Photosynthesis · Chloroplasts: The Sites of Photosynthesis in Plants •Leaves are the major...
Transcript of Photosynthesis · Chloroplasts: The Sites of Photosynthesis in Plants •Leaves are the major...
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Photosynthesis
لجنة الطب البشري
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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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Figure 10.1
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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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BioFlix: Photosynthesis
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(a) Plants
(b) Multicellular
alga
(c) Unicellular
protists
(d) Cyanobacteria
(e) Purple sulfur
bacteria
10 m
1 m
40 m
Figure 10.2
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Figure 10.2a
(a) Plants
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Figure 10.2b
(b) Multicellular alga
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Figure 10.2c
(c) Unicellular protists10 m
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Figure 10.2d
(d) Cyanobacteria 40 m
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Figure 10.2e
(e) Purple sulfur
bacteria1 m
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• 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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• 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
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Figure 10.3
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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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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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Figure 10.4
Mesophyll
Leaf cross section
Chloroplasts Vein
Stomata
Chloroplast Mesophyll
cell
CO2 O2
20 m
Outer
membraneIntermembrane
space
Inner
membrane
1 m
Thylakoid
space
ThylakoidGranumStroma
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Mesophyll
Leaf cross section
Chloroplasts Vein
Stomata
Chloroplast Mesophyll
cell
CO2 O2
20 m
Figure 10.4a
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Outermembrane
Intermembranespace
Innermembrane
1 m
Thylakoidspace
ThylakoidGranumStroma
ChloroplastFigure 10.4b
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Figure 10.4c
Mesophyllcell
20 m
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Figure 10.4d
1 m
GranumStroma
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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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Figure 10.5
Reactants:
Products:
6 CO2
6 H2O 6 O2
12 H2O
C6H12O6
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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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Figure 10.UN01
Energy 6 CO2 6 H2O C6 H12 O6 6 O2
becomes reduced
becomes oxidized
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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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• 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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Light
LightReactions
Chloroplast
NADP
ADP
+ P i
H2O
Figure 10.6-1
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Light
LightReactions
Chloroplast
ATP
NADPH
NADP
ADP
+ P i
H2O
O2
Figure 10.6-2
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Light
LightReactions
CalvinCycle
Chloroplast
ATP
NADPH
NADP
ADP
+ P i
H2O CO2
O2
Figure 10.6-3
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Light
LightReactions
CalvinCycle
Chloroplast
[CH2O](sugar)
ATP
NADPH
NADP
ADP
+ P i
H2O CO2
O2
Figure 10.6-4
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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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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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Figure 10.7
Gammarays
X-rays UV InfraredMicro-waves
Radiowaves
Visible light
Shorter wavelength Longer wavelength
Lower energyHigher energy
380 450 500 550 600 650 700 750 nm
105 nm 103 nm 1 nm 103 nm 106 nm (109 nm) 103 m1 m
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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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Chloroplast
LightReflectedlight
Absorbedlight
Transmittedlight
Granum
Figure 10.8
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• 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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Figure 10.9
Whitelight
Refractingprism
Chlorophyllsolution
Photoelectrictube
Galvanometer
Slit moves topass lightof selectedwavelength.
Greenlight
High transmittance(low absorption):Chlorophyll absorbsvery little green light.
Bluelight
Low transmittance(high absorption):Chlorophyll absorbsmost blue light.
