CAMPBELL BIOLOGY IN FOCUS
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Urry • Cain • Wasserman • Minorsky • Jackson • Reece
Lecture Presentations by Kathleen Fitzpatrick and Nicole Tunbridge
8Photosynthesis
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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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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© 2014 Pearson Education, Inc.
Figure 8.1
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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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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© 2014 Pearson Education, Inc.
Figure 8.2
(a) Plants
(d) Cyanobacteria
(e) Purple sulfurbacteria
(b) Multicellularalga
(c) Unicellular eukaryotes10 µ
m
1 µ
m40
µm
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Figure 8.2a
(a) Plants
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Figure 8.2b
(b) Multicellular alga
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Figure 8.2c
(c) Unicellular eukaryotes10 µ
m
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Figure 8.2d
(d) Cyanobacteria
40 µ
m
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Figure 8.2e
(e) Purple sulfurbacteria
1 µ
m
Concept 8.1: Photosynthesis converts light energy to the chemical energy of food
The structural organization of photosynthetic cells includes enzymes and other molecules grouped together in a membrane
This organization allows for the chemical reactions of photosynthesis to proceed efficiently
Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria
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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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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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© 2014 Pearson Education, Inc.
Figure 8.3Leaf cross section
20 µm
Mesophyll
Stomata
Chloroplasts Vein
CO2 O2
Mesophyll cell
Chloroplast
Stroma
Thylakoid
Thylakoidspace
Outermembrane
Intermembranespace
Inner membrane
Granum
1 µm
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Figure 8.3a
Leaf cross section
Mesophyll
Stomata
Chloroplasts Vein
CO2 O2
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Figure 8.3b
20 µm
Mesophyll cellChloroplast
Stroma
Thylakoid
Thylakoidspace
Outermembrane
Intermembranespace
Inner membrane
Granum
1 µm
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Figure 8.3c
20 µm
Mesophyll cell
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Figure 8.3d
StromaGranum
1 µm
Tracking Atoms Through Photosynthesis: Scientific Inquiry
Photosynthesis is a complex series of reactions that can be summarized as the following equation
6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2O
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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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© 2014 Pearson Education, Inc.
Figure 8.4
Products:
Reactants: 6 CO2
6 O2C6H12O6 6 H2O
12 H2O
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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© 2014 Pearson Education, Inc.
Figure 8.UN01
becomes reduced
becomes oxidized
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 the electron acceptor, NADP+, to NADPH
Generate ATP from ADP by adding a phosphate group, photophosphorylation
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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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Animation: Photosynthesis
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Figure 8.5
Light
CO2H2O
P i
Chloroplast
LightReactions
CalvinCycle
[CH2O](sugar)
O2
ADP
ATP
NADP+
+
NADPH
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Figure 8.5-1
Light
H2O
Chloroplast
LightReactions
P i
ADP
NADP+
+
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Figure 8.5-2
Light
H2O
P i
Chloroplast
LightReactions
O2
ADP
ATP
NADP+
+
NADPH
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Figure 8.5-3
Light
CO2H2O
P i
Chloroplast
LightReactions
CalvinCycle
O2
ADP
ATP
NADP+
+
NADPH
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Figure 8.5-4
Light
CO2H2O
P i
Chloroplast
LightReactions
CalvinCycle
[CH2O](sugar)
O2
ADP
ATP
NADP+
+
NADPH
Concept 8.2: 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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The Nature of Sunlight
Light is a form of electromagnetic energy, also called electromagnetic radiation
Like other electromagnetic energy, light travels in rhythmic waves
Wavelength is the distance between crests of waves
Wavelength determines the type of electromagnetic energy
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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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© 2014 Pearson Education, Inc.
Figure 8.6
Gammarays
10−5 nm 10−3 nm 1 nm 103 nm 106 nm1 m
(109 nm) 103 m
Radiowaves
Micro-wavesX-rays InfraredUV
Visible light
Shorter wavelength Longer wavelength
Lower energyHigher energy
380 450 500 550 650600 700 750 nm
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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Animation: Light and Pigments
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Figure 8.7
Reflectedlight
Light
Absorbedlight
Chloroplast
Granum
Transmittedlight
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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© 2014 Pearson Education, Inc.
