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2. The Process That Feeds the Biosphere
Plants and other photosynthetic organisms contain organelles called chloroplasts
Photosynthesis is the process that converts solar energy into chemical energy within chloroplasts
Photosynthesis occurs in plants, algae, certain other unicellular eukaryotes, and some prokaryotes
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3. Autotrophs
“self-feeders” that sustain themselves without eating anything derived from other organisms
Produce 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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4. (b) Multicellular alga
Figure 10.2
(d) Cyanobacteria 40 µm
(a) Plants
Figure 10.2 Photoautotrophs
(b) Multicellular alga
1 µm
(e) Purple sulfur bacteria
10 µm
(c) Unicellular eukaryotes
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5. Heterotrophs Obtain organic material from other organisms
Consumers of the biosphere
Some eat other living organisms
Decomposers consume dead organic material or feces
Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2
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6. 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 organelles allows for the chemical reactions of photosynthesis
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7. Chloroplasts: The Sites of Photosynthesis in Plants
Leaves are the major locations of photosynthesis in plants
Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf
Each mesophyll cell contains 30–40 chloroplasts
CO2 enters and O2 exits the leaf through microscopic pores called stomata
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8. A chloroplast has an envelope of two membranes surrounding a dense fluid called the stroma
Thylakoids
Connected sacs in the chloroplast that compose a third membrane system
Stacked in columns called grana
Chlorophyll, the pigment that gives leaves their green color, resides in the thylakoid membranes
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9. Mesophyll Chloroplast Mesophyll cell Outer membrane Thylakoid 20 µm
Leaf cross section
Chloroplasts
Vein
Mesophyll
Stomata
CO2 O2
Chloroplast
Mesophyll
cell
Outer
membrane
Figure 10.4 Zooming in on the location of photosynthesis in a plant
Thylakoid
20 µm
Thylakoid
space
Intermembrane
space
Stroma
Granum
Inner
membrane
Chloroplast
1 µm
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10. 6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2O
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
Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product
Reactants:
6 CO2
12 H2O
Products:
6 H2O
6 O2
C6H12O6
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11. Photosynthesis as a Redox Process
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
becomes reduced
becomes oxidized
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12. The Two Stages of Photosynthesis: A Preview
Light reactions (The photo part)
In the thylakoids
Split H2O
Release O2
Reduce NADP+ to NADPH
Generate ATP from ADP by photophosphorylation
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13. The Two Stages of Photosynthesis: A Preview
Calvin Cycle (The synthesis part)
In the stroma
Forms sugar from CO2, using ATP and NADPH
Begins with carbon fixation, incorporating CO2 into organic molecules
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14. CALVIN CYCLE LIGHT REACTIONS [CH2O] (sugar)
CO2
NADP+
ADP + P
CALVIN
CYCLE
LIGHT
REACTIONS i
ATP
Thylakoid
Stroma
Figure 10.6_4 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (step 4)
NADPH
Chloroplast O2
[CH2O]
(sugar)
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15. Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH
Light is electromagnetic energy, also called electromagnetic radiation
Electromagnetic energy travels in waves
Wavelength is the distance between crests of electromagnetic waves
Wavelength determines the type of electromagnetic energy
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16. Visible light also includes the wavelengths that drive photosynthesis
The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation (Radio Waves  Gamma)
Visible light consists of wavelengths (380 nm to 750 nm) that produce colors we can see
Visible light also includes the wavelengths that drive photosynthesis
Light also behaves as though it consists of discrete particles, called photons
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17. Gamma rays Micro- waves Radio waves
10–5 nm
10–3 nm
1 nm
103 nm
106 nm
(109 nm)
103 m
Gamma
rays
Micro-
waves
Radio
waves
X-rays UV
Infrared
Visible light
Figure 10.7 The electromagnetic spectrum
380
450
500
550
600
650
700
750 nm
Shorter wavelength
Longer wavelength
Higher energy
Lower energy
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18. 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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19. Light Reflected light Chloroplast Absorbed Granum light Transmitted
Figure 10.8 Why leaves are green: interaction of light with chloroplasts
Absorbed
light
Granum
Transmitted
light
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20. Animation: Light Energy and Pigments
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21. The high transmittance (low absorption) reading indicates
White
light
Refracting
prism
Chlorophyll
solution
Photoelectric
tube
Spectrometer 2 3 1 4
100
The high transmittance (low
absorption) reading indicates
that chlorophyll absorbs
very little green light.
Slit moves to pass light
of selected wavelength.
Green
light
Figure 10.9 Research method: determining an absorption spectrum
100
The low transmittance (high
absorption) reading indicates
that chlorophyll absorbs
most blue light.
