1. The Sun The Ultimate Energy Source For Life on Planet Earth
Photosynthesis
The Sun
The Ultimate Energy Source
For Life on Planet Earth

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
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3. Pyramid of Biomass Consumers Heterotrophs Producers Autotrophs
Trophic Levels
Consumers
The Sun
Heterotrophs
Producers
Autotrophs
Photosynthesis (P/S)

4. Autotrophs
Autotrophs sustain themselves without eating anything derived from other organisms
Autotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules
Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules
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5. 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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BioFlix: Photosynthesis

6. Purple sulfur bacteria 1 m
Figure 10.2
(b)
Multicellular alga
(a) Plants
Figure 10.2 Photoautotrophs.
(d) Cyanobacteria
40 m
(c)
Unicellular protists
10 m
(e)
Purple sulfur bacteria
1 m 6

7. Heterotrophs
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 O2
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8. Fossil Fuels
The Earth’s supply of fossil fuels was formed from the remains of organisms that died hundreds of millions of years ago
In a sense, fossil fuels represent stores of solar energy from the distant past (“Ancient Sunlight”)
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9. Concept 10.1: Photosynthesis converts light energy to the chemical energy of food
Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria
The structural organization of these cells allows for the chemical reactions of photosynthesis
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10. Photosynthesis (net consumption of water)
6CO2 + 6H2O C6H12O6 + 6O2
Light
Chlorophyll

11. Photosynthesis (tracking atoms)

12. How Does CO2 Get In The Plant?
CO2 enters the ____________
What is the source of CO2? ______
Air
78.08 %N2
20.95 %O2
0.03 %CO2
argon, helium, hydrogen, ozone, methane

13. How Does Water Get In The Plant?
Water enters through the __________
Most water is lost from the plant through the _________
To reduce water loss leaves are covered with a waxy cuticle (plant “chapstick”)

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
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15. The Leaf is Covered with a Waxy Cuticle: “Plant Chapstick”
If water cannot get out of
the leaf through the waxy
cuticle what cannot get
into the leaf for P/S?
CO2 Enters
H2O Exits
Mesophyll cell or photosynthetic cell: Note the chloroplasts

16. Chloroplasts also contain stroma, a dense interior fluid
CO2 enters and O2 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
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18. H2OExits
2 Guard Cells
surrounding a
pore
CO2 Enters
Open Stomate
Transpiration=
Evaporation of water from plant
stomate
Closed Stomate

19. Stomata On A Pine Needle

20. Transpiration
The evaporative water loss by a plant, primarily through stomata
Transpiration
Rate
Closed Partially Fully
Open Open
Degree of Stomatal Opening

21. Photosynthesis and Transpiration
High Rates of P/S are associated with high transpiration rates
Rate of
P/S
Closed Partially Fully
Open Open
Degree of Stomatal Opening

22. Tracking Atoms Through Photosynthesis: Scientific Inquiry
Photosynthesis is a complex series of reactions that can be summarized as the following equation:
6 CO H2O + Light energy  C6H12O6 + 6 O2 + 6 H2O
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23. The Splitting of Water
Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product
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24. Photosynthesis as a Redox Process
Photosynthesis reverses the direction of electron flow compared to respiration
Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced
Photosynthesis is an endergonic process; the energy boost is provided by light
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25. Energy  6 CO2  6 H2O C6 H12 O6  6 O2 becomes reduced
Figure 10.UN01
becomes reduced
Energy  6 CO2  6 H2O
C6 H12 O6  6 O2
becomes oxidized
Figure 10.UN01 In-text figure, p. 188 25

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 H2O
Release O2
Reduce NADP+ to NADPH
Generate ATP from ADP by photophosphorylation
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27. The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH
The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules
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28. Concept 10.2: 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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29. Structure of a Chloroplast

30. Chlorophyll Chlorophyll is a green pigment
Pigments absorb light energy
The color you see is the color that is reflected
White versus Black
Why is chlorophyll green?
Would you expect green light to be an effective color of light for P/S?

31. 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
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32. Light: Energy is Inversely Proportional to Wavelength

33. 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
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34. Chlorophyll Reflects and Transmits Green Light
Chlorophyll absorbs blue and red light
Accessory pigments such as carotenoids absorb green light and pass the energy to chlorophyll

35. 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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36. Absorption Spectra An absorption spectrum is a
graph plotting a pigment’s light absorption versus wavelength

37. light work best for photosynthesis
The absorption spectrum of chlorophyll a suggests that violet-blue and red
light work best for photosynthesis
For the Cell Biology Video Space-Filling Model of Chlorophyll a, go to Animation and Video Files.
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38. Photosynthesis is a Two Step Process
6CO2 + 6H2O C6H12O6 + 6O2
The Light Reactions: Light Dependant
Photochemical
Convert light energy into chemical energy
The Light Independent
Temperature Dependant
Use the chemical energy created in the light reactions to convert CO2 to glucose

39. Photosynthesis Overview (2 step process)

40. Locations of Reactions
The Light Dependent Reactions occur on the grana
The Light Independent Reactions take place in the stroma

41. High Potential Energy Molecules
NADP e- (2H) NADPH H+ e- e-
P P
NADPH H+ NADP e-
Is this an example of oxidation or reduction?
(oxidation = loss of e-, reduction = gain of e-)
Does the formation of NADPH require or release energy?

42. High Potential Energy Molecules
NADP e- (2H) NADPH H+
Reduction (endergonic)
NADPH H+ NADP e-
Oxidation (exergonic)
Does the formation of NADPH require or release energy?

43. High Potential Energy Molecules
Adenosine Triphosphate
Adenosine Diphosphate + P + Energy
High P.E.
Low P.E.
Low P.E.
High P.E.