Figure 8.8
Refractingprism
Whitelight
Greenlight
Bluelight
Chlorophyllsolution
Photoelectrictube
Galvanometer
Slit moves to passlight of selectedwavelength.
The low transmittance (highabsorption) reading indicatesthat chlorophyll absorbs mostblue light.
The high transmittance (lowabsorption) reading indicatesthat chlorophyll absorbs verylittle green light.
Technique
1
2
4
3
An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength
The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis
Accessory pigments include chlorophyll b and a group of pigments called carotenoids
An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process
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© 2014 Pearson Education, Inc.
Figure 8.9
Chloro-phyll a
Rat
e o
fp
ho
tosy
nth
esis
(mea
sure
d b
y O
2
rele
ase)
Results
Ab
so
rpti
on
of
lig
ht
by
chlo
rop
last
pig
men
ts
Chlorophyll b
Carotenoids
Filamentof alga
Aerobic bacteria
(a) Absorption spectra
(b) Action spectrum
(c) Engelmann’s experiment
400 700600500
400 700600500
400 700600500
Wavelength of light (nm)
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Figure 8.9a
Chloro-phyll a
Ab
sorp
tio
n o
f li
gh
tb
y ch
loro
pla
stp
igm
ents
Chlorophyll b
Carotenoids
(a) Absorption spectra
400 700600500
Wavelength of light (nm)
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Figure 8.9b
(b) Action spectrum
400 700600500
Rat
e o
fp
ho
tosy
nth
esi
s(m
eas
ure
d b
y O
2
rele
ase
)
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Figure 8.9c
Filamentof alga
Aerobic bacteria
(c) Engelmann’s experiment400 700600500
The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W. Engelmann
In his experiment, he exposed different segments of a filamentous alga to different wavelengths
Areas receiving wavelengths favorable to photosynthesis produced excess O2
He used the growth of aerobic bacteria clustered along the alga as a measure of O2 production
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Chlorophyll a is the main photosynthetic pigment
Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis
A slight structural difference between chlorophyll a and chlorophyll b causes them to absorb slightly different wavelengths
Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll
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Video: Chlorophyll Model
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Figure 8.10
Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts; H atoms notshown
Porphyrin ring:light-absorbing“head” of molecule;note magnesiumatom at center
CH3 in chlorophyll aCHO in chlorophyll bCH3
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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© 2014 Pearson Education, Inc.
Figure 8.11
Photon(fluorescence)
Groundstate
(b) Fluorescence
Excitedstate
Chlorophyllmolecule
Photon
Heat
e−
(a) Excitation of isolated chlorophyll molecule
En
erg
y o
f el
ectr
on
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Figure 8.11a
(b) Fluorescence
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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© 2014 Pearson Education, Inc.
Figure 8.12
(b) Structure of a photosystem(a) How a photosystem harvests light
Chlorophyll STROMA
THYLAKOIDSPACE
Proteinsubunits
STROMA
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
PhotosystemPhoton
Light-harvestingcomplexes
Reaction-centercomplex
Primaryelectronacceptor
Special pair ofchlorophyll amolecules
Transferof energy
Pigmentmolecules
Th
ylak
oid
mem
bra
ne
Th
ylak
oid
mem
bra
nee−
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Figure 8.12a
(a) How a photosystem harvests light
STROMA
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
PhotosystemPhoton
Light-harvestingcomplexes
Reaction-centercomplex
Primaryelectronacceptor
Special pair ofchlorophyll amolecules
Transferof energy
Pigmentmolecules
Th
ylak
oid
mem
bra
ne
e−
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Figure 8.12b
(b) Structure of a photosystem
Chlorophyll STROMA
THYLAKOID SPACE
Proteinsubunits
Th
ylak
oid
mem
bra
ne
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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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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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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Linear Electron Flow
Linear electron flow involves the flow of electrons through both photosystems to produce ATP and NADPH using light energy
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Linear electron flow can be broken down into a series of steps
1. A photon hits a pigment and its energy is passed among pigment molecules until it excites P680
2. An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680+)
3. H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680; O2 is released as a by-product
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© 2014 Pearson Education, Inc.
