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Overview: The Process That Feeds the Biosphere Photosynthesis
converts solar energy into chemical energy
Directly or indirectly, photosynthesis nourishes almost the entire living world
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Autotrophs are the producers of the biosphere
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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These organisms feed not only themselves but also most of the living world
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© 2011 Pearson Education, Inc.
BioFlix: Photosynthesis
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(a) Plants(b) Multicellular
alga
(c) Unicellularprotists
(d) Cyanobacteria
(e) Purple sulfurbacteria
10 m
1 m
40 m
Figure 10.2
Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2
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Figure 10.3
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Photosynthesis converts light energy to the chemical energy of foodThe structure and organization of plant cells allows for photosynthesis to occur thanks to chloroplasts
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Chloroplasts: The Sites of Photosynthesis in PlantsLeaves are the major
locations of photosynthesis
Their green color is from chlorophyll, the green pigment within chloroplasts
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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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Outermembrane
IntermembranespaceInnermembrane
1 m
Thylakoidspace
ThylakoidGranumStroma
ChloroplastFigure 10.4b
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Figure 10.4c
Mesophyllcell
20 m
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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
© 2011 Pearson Education, Inc.
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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 ProcessPhotosynthesis reverses the direction of electron
flow compared to respirationPhotosynthesis is an endergonic process; the
energy boost is provided by light
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)
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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
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Light
LightReactions
Chloroplast
ATP
NADPH
NADP
ADP
+ P i
H2O
O2
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Light
LightReactions
CalvinCycle
Chloroplast
ATP
NADPH
NADP
ADP
+ P i
H2O CO2
O2
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Light
LightReactions
CalvinCycle
Chloroplast
[CH2O](sugar)
ATP
NADPH
NADP
ADP
+ P i
H2O CO2
O2
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The light reactions convert solar energy to the chemical energy of ATP and NADPHThylakoids turn light into chemical energy
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The Nature of Sunlight
Light is a form of electromagnetic energy
Wavelength determines the type of electromagnetic energy
Each color is seen due to them being different wavelengths
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Figure 10.7
Gammarays X-rays UV Infrared
Micro-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
Wavelengths that are not absorbed are reflected or transmitted
Leaves appear green because chlorophyll reflects and transmits green light
Chloroplast
LightReflectedlight
Absorbedlight
Transmittedlight
Granum
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© 2011 Pearson Education, Inc.
Animation: Light and Pigments
Right-click slide / select “Play”
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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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(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
me
nts
Ra
te o
f p
ho
tos
yn
the
sis
(m
ea
su
red
by
O2
rele
as
e)
Aerobic bacteria
Filamentof alga
400 500 600 700
400 500 600 700
400 500 600 700
RESULTS
action spectrum profiles the relative effectiveness of different wavelengths
absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength
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• The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W. Engelmann
• 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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Structure of Chlorophyll
Hydrocarbon tail(H atoms not shown)
Porphyrin ring
CH3
CH3 in chlorophyll aCHO in chlorophyll b
• Chlorophyll a is the main photosynthetic pigment
• Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis, or to absorb excess energy
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Excitation of Chlorophyll by LightWhen 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
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Figure 10.12
Excitedstate
Heat
e
Photon(fluorescence)
Groundstate
PhotonChlorophyll
molecule
En
erg
y o
f el
ectr
on
Fluorescence
If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat
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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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(a) How a photosystem harvests light
Th
ylak
oid
mem
bra
ne
PhotonPhotosystem STROMA
Light-harvestingcomplexes
Reaction-centercomplex
Primaryelectronacceptor
Transferof energy
Special pair ofchlorophyll amolecules
Pigmentmolecules
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
e