Blue
light
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22. There are three types of pigments in chloroplasts:
Chlorophyll a, the key light-capturing pigment
Chlorophyll b, an accessory pigment
Carotenoids, a separate group of accessory pigments
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23. An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength
An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process
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24. interacts with hydrophobic regions of proteins inside
CH3 in chlorophyll a
CHO in chlorophyll b
CH3
Porphyrin ring:
light-absorbing
“head” of molecule;
note magnesium
atom at center
Figure Structure of chlorophyll molecules in chloroplasts of plants
Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts; H atoms not
shown
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25. 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, excess energy is released as heat
In isolation, some pigments also emit light, an afterglow called fluorescence
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26. 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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27. A primary electron acceptor (PEA) 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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28. (INTERIOR OF THYLAKOID)
Photosystem
STROMA
Photon
Light-
harvesting
complexes
Reaction-
center
complex
Primary
electron
acceptor
Thylakoid membrane e–
Figure 10.13a The structure and function of a photosystem (part 1: mechanism)
Transfer
of energy
Special pair of chloro-
phyll a molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
(a) How a photosystem harvests light
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29. There are two types of photosystems in the thylakoid membrane
Photosystem II (PS II) functions first (the numbers reflect order of discovery)
The reaction-center chlorophyll a of PS II is called P680
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30. 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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31. LIGHT REACTIONS CALVIN CYCLE
H2O
Light
NADP+
ADP
LIGHT
REACTIONS
ATP
NADPH
CO2
CALVIN
CYCLE O2
[CH2O] (sugar)
Figure 10.UN02 In-text figure, light reaction schematic, p. 197
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32. Linear Electron Flow
During the light reactions, there are two possible routes for electron flow:
Linear Electron Flow
Cyclic Electron Flow
Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy
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33. There are eight steps in linear electron flow:
A photon hits a pigment in a light-harvesting complex of PS II, 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+)
H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680
P680+ is the strongest known biological oxidizing agent
The H+ are released into the thylakoid space
O2 is released as a by-product of this reaction
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34. 2 H+ Photosystem II (PS II)
Primary
electron
acceptor 2
2 H+
H2O e– + 1 /2 3 O2 e– 1 e–
P680
Light
Figure 10.14_2 How linear electron flow during the light reactions generates ATP and NADPH (step 2)
Pigment
molecules
Photosystem II
(PS II)
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35. 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
Potential energy stored in the proton gradient drives production of ATP by chemiosmosis
In PS I (like PS II), transferred light energy excites P700, which loses an electron to the primary 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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36. 2 H+ Photosystem I (PS I) Photosystem II (PS II)4
Electron
transport chain
Primary
electron
acceptor
Primary
electron
acceptor Pq e– 2
2 H+
Cytochrome
complex
H2O e– + 1 /2 3 O2 Pc e–
P700 1 e– 5
P680
Light
Light 6
ATP
Figure 10.14_4 How linear electron flow during the light reactions generates ATP and NADPH (step 4)
Pigment
molecules
Photosystem I
(PS I)
Photosystem II
(PS II)
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37. Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)
NADP+ reductase catalyzes the transfer of electrons to NADP+, reducing 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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38. NADP+ + H+ 2 H+ Photosystem I (PS I) Photosystem II (PS II)7
Electron
transport chain 4
Electron
transport chain
Primary
electron
acceptor Fd 8
NADP+
+ H+
Primary
electron
acceptor e– e–
NADP+
reductase Pq e– 2
NADPH
2 H+
Cytochrome
complex
H2O e– + 1 /2 3 O2 Pc e–
P700 1 e– 5
P680
Light
Light 6
ATP
Figure 10.14_5 How linear electron flow during the light reactions generates ATP and NADPH (step 5)
Pigment
molecules
Photosystem I
(PS I)
Photosystem II
(PS II)
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39. Cyclic Electron Flow
In cyclic electron flow, electrons cycle back from Fd to the PS I reaction center via a plastocyanin molecule (Pc)
Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH
No oxygen is released
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40. Photosystem I Photosystem II Primary acceptor Primary Fd acceptor Fd
NADP+ Pq