Figure 8.UN02
CalvinCycle
NADPH
NADP+
ATP
ADP
Light
CO2
[CH2O] (sugar)
LightReactions
O2
H2O
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Figure 8.13-1
Primaryacceptor
Photosystem II(PS II)
Light
P680
Pigmentmolecules
1
2e−
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Figure 8.13-2
Primaryacceptor
2 H+
O2
+
Photosystem II(PS II)
H2O
Light
/2
1
P680
Pigmentmolecules
1
2
3
e−
e−
e−
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Figure 8.13-3
Primaryacceptor
2 H+
O2
+
ATP
Photosystem II(PS II)
H2O
Light
/2
1
P680
Pq
Electrontransportchain
Cytochromecomplex
Pc
Pigmentmolecules
1
2
3
4
5
e−
e−
e−
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Figure 8.13-4
Primaryacceptor
2 H+
O2
+
ATP
Photosystem II(PS II)
H2O
Light
/2
1
P680
Pq
Electrontransportchain
Cytochromecomplex
Pc
Pigmentmolecules
Primaryacceptor
Photosystem I(PS I)
P700
Light1
2
3
4
5
6
e−
e−
e−
e−
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Figure 8.13-5
Primaryacceptor
2 H+
O2
+
ATP
NADPH
Photosystem II(PS II)
H2Oe−
e−
e−
Light
/2
1
P680
Pq
Electrontransportchain
Cytochromecomplex
Pc
Pigmentmolecules
Primaryacceptor
Photosystem I(PS I)
e−
P700
e−
e−
Fd
Light
Electrontransportchain
H++NADP+
NADP+
reductase
1
2
3
4
5
6
7
8
4. Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I
5. 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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6. In PS I (like PS II), transferred light energy excites P700, causing it to lose an electron to an electron acceptor (we now call it P700+)
P700+ accepts an electron passed down from PS II via the electron transport chain
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7. Excited electrons “fall” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)
8. The electrons are transferred to NADP+, reducing it to NADPH, and become available for the reactions of the Calvin cycle
This process also removes an H+ from the stroma
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The energy changes of electrons during linear flow can be represented in a mechanical analogy
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© 2014 Pearson Education, Inc.
Figure 8.14
Photosystem II Photosystem I
NADPH
Millmakes
ATP
Ph
oto
n
Ph
oto
n
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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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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© 2014 Pearson Education, Inc.
Figure 8.15
Electrontransport
chain
Higher [H+]P i
H+
CHLOROPLASTSTRUCTURE
Inter-membrane
space
MITOCHONDRIONSTRUCTURE
Thylakoidspace
Innermembrane
Matrix
Key
Lower [H+]
Thylakoidmembrane
Stroma
ATP
ATPsynthase
ADP +
H+ Diffusion
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Figure 8.15a
Electrontransport
chain
Higher [H+] H+
CHLOROPLASTSTRUCTURE
Inter-membrane
space
MITOCHONDRIONSTRUCTURE
Thylakoidspace
Innermembrane
MatrixKey
Lower [H+]
Thylakoidmembrane
Stroma
ATP
ATPsynthase
ADP +
H+ Diffusion
Pi
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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© 2014 Pearson Education, Inc.
Figure 8.UN02
CalvinCycle
NADPH
NADP+
ATP
ADP
Light
CO2
[CH2O] (sugar)
LightReactions
O2
H2O
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Figure 8.16
Photosystem II Photosystem I
ToCalvinCycle
H+
THYLAKOID SPACE(high H+ concentration)
Thylakoidmembrane
STROMA(low H+ concentration)
ATPsynthase
NADPH
e−
LightNADP+
ATPADP
+
NADP+
reductase
FdH++
Pq
Pc
Cytochromecomplex
4 H+
Light
+2 H+O2
H2O/21
4 H+
e−
1
2
3
Pi
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Figure 8.16a
Photosystem II Photosystem I
H+
THYLAKOID SPACE(high H+ concentration)
Thylakoidmembrane
STROMA(low H+ concentration)
ATPsynthase
e−
Light
ATPADP
+
Fd
Pq
Pc
Cytochromecomplex
4 H+
Light
+2 H+
O2
H2O/2
1
4 H+
e−
1
2
Pi
© 2014 Pearson Education, Inc.