Filled with various pigments
REDOX
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Figure 10.13b
(b) Structure of photosystem II
Th
ylak
oid
mem
bra
ne Chlorophyll STROMA
Proteinsubunits THYLAKOID
SPACE
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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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Light Reactions
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Primaryacceptor
P680
Light
Pigmentmolecules
Photosystem II(PS II)
1
2e
• Photosystem II (PS II) functions first and is best at absorbing a wavelength of 680 nm
• The reaction-center chlorophyll a of PS II is called P680
• It is P680+ naturally
Types of Photosystems in the Thylakoid Membrane
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Primaryacceptor
H2O
O2
2 H
+1/2
P680
Light
Pigmentmolecules
Photosystem II(PS II)
1
2
3
e
e
e
P680+ is a very strong oxidizing agent
H2O is split by enzymes, and electrons are transferred from the H atoms to P680+ P680
O2 is released
P680+ Activation: Photolysis
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Cytochromecomplex
Primaryacceptor
H2O
O2
2 H
+
1/2
P680
Light
Pigmentmolecules
Photosystem II(PS II)
Pq
Pc
ATP
1
2
3
5
Electron transport chain
e
e
e
4
Energy released by “the fall” creates a proton gradient across thylakoid membrane
Diffusion of H+ (protons) across the membrane drives ATP synthesis
Electrons Are Passed Between Photosystems by ETC & Generate ATP
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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
Electron transport chain
P700
Light
e
e
4
e
e
PSI Accepts Electrons From PSII
Transferred light energy excites P700, which loses an electron to an electron acceptor, creating P700+ (ready to take on more e-)
Reaction Center for PSI is P700
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The primary electron acceptor of PS I passes electrons to the protein ferredoxin (Fd)
The electrons are then transferred to NADP+ and reduce it to NADPH, for the reactions of the Calvin cycle
This process also removes an H+ from the stroma
Photosystem I Redox
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Linear Electron Flow
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
Electron transport chain
Electron
transport
chain
P700
Light
+ HNADP
NADPH
NADP
reductase
Fd
e
e
e
e
4
e
e
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Light Reaction: Review
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Photosystem II Photosystem I
Millmakes
ATP
ATP
NADPH
e
e
e
ee
e
e
Ph
oto
n
Ph
oto
n
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Photosystem I
Primaryacceptor
Cytochromecomplex
Fd
Pc
ATP
Primaryacceptor
Pq
Fd
NADPH
NADP
reductase
NADP
+ H
Photosystem II
Cyclic Electron Flow
• Only photosystem I used: produces ATP, but not NADPH, no O2 released
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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 MitochondriaChloroplasts 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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Mitochondrion Chloroplast
MITOCHONDRIONSTRUCTURE
CHLOROPLASTSTRUCTURE
Intermembranespace
Innermembrane
Matrix
Thylakoidspace
Thylakoidmembrane
Stroma
Electrontransport
chain
H Diffusion
ATPsynthase
H
ADP P iKey Higher [H ]
Lower [H ]
ATP
Figure 10.17
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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 I
Light
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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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
The Light Reactions: in Summary
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The Calvin CycleGenerates sugars and starting materials through the use of ATP and electrons from 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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Ribulose – 1,5 bisphosphate Carboxylase/Oxygenase RuBisCO
The Rubisco enzyme is the most common enzyme on the planet!
Vital for Glucose formation!
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Input
3 (Entering oneat a time)
CO2
Phase 1: Carbon fixation
Rubisco
3 P P
P6
Short-livedintermediate
3-Phosphoglycerate3 P P
Ribulose bisphosphate(RuBP)
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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 P1,3-Bisphosphoglycerate
CalvinCycle
6 NADPH
6 NADP
6 P i
6 P
Phase 2: Reduction
Glyceraldehyde 3-phosphate(G3P)
3 P PRibulose bisphosphate
(RuBP)
1 PG3P
(a sugar)Output
Glucose andother organiccompounds
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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 P1,3-Bisphosphoglycerate
CalvinCycle
6 NADPH
6 NADP
6 P i
6 P
Phase 2: Reduction
Glyceraldehyde 3-phosphate(G3P)
P5G3P
ATP
3 ADP
Phase 3:Regeneration ofthe CO2 acceptor(RuBP)
3 P PRibulose bisphosphate
(RuBP)
1 PG3P
(a sugar)Output
Glucose andother organiccompounds
3
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Calvin Cycle: Recap
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The Importance of Photosynthesis: A ReviewThe energy entering chloroplasts as sunlight gets
stored as chemical energy in organic compoundsSugar 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
Electron transport
chain
Electron transport
chain
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Regeneration ofCO2 acceptor
Carbon fixation
Reduction
CalvinCycle
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