1. Chapter 10
Photosynthesis

3. Overview: The Process That Feeds the Biosphere Photosynthesis
Is the process that converts solar energy into chemical energy

4. 1. Plants and other autotrophs make their own energy (aka producers)

5. Plants are photoautotrophs
They use the energy of sunlight to make organic molecules from water and carbon dioxide
Figure 10.1

6. (b) Multicellular algae (c) Unicellular protist
Photosynthesis
Occurs in plants, algae, certain other protists, and some prokaryotes
These organisms use light energy to drive the
synthesis of organic molecules from carbon dioxide
and (in most cases) water. They feed not only
themselves, but the entire living world. (a) On
land, plants are the predominant producers of
food. In aquatic environments, photosynthetic
organisms include (b) multicellular algae, such
as this kelp; (c) some unicellular protists, such
as Euglena; (d) the prokaryotes called
cyanobacteria; and (e) other photosynthetic
prokaryotes, such as these purple sulfur
bacteria, which produce sulfur (spherical
globules) (c, d, e: LMs).
(a) Plants
(b) Multicellular algae
(c) Unicellular protist
10 m
40 m
(d) Cyanobacteria
1.5 m
(e) Pruple sulfur
bacteria
Figure 10.2

7. 2. Heterotrophs obtain their organic material from other organisms (aka consumers)

8. Concept 1: Photosynthesis converts light energy to the chemical energy of food

9. Chloroplasts: The Sites of Photosynthesis in Plants
3. Usually happens in the leaves of plants
Are the major sites of photosynthesis
Vein
Leaf cross section
Figure 10.3
Mesophyll
CO2 O2
Stomata

10. 3. Chloroplasts Are the organelles in which photosynthesis occurs
Contain thylakoids
and grana
Chloroplast
Mesophyll
5 µm
Outer
membrane
Intermembrane
space
Inner
Thylakoid
Granum
Stroma
1 µm

11. Tracking Atoms Through Photosynthesis: Scientific Inquiry
Photosynthesis is summarized as
6 CO2 + 6 H2O + Light energy  C6H12O6 + 6 O2

12. Chloroplasts split water into
The Splitting of Water
Chloroplasts split water into
Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules
6 CO2
12 H2O
Reactants:
Products:
C6H12O6
6 H2O
6 O2
Figure 10.4

13. Photosynthesis as a Redox Process
Photosynthesis is a redox process
Water is oxidized, carbon dioxide is reduced

14. The Two Stages of Photosynthesis: A Preview
Photosynthesis consists of two processes
The light reactions
The Calvin cycle

15. The light reactions occur in the thylakoids
4. Split water, release oxygen, produce ATP, and turns NADP+ into NADPH

16. The Calvin cycle occurs in the stroma
5. Forms sugar from carbon dioxide, using ATP for energy and NADPH for its electrons

17. The Calvin cycle occurs in the stroma
5. Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power

18. An overview of photosynthesis
CO2
Light
LIGHT REACTIONS
CALVIN
CYCLE
Chloroplast
[CH2O]
(sugar)
NADPH
NADP 
ADP
+ P O2
Figure 10.5
ATP

19. Concept 2: The light reactions convert solar energy to the chemical energy of ATP and NADPH

20. The Nature of Sunlight Light
Is a form of electromagnetic energy, which travels in waves

21. Wavelength Is the distance between the crests of waves
Determines the type of electromagnetic energy

22. The electromagnetic spectrum
Is the entire range of electromagnetic energy, or radiation
Gamma
rays
X-rays UV
Infrared
Micro-
waves
Radio
10–5 nm
10–3 nm
1 nm
103 nm
106 nm
1 m
103 m
380
450
500
550
600
650
700
750 nm
Visible light
Shorter wavelength
Higher energy
Longer wavelength
Lower energy
Figure 10.6

23. The visible light spectrum
Includes the colors of light we can see
Includes the wavelengths that drive photosynthesis

24. Photosynthetic Pigments: The Light Receptors
6. Pigments are substances that absorb specific wavelengths (colors) of light
- They absorb some colors of light better than others

25. Reflect light, which include the colors we see
Reflected
Chloroplast
Absorbed
light
Granum
Transmitted
Figure 10.7

26. The spectrophotometer
Is a machine that sends light through pigments and measures the fraction of light transmitted at each wavelength

27. An absorption spectrum
Is a graph plotting light absorption versus wavelength
Figure 10.8
White
light
Refracting
prism
Chlorophyll
solution
Photoelectric
tube
Galvanometer
Slit moves to
pass light
of selected
wavelength
Green
The high transmittance
(low absorption)
reading indicates that
chlorophyll absorbs
very little green light.
The low transmittance
(high absorption) reading
chlorophyll absorbs most blue light.
Blue 1 2 3 4
100