Figure 8.16b
2
3Photosystem I
ToCalvinCycle
H+
ATPsynthase
NADPH
LightNADP+
ATPADP
+
NADP+
reductase
FdH++
Pc
Cytochromecomplex
4 H+
THYLAKOID SPACE(high H+ concentration)
STROMA(low H+ concentration)
Pi
Concept 8.3: 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
Unlike the citric acid cycle, the Calvin cycle is anabolic
It builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH
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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
Reduction
Regeneration of the CO2 acceptor
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Phase 1, carbon fixation, involves the incorporation of the CO2 molecules into ribulose bisphosphate (RuBP) using the enzyme rubisco
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© 2014 Pearson Education, Inc.
Figure 8.UN03
CalvinCycle
NADPH
NADP+
ATP
ADP
Light
CO2
[CH2O] (sugar)
LightReactions
O2
H2O
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Figure 8.17-1Input 3
CalvinCycle
as 3 CO2
Rubisco Phase 1: Carbon fixation
RuBP 3-Phosphoglycerate6
3
3 P
P
P P
P
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Figure 8.17-2
6 P i
NADPH
Input 3
ATP
CalvinCycle
as 3 CO2
Rubisco Phase 1: Carbon fixation
Phase 2: Reduction
G3POutput
Glucose and other organiccompounds
G3P
RuBP 3-Phosphoglycerate
1,3-Bisphosphoglycerate
6 ADP
6
6
6
6
3
6 NADP+
6
3
1
P
P P
P
P
P P
P
P
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Figure 8.17-3
6 P i
NADPH
Input 3
ATP
CalvinCycle
as 3 CO2
Rubisco Phase 1: Carbon fixation
Phase 2: Reduction
Phase 3: Regenerationof RuBP
G3POutput
Glucose and other organiccompounds
G3P
RuBP 3-Phosphoglycerate
1,3-Bisphosphoglycerate
6 ADP
6
6
6
6 P
3
P P
P
6 NADP+
6 P
5 P
G3P
ATP
3 ADP
3
3 P P
1 P
P
Phase 2, reduction, involves the reduction and phosphorylation of 3-phosphoglycerate to G3P
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Phase 3, regeneration, involves the rearrangement of G3P to regenerate the initial CO2 receptor, RuBP
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Evolution of Alternative Mechanisms of Carbon Fixation in Hot, Arid Climates
Adaptation to dehydration is a problem for land 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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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 decreases photosynthetic output by consuming ATP, O2, and organic fuel and releasing CO2 without producing any ATP or sugar
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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
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C4 plants minimize the cost of photorespiration by incorporating CO2 into a four-carbon compound
An enzyme in the mesophyll cells has a high affinity for CO2 and can fix carbon 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
C4 Plants
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© 2014 Pearson Education, Inc.
Figure 8.18
Bundle-sheathcell
Sugarcane
CO2
Pineapple
CO2
(a) Spatial separation of steps
C4
CO2CO2
CAM
Day
Night
Sugar
CalvinCycle
CalvinCycle
Sugar
Organicacid
Organicacid
Mesophyllcell
(b) Temporal separation of steps
1
2
1
2
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Figure 8.18a
Sugarcane
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Figure 8.18b
Pineapple
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Figure 8.18c
Bundle-sheathcell
CO2CO2
(a) Spatial separation of steps
C4
CO2CO2
CAM
Day
Night
Sugar
CalvinCycle
CalvinCycle
Sugar
Organicacid
Organicacid
Mesophyllcell
(b) Temporal separation of steps
1
2
1
2
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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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 the chloroplasts and 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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© 2014 Pearson Education, Inc.
Figure 8.19
Photosystem IIElectron transport chain
CalvinCycle
NADPH
LightNADP+
ATP
CO2H2O
ADP+
3-Phosphpglycerate
G3P
RuBP
Sucrose (export)
Starch(storage)
Chloroplast
O2
LightReactions:
Photosystem IElectron transport chain
Pi
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Figure 8.UN04
© 2014 Pearson Education, Inc.
Figure 8.UN05
Photosystem II
Photosystem I
NADP+
ATP
Fd
H++Pq
Cytochromecomplex
O2
H2O
Pc
NADP+
reductaseNADPH
Primaryacceptor
Electron transport
chain
Primaryacceptor
Electron transport
chain
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Figure 8.UN06
CalvinCycle
Regeneration ofCO2 acceptor
Carbon fixation
Reduction
1 G3P (3C)
3 CO2
3 × 5C 6 × 3C
5 × 3C
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Figure 8.UN07
pH 7
pH 4 pH 8
pH 4