TECHNIQUE
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• An absorption spectrum is a graph plotting a
pigment’s light absorption versus wavelength
• The absorption spectrum of chlorophyll asuggests that violet-blue and red light work best
for photosynthesis
• An action spectrum profiles the relative
effectiveness of different wavelengths of
radiation in driving a process
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(b) Action spectrum
(a) Absorptionspectra
Engelmann’sexperiment
(c)
Chloro-phyll a Chlorophyll b
Carotenoids
Wavelength of light (nm)
Ab
so
rpti
on
of
lig
ht
by
ch
loro
pla
st
pig
men
tsR
ate
of
ph
oto
syn
the
sis
(m
ea
su
red
by O
2re
lea
se
)
Aerobic bacteria
Filamentof alga
400 500 600 700
400 500 600 700
400 500 600 700
RESULTSFigure 10.10
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• 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
• Accessory pigments called carotenoids absorb
excessive light that would damage chlorophyll
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Figure 10.11
Hydrocarbon tail(H atoms not shown)
Porphyrin ring
CH3
CH3 in chlorophyll a
CHO in chlorophyll b
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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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Figure 10.12
Excitedstate
Heat
e
Photon(fluorescence)
Groundstate
Photon
Chlorophyllmolecule
En
erg
y o
f ele
ctr
on
(a) Excitation of isolated chlorophyll molecule (b) Fluorescence
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Figure 10.12a
(b) Fluorescence
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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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Figure 10.13
(b) Structure of photosystem II(a) How a photosystem harvests light
Th
yla
ko
id m
em
bra
ne
Th
yla
ko
id m
em
bra
ne
Photon
PhotosystemSTROMA
Light-harvestingcomplexes
Reaction-centercomplex
Primaryelectronacceptor
Transferof energy
Special pair ofchlorophyll amolecules
Pigmentmolecules
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
Chlorophyll STROMA
Proteinsubunits
THYLAKOIDSPACE
e
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Figure 10.13a
(a) How a photosystem harvests light
Th
yla
ko
id m
em
bra
ne
PhotonPhotosystem STROMA
Light-harvestingcomplexes
Reaction-centercomplex
Primaryelectronacceptor
Transferof energy
Special pair ofchlorophyll amolecules
Pigmentmolecules
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
e
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Figure 10.13b
(b) Structure of photosystem II
Th
yla
ko
id m
em
bra
ne
Chlorophyll STROMA
Proteinsubunits
THYLAKOIDSPACE
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• 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
• During the light reactions, there are two possible routes for electron flow: cyclic and linear
• Linear electron flow, the primary pathway,
involves both photosystems and produces ATP
and NADPH using light energy
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• 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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Figure 10.14-1
Primaryacceptor
P680
Light
Pigmentmolecules
Photosystem II(PS II)
1
2e
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• P680+ is a very strong oxidizing agent
• 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 of this reaction
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Figure 10.14-2
Primaryacceptor
H2O
O2
2 H
+1/2
P680
Light
Pigmentmolecules
Photosystem II(PS II)
1
2
3
e
e
e
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• 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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Figure 10.14-3
Cytochromecomplex
Primaryacceptor
H2O
O2
2 H
+1/2
P680
Light
Pigmentmolecules
Photosystem II(PS II)
Pq
Pc
ATP
1
2
3
5
e
e
e
4
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• 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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Figure 10.14-4
Cytochromecomplex
Primaryacceptor
Primaryacceptor
H2O
O2
2 H
+1/2
P680
Light
Pigmentmolecules
Photosystem II(PS II)
Photosystem I(PS I)
Pq
Pc
ATP
1
2
3
5
6
P700
Light
e
e
4
e
e
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• 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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Figure 10.14-5
Cytochromecomplex
Primaryacceptor
Primaryacceptor
H2O
O2
2 H
+1/2
P680
Light
Pigmentmolecules
Photosystem II(PS II)
Photosystem I(PS I)
Pq
Pc
ATP
1
2
3
5
6
7
8
P700
Light
+ HNADP
NADPH
NADP
reductase
Fd
e
e
ee
4
e
e
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Photosystem II Photosystem I
Millmakes
ATP
ATP
NADPH
e
e
e
ee
e
e
Figure 10.15
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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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Figure 10.16
Photosystem I
Primaryacceptor
Cytochromecomplex
Fd
Pc
ATP
Primaryacceptor
Pq
Fd
NADPH
NADP
reductase
NADP
+ H
Photosystem II
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• Some organisms such as purple sulfur bacteria
have PS I but not PS II
• Cyclic electron flow is thought to have evolved
before linear electron flow
• Cyclic electron flow may protect cells from
light-induced damage
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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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Mitochondrion Chloroplast