28. The absorption spectra of chloroplast pigments
Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis

29. Wavelength of light (nm)
The absorption spectra of three types of pigments in chloroplasts
Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below.
EXPERIMENT
RESULTS
Absorption of light by
chloroplast pigments
Chlorophyll a
(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments.
Wavelength of light (nm)
Chlorophyll b
Carotenoids
Figure 10.9

30. The action spectrum of a pigment
Profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis
(measured by O2 release)
Rate of photosynthesis
Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids.
(b)

31. The action spectrum for photosynthesis
Was first demonstrated by Theodor W. Engelmann
400
500
600
700
Aerobic bacteria
Filament
of alga
Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most.
Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b.
(c)
Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis.
CONCLUSION

32. 7. Chlorophyll a 8. Chlorophyll b Is the main photosynthetic pigment
Is an accessory pigment C CH
CH2 N
H3C Mg H
CH3 O
CHO
in chlorophyll a
in chlorophyll b
Porphyrin ring:
Light-absorbing
“head” of molecule
note magnesium
atom at center
Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts: H atoms not
shown
Figure 10.10

33. Other accessory pigments
Absorb different wavelengths of light and pass the energy to chlorophyll a

34. Photosystems – groups of pigments found in thylakoid membranes that work with integral proteins
Photosystem 1 and 2

35. Excitation of Chlorophyll by Light
1. When a pigment in photosystem 2 absorbs light, its electron jumps to a higher, unstable energy level
Excited
state
Energy of election
Heat
Photon
(fluorescence)
Chlorophyll
molecule
Ground e–
Figure A

36. Excitation of Chlorophyll by Light
2. Photosystem 2 then ejects these excited electrons
Excited
state
Energy of election
Heat
Photon
(fluorescence)
Chlorophyll
molecule
Ground e–
Figure A

37. 3. To replace it’s lost electrons, photosystem 2 steals electrons from H2O, which then breaks down into H+ and O2 (oxygen gas)

38. 4. Electrons from photosystem 2 move through thylakoid membrane in electron transport chain (ETC) to photosystem 1

39. Photosystem 1
5. In photosystem 1, the electrons from photosystem 2 get added to NADP+, which turns it into NADPH

40. 6. Leftover H+ from photosystem 2 get moved to other side of thylakoid, which creates a gradient
There are more H+ inside the cell than outside the cell
Where do protons want to go if there are more inside the cell than outside the cell?

41. 6. Leftover H+ from photosystem 2 get moved to other side of thylakoid, which creates a gradient
There are more H+ inside the cell than outside the cell
Where do protons want to go if there are more inside the cell than outside the cell?
OUTSIDE THE CELL!!!!!

43. 7. Protons leave the thylakoid through ATP synthase
ATP synthase – integral protein enzyme that phosphorylates ADP into ATP

45. So what gets made in the light reaction?
Oxygen
ATP
NADPH

46. Dark Reaction (Calvin Cycle)
8. ATP and NADPH move into stroma of the chloroplast

47. 9. ATP and the electrons carried by NADPH are used to power Calvin cycle (dark reaction)
This process takes carbon from carbon dioxide to make glucose for the cell

48. Calvin Cycle / Dark Reaction
10. Calvin cycle takes carbon from carbon dioxide to make glucose for the cell

49. The thylakoid membrane
Is populated by two types of photosystems, I and II

50. Noncyclic Electron Flow
Is the primary pathway of energy transformation in the light reactions

51. Electron transport chain
Produces NADPH, ATP, and oxygen
Figure 10.13
Photosystem II
(PS II)
Photosystem-I
(PS I)
ATP
NADPH
NADP+
ADP
CALVIN
CYCLE
CO2
H2O O2
[CH2O] (sugar)
LIGHT
REACTIONS
Light
Primary
acceptor Pq
Cytochrome
complex PC e
P680 e– +
2 H+ Fd
reductase
Electron
Transport
chain
Electron transport chain
P700
+ 2 H+
+ H+ 7 4 2 8 3 5 1 6

52. A mechanical analogy for the light reactions
Mill
makes
ATP e–
Photon
Photosystem II
Photosystem I
NADPH
Figure 10.14 

53. Under certain conditions
Cyclic Electron Flow
Under certain conditions
Photoexcited electrons take an alternative path

54. In cyclic electron flow
Only photosystem I is used
Only ATP is produced
Primary
acceptor Pq Fd
Cytochrome
complex Pc
NADP+
reductase
NADPH
ATP
Figure 10.15
Photosystem II
Photosystem I

55. A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
Generate ATP by the same basic mechanism: chemiosmosis
But use different sources of energy to accomplish this