44. The Light Reactions
Photochemical: Light energy is converted to chemical energy in the form of two high potential energy molecules.

45. The Light Reactions
The Two High Potential Energy Molecules Produced are:
__________
The Electron Source is?__________
When water gives up electrons what waste product is produced?

46. 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
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47. Photon (fluorescence)
Figure 10.12
Excited state e
Heat
Energy of electron
Photon (fluorescence)
Photon
Figure Excitation of isolated chlorophyll by light.
Ground state
Chlorophyll molecule
(a) Excitation of isolated chlorophyll molecule
(b) Fluorescence 47

48. 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
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49. THYLAKOID SPACE (INTERIOR OF THYLAKOID)
Figure 10.13a
Photosystem
STROMA
Photon
Light- harvesting complexes
Reaction- center complex
Primary electron acceptor e
Thylakoid membrane
Figure The structure and function of a photosystem.
Transfer of energy
Special pair of chlorophyll a molecules
Pigment molecules
THYLAKOID SPACE (INTERIOR OF THYLAKOID)
(a) How a photosystem harvests light 49

50. 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
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51. 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
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52. Photosystem I (PS I) is best at absorbing a wavelength of 700 nm
The reaction-center chlorophyll a of PS I is called P700
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53. Noncyclic (Linear) Electron Flow
During the light reactions, there are two possible routes for electron flow: cyclic and linear
Noncyclic (linear) electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy
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54. The Model For The Light Reactions Noncyclic Electron Flow

55. A photon hits a pigment and its energy is passed among pigment molecules until it excites P680
An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680+)
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56. Diffusion of H+ (protons) across the membrane drives ATP synthesis
Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I
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
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57. In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor
P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain
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58. The electrons are then transferred to NADP+ and reduce it to NADPH
Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)
The electrons are then transferred to NADP+ and reduce it to NADPH
The electrons of NADPH are available for the reactions of the Calvin cycle
This process also removes an H+ from the stroma
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60. The Model For The Light Reactions Cyclic Electron Flow

61. Cyclic Electron Flow
Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH
No oxygen is released
Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle
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62. The Light Reactions are Light Dependent
Can you think of a habitat
or ecosystem where P/S might
be limited?
Light Saturation
Rate of
P/S
Low Med High
Light Intensity

63. The Light Independent Reactions
What is the “fuel” that drives the light independent reactions? __________
Where was this “fuel” produced?_______________________

64. 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 H2O to NADPH
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65. STROMA (low H concentration) Cytochrome complex NADP reductase
Figure 10.18
STROMA (low H concentration)
Cytochrome complex
NADP reductase
Photosystem II
Photosystem I
Light 3
Light
4 H+
NADP + H Fd Pq
NADPH 2 Pc
H2O 1
1/2 O2
THYLAKOID SPACE (high H concentration)
+2 H+
4 H+
To Calvin Cycle
Figure The light reactions and chemiosmosis: the organization of the thylakoid membrane.
Thylakoid membrane
ATP synthase
ADP + P i
ATP
STROMA (low H concentration) H+ 65

66. Concept 10.3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar
The Calvin cycle regenerates its starting material after molecules enter and leave the cycle
The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH
Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate (G3P)
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67. Calvin Cycle (Step One)

68. Figure 10.19 The Calvin cycle.
Input 3
(Entering one at a time)
CO2
Phase 1: Carbon fixation
Rubisco 3 P P
Short-lived intermediate 3 P P 6 P
Ribulose bisphosphate (RuBP)
3-Phosphoglycerate 6
ATP
6 ADP
Calvin Cycle 6 P P
1,3-Bisphosphoglycerate
6 NADPH
6 NADP
6 P i
Figure The Calvin cycle. 6 P
Glyceraldehyde 3-phosphate (G3P)
Phase 2: Reduction 1 P
G3P (a sugar)
Glucose and other organic compounds
Output 68

70. Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates
Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis
On hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesis
The closing of stomata reduces access to CO2 and causes O2 to build up
These conditions favor an apparently wasteful process called photorespiration
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71. Photorespiration: An Evolutionary Relic?
In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (3-phosphoglycerate)
In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle, producing a two-carbon compound
Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar
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72. Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2
Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle
In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle
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73. C4 Plants
C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells
This step requires the enzyme PEP carboxylase
PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low
These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle
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74. Saltbush
Saltbush
can live in
salty soils since
it has salt
glands

75. Saltbush – A C4 Plant

76. Kranz Anatomy
Note that the
chloroplasts
are in the
center of the
leaf

78. CAM Plants
Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon
CAM plants open their stomata at night, incorporating CO2 into organic acids
Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle
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79. Cactus – A Stem Succulent
CAM P/s
Green
Stem

82. 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 structures such as roots, tubers, seeds, and fruits
In addition to food production, photosynthesis produces the O2 in our atmosphere
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83. Electron transport chain
Figure 10.17
Mitochondrion
Chloroplast
MITOCHONDRION STRUCTURE
CHLOROPLAST STRUCTURE H
Diffusion
Intermembrane space
Thylakoid space
Electron transport chain
Inner membrane
Thylakoid membrane
Figure Comparison of chemiosmosis in mitochondria and chloroplasts.
ATP synthase
Matrix
Stroma
ADP  P i
ATP
Key
Higher [H ] H
Lower [H ] 83

84. 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
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85. 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
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86. Climate Change
In the last 150 years since the Industrial Revolution, CO2 levels have risen greatly
Increasing levels of CO2 may affect C3 and C4 plants differently, perhaps changing the relative abundance of these species
The effects of such changes are unpredictable and a cause for concern
350.org, davidsuzuki.org
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