NADP+
reductase
+ H+
Cytochrome
complex
NADPH Pc
Figure Cyclic electron flow
Photosystem I
Photosystem II
ATP
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41. 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
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42. 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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43. Electron transport chain
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 ] 43

44. (high H+ concentration) +2 H+ 4 H+
Cytochrome
complex
NADP+
reductase
Photosystem II
Photosystem I
Light
Light
4 H+ 3
NADP+ + H+ Fd Pq
NADPH Pc e– 2 e–
H2O 1 1 / 2 O2
THYLAKOID SPACE
(high H+ concentration)
+2 H+
4 H+
CALVIN
CYCLE
Figure The light reactions and chemiosmosis: Current model of the organization of the thylakoid membrane
Thylakoid
membrane
ATP
synthase
STROMA
(low H+ concentration)
ADP +
ATP H+ P i
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45. Concept 10.3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar
The Calvin cycle, like the citric acid 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 (Anabolic Process)
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46. CALVIN CYCLE LIGHT REACTIONS
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
Figure 10.UN03 In-text figure, Calvin cycle schematic, p. 202
NADPH O2
[CH2O] (sugar)
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47. The Calvin cycle has three phases:
Carbon enters the cycle as CO2 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 CO2
The Calvin cycle has three phases:
Carbon fixation (catalyzed by rubisco)
Reduction
Regeneration of the CO2 acceptor (RuBP)
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48. Input 3 CO2, entering one per cycle
ribulose-1,5-bisphosphate-carboxylase/oxygenase 
Phase 1: Carbon fixation
Rubisco 3 P P 3 P P 6 P
3-Phosphoglycerate
RuBP
Calvin
Cycle
Figure 10.19_1 The Calvin cycle (step 1)
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49. Input 3 CO2, entering one per cycle
Phase 1: Carbon fixation
Rubisco 3 P P 3 P P 6 P
3-Phosphoglycerate
RuBP 6
ATP
6 ADP
Calvin
Cycle 6 P P
1,3-Bisphosphoglycerate 6
NADPH
6 NADP+ 6 P i
Figure 10.19_2 The Calvin cycle (step 2) 6 P
G3P
Phase 2:
Reduction 1 P
Glucose and
other organic
compounds
G3P
Output
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50. Input 3 CO2, entering one per cycle
Phase 1: Carbon fixation
Rubisco 3 P P 3 P P 6 P
3-Phosphoglycerate
RuBP 6
ATP
6 ADP
3 ADP
Calvin
Cycle 6 P P 3
ATP
1,3-Bisphosphoglycerate 6
NADPH
Phase 3:
Regeneration
of RuBP
6 NADP+ 6 P i 5 P
Figure 10.19_3 The Calvin cycle (step 3)
G3P 6 P
G3P
Phase 2:
Reduction 1 P
Glucose and
other organic
compounds
G3P
Output
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51. 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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52. Photorespiration: An Evolutionary Relic?
In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (3-phosphoglycerate)
When stomata close CO2 is unavailable
Rubisco adds O2 instead of CO2 in the Calvin cycle, RuBP splits producing a two-carbon compound
Photorespiration consumes O2 and ATP
Releases CO2 (later) without producing ATP or sugar
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53. Photorespiration (rubisco) first evolved at a time when the atmosphere had far less O2 and more CO2
Photorespiration limits the build up of NADPH in the absence of the Calvin cycle
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54. There are two distinct types of cells in the leaves of C4 plants:
C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds
There are two distinct types of cells in the leaves of C4 plants:
Bundle-sheath cells are arranged in tightly packed sheaths around the veins of the leaf
Mesophyll cells are loosely packed between the bundle sheath and the leaf surface
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55. Sugar production in C4 plants occurs in a three-step process:
The production of the four-carbon precursors is catalyzed by the enzyme PEP carboxylase in the mesophyll cells
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
Within the bundle-sheath cells, they release CO2 that is then used in the Calvin cycle
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56. C4 leaf anatomy The C4 pathway Mesophyll cell Photo- synthetic
cells of
C4 plant
leaf
Mesophyll
cell
CO2
PEP carboxylase
Bundle-
sheath
cell
Oxaloacetate (4C)
PEP (3C)
ADP
Vein
(vascular
tissue)
Malate (4C)
ATP
Pyruvate
(3C)
Plasmodesma
CO2
Bundle-
sheath
cell
Stoma
Calvin
Cycle
Figure C4 leaf anatomy and the C4 pathway
Sugar
Vascular
tissue
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57. 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 that are stored in the vacuoles
Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle
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58. Organic Calvin Cycle Calvin Cycle
Sugarcane
Pineapple 1 1
CO2
CO2 C4
CAM
Mesophyll
cell
Organic
acid
Organic
acid
Night
CO2 2
CO2 2
Figure C4 and CAM photosynthesis compared
Bundle-
sheath
cell
Day
Calvin
Cycle
Calvin
Cycle
Sugar
Sugar
(a) Spatial separation of steps
(b) Temporal separation of steps
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