56. The spatial organization of chemiosmosis
Differs in chloroplasts and mitochondria
Key
Higher [H+]
Lower [H+]
Mitochondrion
Chloroplast
MITOCHONDRION
STRUCTURE
Intermembrance
space
Membrance
Matrix
Electron
transport
chain H+
Diffusion
Thylakoid
Stroma
ATP P
ADP+
Synthase
CHLOROPLAST
Figure 10.16

57. In both organelles ATP synthase
Redox reactions of electron transport chains generate a H+ gradient across a membrane
ATP synthase
Uses this proton-motive force to make ATP

58. The light reactions and chemiosmosis: the organization of the thylakoid membrane
REACTOR
NADP+
ADP
ATP
NADPH
CALVIN
CYCLE
[CH2O] (sugar)
STROMA
(Low H+ concentration)
Photosystem II
H2O
CO2
Cytochrome
complex O2 1
1⁄2 2
Photosystem I
Light
THYLAKOID SPACE
(High H+ concentration)
Thylakoid
membrane
synthase Pq Pc Fd
reductase
+ H+
NADP+ + 2H+ To
Calvin
cycle P 3 H+
2 H+
+2 H+
Figure 10.17

59. Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO2 to sugar
Is similar to the citric acid cycle
Occurs in the stroma

60. The Calvin cycle has three phases
Carbon fixation
Reduction
Regeneration of the CO2 acceptor

61. The Calvin cycle Figure 10.18 Input Light 3 CO2 CALVIN CYCLE
(G3P)
Input
(Entering one
at a time)
CO2 3
Rubisco
Short-lived intermediate
3 P P
Ribulose bisphosphate (RuBP)
3-Phosphoglycerate
6 P 6
1,3-Bisphoglycerate
6 NADPH
6 NADPH+
Glyceraldehyde-3-phosphate
6 ATP
ATP
3 ADP
CALVIN
CYCLE 5 1
G3P (a sugar) Output
Light
H2O
LIGHT REACTION
ATP
NADPH
NADP+
ADP
[CH2O] (sugar)
CALVIN CYCLE
Figure 10.18 O2
6 ADP
Glucose and other organic compounds
Phase 1: Carbon fixation
Phase 3: Regeneration of the CO2 acceptor (RuBP)
Phase 2: Reduction

62. Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates

63. On hot, dry days, plants close their stomata
Conserving water but limiting access to CO2
Causing oxygen to build up

64. Photorespiration: An Evolutionary Relic?
In photorespiration
O2 substitutes for CO2 in the active site of the enzyme rubisco
The photosynthetic rate is reduced

65. C4 plants minimize the cost of photorespiration
By incorporating CO2 into four carbon compounds in mesophyll cells

66. These four carbon compounds
Are exported to bundle sheath cells, where they release CO2 used in the Calvin cycle

67. C4 leaf anatomy and the C4 pathway
CO2
Mesophyll cell
Bundle-
sheath
cell
Vein
(vascular tissue)
Photosynthetic
cells of C4 plant
leaf
Stoma
Mesophyll
C4 leaf anatomy
PEP carboxylase
Oxaloacetate (4 C)
PEP (3 C)
Malate (4 C)
ADP
ATP
Sheath
Pyruate (3 C)
CALVIN
CYCLE
Sugar
Vascular
tissue
Figure 10.19

68. CAM Plants
CAM plants
Open their stomata at night, incorporating CO2 into organic acids

69. During the day, the stomata close
And the CO2 is released from the organic acids for use in the Calvin cycle

70. The CAM pathway is similar to the C4 pathway
Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in different
types of cells.
(a)
Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different times.
(b)
Pineapple
Sugarcane
Bundle-
sheath cell
Mesophyll Cell
Organic acid
CALVIN CYCLE
Sugar
CO2 C4
CAM
CO2 incorporated
into four-carbon
organic acids
(carbon fixation)
Night
Day 1 2
Organic acids
release CO2 to
Calvin cycle
Figure 10.20

71. The Importance of Photosynthesis: A Review
A review of photosynthesis
Light reactions:
• Are carried out by molecules in the
thylakoid membranes
• Convert light energy to the chemical
energy of ATP and NADPH
• Split H2O and release O2 to the
atmosphere
Calvin cycle reactions:
• Take place in the stroma
• Use ATP and NADPH to convert
CO2 to the sugar G3P
• Return ADP, inorganic phosphate, and NADP+ to the light reactions O2
CO2
H2O
Light
Light reaction
Calvin cycle
NADP+
ADP
ATP
NADPH
+ P 1
RuBP
3-Phosphoglycerate
Amino acids
Fatty acids
Starch
(storage)
Sucrose (export)
G3P
Photosystem II
Electron transport chain
Photosystem I
Chloroplast
Figure 10.21

72. Organic compounds produced by photosynthesis
Provide the energy and building material for ecosystems