MITOCHONDRIONSTRUCTURE
CHLOROPLASTSTRUCTURE
Intermembranespace
Innermembrane
Matrix
Thylakoidspace
Thylakoidmembrane
Stroma
Electrontransport
chain
H Diffusion
ATPsynthase
H
ADP P i
Key Higher [H ]
Lower [H ]
ATP
Figure 10.17
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• 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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Figure 10.18
STROMA(low H concentration)
STROMA(low H concentration)
THYLAKOID SPACE(high H concentration)
Light
Photosystem II
Cytochromecomplex
Photosystem ILight
NADP
reductase
NADP + H
ToCalvinCycle
ATPsynthase
Thylakoidmembrane
2
1
3
NADPH
Fd
Pc
Pq
4 H+
4 H++2 H+
H+
ADP
+
P i
ATP
1/2
H2OO2
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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
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• Carbon enters the cycle as CO2 and leaves as
a sugar named glyceraldehyde 3-phospate
(G3P)
• For net synthesis of 1 G3P, the cycle must take
place three times, fixing 3 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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Input
3 (Entering oneat a time)
CO2
Phase 1: Carbon fixation
Rubisco
3 P P
P6
Short-livedintermediate
3-Phosphoglycerate
3 P P
Ribulose bisphosphate(RuBP)
Figure 10.19-1
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Input
3 (Entering oneat a time)
CO2
Phase 1: Carbon fixation
Rubisco
3 P P
P6
Short-livedintermediate
3-Phosphoglycerate6
6 ADP
ATP
6 P P
1,3-Bisphosphoglycerate
CalvinCycle
6 NADPH
6 NADP
6 P i
6 P
Phase 2: Reduction
Glyceraldehyde 3-phosphate(G3P)
3 P P
Ribulose bisphosphate(RuBP)
1 P
G3P(a sugar)
Output
Glucose andother organiccompounds
Figure 10.19-2
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Input
3 (Entering oneat a time)
CO2
Phase 1: Carbon fixation
Rubisco
3 P P
P6
Short-livedintermediate
3-Phosphoglycerate6
6 ADP
ATP
6 P P
1,3-Bisphosphoglycerate
CalvinCycle
6 NADPH
6 NADP
6 P i
6 P
Phase 2: Reduction
Glyceraldehyde 3-phosphate(G3P)
P5
G3P
ATP
3 ADP
Phase 3:Regeneration ofthe CO2 acceptor(RuBP)
3 P P
Ribulose bisphosphate(RuBP)
1 P
G3P(a sugar)
Output
Glucose andother organiccompounds
3
Figure 10.19-3
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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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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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• 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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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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Figure 10.20
C4 leaf anatomy The C4 pathway
Photosyntheticcells of C4
plant leaf
Mesophyll cell
Bundle-sheathcell
Vein(vascular tissue)
Stoma
Mesophyll cell
PEP carboxylaseCO2
Oxaloacetate (4C) PEP (3C)
Malate (4C)
Pyruvate (3C)
CO2
Bundle-sheathcell
CalvinCycle
Sugar
Vasculartissue
ADP
ATP
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Figure 10.20a
C4 leaf anatomy
Photosyntheticcells of C4
plant leaf
Mesophyll cell
Bundle-sheathcell
Vein(vascular tissue)
Stoma
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Figure 10.20bThe C4 pathway
Mesophyll cell PEP carboxylase
CO2
Oxaloacetate (4C) PEP (3C)
Malate (4C)
Pyruvate (3C)
CO2
Bundle-sheathcell
CalvinCycle
Sugar
Vasculartissue
ADP
ATP
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• 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
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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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Sugarcane
Mesophyllcell
Bundle-sheathcell
C4 CO2
Organic acid
CO2
CalvinCycle
Sugar
(a) Spatial separation of steps (b) Temporal separation of steps
CO2
Organic acid
CO2
CalvinCycle
Sugar
Day
Night
CAM
Pineapple
CO2 incorporated
(carbon fixation)
CO2 released
to the Calvin
cycle
2
1
Figure 10.21
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Figure 10.21a
Sugarcane
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Figure 10.21b
Pineapple
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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 structures
such as roots, tubers, seeds, and fruits
• In addition to food production, photosynthesis
produces the O2 in our atmosphere
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Light
LightReactions:
Photosystem IIElectron transport chain
Photosystem IElectron transport chain
NADP
ADP
+ P i
RuBP
ATP
NADPH
3-Phosphoglycerate
CalvinCycle
G3PStarch(storage)
Sucrose (export)
Chloroplast
H2O CO2
O2
Figure 10.22
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Figure 10.UN02
Primaryacceptor
Primaryacceptor
Cytochromecomplex
NADP
reductase
Photosystem II
Photosystem IATP
Pq
Pc
Fd
NADP
+ H
NADPH
H2O
O2
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Regeneration of
CO2 acceptor
Carbon fixation
Reduction
Calvin
Cycle
1 G3P (3C)
5 3C
3 5C 6 3C
3 CO2
Figure 10.UN03
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Figure 10.UN04
pH 7
pH 4
pH 4
pH 8
ATP
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Figure 10.UN05
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Figure 10.UN06
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Figure 10.UN07
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Figure 10.UN08