb light, increasing their energy level.
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. The light-dependent reactions take place within the thylakoid membranes of chloroplasts.

37. These high-energy electrons are passed on to the electron transport chain.
Photosystem II
Electron carriers
High-energy electron

38. Enzymes on the thylakoid membrane break water molecules into:
Photosystem II
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
Electron carriers
High-energy electron

39. hydrogen ions oxygen atoms energized electrons Photosystem II + O2
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
Electron carriers
High-energy electron

40. The energized electrons from water replace the high-energy electrons that chlorophyll lost to the electron transport chain.
Photosystem II
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
High-energy electron

41. As plants remove electrons from water, oxygen is left behind and is released into the air.
Photosystem II
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. The light-dependent reactions take place within the thylakoid membranes of chloroplasts.
High-energy electron

42. The hydrogen ions left behind when water is broken apart are released inside the thylakoid membrane.
Photosystem II
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
High-energy electron

43. Energy from the electrons is used to transport H+ ions from the stroma into the inner thylakoid space.
Photosystem II
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.

44. High-energy electrons move through the electron transport chain from photosystem II to photosystem I.
Photosystem II
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
Photosystem I

45. Pigments in photosystem I use energy from light to re-energize the electrons.
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
Photosystem I

46. NADP+ then picks up these high-energy electrons, along with H+ ions, and becomes NADPH.
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
2 NADP+ 2 2
NADPH

47. As electrons are passed from chlorophyll to NADP+, more H+ ions are pumped across the membrane.
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
2 NADP+ 2 2
NADPH

48. Soon, the inside of the membrane fills up with positively charged hydrogen ions, which makes the outside of the membrane negatively charged.
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
2 NADP+ 2 2
NADPH

49. The difference in charges across the membrane provides the energy to make ATP
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
2 NADP+ 2 2
NADPH

50. H+ ions cannot cross the membrane directly.
ATP synthase
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
2 NADP+ 2 2
NADPH

51. The cell membrane contains a protein called ATP synthase that allows H+ ions to pass through it
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
2 NADP+ 2 2
NADPH

52. As H+ ions pass through ATP synthase, the protein rotates.
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
2 NADP+ 2 2
NADPH

53. As it rotates, ATP synthase binds ADP and a phosphate group together to produce ATP.
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
ADP
2 NADP+ 2 2
NADPH

54. Because of this system, light-dependent electron transport produces not only high-energy electrons but ATP as well.
ATP synthase
+ O2
2H2O
The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.
ADP
2 NADP+ 2 2
NADPH

55. Making Sugar from CO2: The Calvin–Benson Cycle
The Calvin–Benson cycle makes sugar from CO2.
This pathway was elucidated through use of radioactive tracers Review Figure 8.15 34

56. figure jpg
Figure 8.15
Figure 8.15

57. Making Sugar from CO2: The Calvin–Benson Cycle
The Calvin–Benson cycle has three phases: fixation of CO2, reduction and carbohydrate production, and regeneration of RuBP.
RuBP is the initial CO2 acceptor, 3PG is the first stable product of CO2 fixation.
Rubisco catalyzes the reaction of CO2 and RuBP to form 3PG.
Review Figures 8.16, 8.17 36

58. figure jpg
Figure 8.16
Figure 8.16

59. figure jpg
Figure 8.17
Figure 8.17

60. Photorespiration and Its Evolutionary Consequences
Rubisco catalyzes a reaction between O2 and RuBP in addition to that of CO2 and RuBP.
Photorespiration significantly reduces photosynthesis efficiency.
Reactions that constitute photorespiration are distributed over chloroplast, peroxisome, and mitochondria organelles. 39

61. Photorespiration and Its Evolutionary Consequences
At high temperatures and low CO2 concentrations, the oxygenase function of rubisco is favored. 40

62. Photorespiration and Its Evolutionary Consequences
C4 plants bypass photorespiration.
PEP carboxylase in mesophyll chloroplasts initially fixes CO2 in four-carbon acids, which diffuse into bundle sheath cells, where their decarboxylation produces locally high concentrations of CO2.
Review Figures 8.19 41

63. figure jpg
Figure 8.19
Figure 8.19

64. Photorespiration and Its Evolutionary Consequences
CAM plants operate much like C4 plants, but their initial CO2 fixation by PEP carboxylase is temporally separated from the Calvin–Benson cycle, rather than spatially separated.
Review Figure 8.21 43

65. figure jpg
Figure 8.21
Figure 8.21

66. Metabolic Pathways in Plants
Plants respire in light and darkness, but photosynthesize only in light.
A plant must photosynthesize more than it respires, giving it a net gain of reduced energy-rich compounds. 45

67. Metabolic Pathways in Plants
Photosynthesis and respiration are linked through the Calvin–Benson cycle, the citric acid cycle, and glycolysis.
Review Figure 8.22 46

68. figure 08-22a.jpg
Figure 8.22 – Part 1
Figure 8.22 – Part 1

69. figure 08-22b.jpg
Figure 8.22 – Part 2
Figure 8.22 – Part 2