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Ch. 10
• Photosynthesis
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Photosynthesis
• Energy flows into ecosystem as sunlight, • Feeds the Biosphere• Converts solar E into chemical E
Light energy
ECOSYSTEM
CO2 + H2O
Photosynthesisin chloroplasts
Cellular respirationin mitochondria
Organicmolecules
+ O2
ATP
powers most cellular work
HeatenergyFigure 9.2
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Energy Transformations
• Photoautotrophs (producers)– Use E of sunlight to make organic molecules
from water and CO2
(a) Plants
(b) Multicellular algae
(c) Unicellular protist 10 m
40 m(d) Cyanobacteria
1.5 m(e) Pruple sulfurbacteria
Figure 10.2
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• Photosynthesis converts light E to the chemical E of food
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Heterotrophs
• Obtain organic material f/ other organisms• Consumers of the biosphere
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Chloroplasts: Site of Photosynthesis (plants)
• Leaf
Vein
Leaf cross section
Figure 10.3
Mesophyll
CO2 O2Stomata
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Leaf Anatomy
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Chloroplasts
• Chloroplast Structure– Contain grana which consisting of thylakoid stacks
Chloroplast
Mesophyll
5 µm
Outermembrane
Intermembranespace
Innermembrane
Thylakoidspace
ThylakoidGranumStroma
1 µm
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Chloroplasts
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Photosynthesis summary reaction
6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O
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Chloroplasts split water into
• H2 and O2, incorporating the e- of H2 into sugar molecules
6 CO2 12 H2OReactants:
Products: C6H12O6
6 H2O
6 O2
Figure 10.4
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Photosynthesis as a Redox Process
• Water is oxidized, CO2 is reduced
• Protons and Electron are taken from water and added to CO2
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The Two Stages of Photosynthesis: A Preview
• Light Reactions– Occurs on thylakoid membranes– Converts solar E to chemical E
• Dark Reaction (Calvin Cycle)– Occurs in the stroma– Forms sugar from carbon
dioxide, using ATP for energy and NADPH for reducing power
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Overview of photosynthesis
H2O CO2
Light
LIGHT REACTIONS
CALVINCYCLE
Chloroplast
[CH2O](sugar)
NADPH
NADP
ADP
+ P
O2Figure 10.5
ATP
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Lets Talk about Light
• Form of electromagnetic E, travels in waves
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Wavelength ()
• Distance between the crests of waves
• Determines the type of electromagnetic E
More
Power
ful
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Electromagnetic spectrum
Entire range of electromagnetic E, or radiation
Gammarays X-rays UV Infrared
Micro-waves
Radiowaves
10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m
106 nm 103 m
380 450 500 550 600 650 700 750 nm
Visible light
Shorter wavelength
Higher energy
Longer wavelength
Lower energy
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• Visible light spectrum
– Colors of light we can see
– ’s that drive photosynthesis
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Photosynthetic Pigments: The Light Receptors
• Substances that absorb visible light
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Pigments
Reflect light, which include the colors we see
Light
ReflectedLight
Chloroplast
Absorbedlight
Granum
Transmittedlight
Figure 10.7
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Transmitted vs. Absorbed Light
Figure 10.8
Whitelight
Refractingprism
Chlorophyllsolution
Photoelectrictube
Galvanometer
Slit moves topass lightof selectedwavelength
Greenlight The high transmittance
(low absorption)reading indicates thatchlorophyll absorbsvery little green light.
The low transmittance(high absorption) readingchlorophyll absorbs most blue light.
Bluelight
1
2 3
4
0 100
0 100
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Absorption spectra of 3 types of pigments
Ab
sorp
tion
of
ligh
t b
ych
loro
pla
st p
igm
en
tsChlorophyll a
Wavelength of light (nm)
Chlorophyll b
Carotenoids
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Action spectrum of a pigment
• Effectiveness of different of radiation in driving photosynthesis
Rat
e o
f ph
otos
ynth
esis
(mea
sure
d by
O2 r
elea
se)
Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids.
(b)
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First demonstrated by Theodor W. Engelmann
400 500 600 700
Aerobic bacteria
Filamentof alga
Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had
been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most.Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b.
(c)
Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis.
CONCLUSION
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Chlorophylls: Photosynthetic Pigments
• Chlorophyll a
– Main photosynthetic pigment
• Chlorophyll b
– Accessory pigment
C
CH
CH2
CC
CC
C
CNNC
H3C
C
C
C
C C
C
C
C
N
CC
C
C N
MgH
H3C
H
C CH2CH3
H
CH3C
HH
CH2
CH2
CH2
H CH3
C O
O
O
O
O
CH3
CH3
CHO
in chlorophyll a
in chlorophyll b
Porphyrin ring:Light-absorbing“head” of moleculenote magnesiumatom at center
Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts: H atoms notshown
Figure 10.10
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Assesory proteins
• Other accessory pigments– Absorb different s of light and pass the E to
chlorophyll a
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Excitation of Chlorophyll by Light
• When a pigment absorbs light electrons go from their ground state to an excited state (unstable)
Excitedstate
Ene
rgy
of e
lect
ion
Heat
Photon(fluorescence)
Chlorophyllmolecule
GroundstatePhoton
e–
Figure 10.11 A
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Photosystems I and II
MillmakesATP
ATP
e–
e–e–
e–
e–
Pho
ton
Photosystem II Photosystem I
e–
e–
NADPH
Pho
ton
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Starter Question
• Compare and contrast the electron transport chain in cellular respiration with the light reactions in photosynthesis. Be sure to indicate similarities and differences
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Photosystem II and I: Site of Photophosphorylation
• Proton Motive Force? Non Cyclic Flow to Calvin Cycle
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Chemiosmosis in Chloroplasts v. Mitochondria
• Spatial organization of chemiosmosisKey
Higher [H+]Lower [H+]
Mitochondrion Chloroplast
MITOCHONDRIONSTRUCTURE
Intermembrancespace
Membrance
Matrix
Electrontransport
chain
H+ DiffusionThylakoidspace
Stroma
ATPH+
PADP+
ATPSynthase
CHLOROPLASTSTRUCTURE
Figure 10.16
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Light reactions and chemiosmosis: Cyclic Flow
LIGHTREACTOR
NADP+
ADP
ATP
NADPH
CALVINCYCLE
[CH2O] (sugar)STROMA(Low H+ concentration)
Photosystem II
LIGHT
H2O CO2
Cytochromecomplex
O2
H2OO2
1
1⁄2
2
Photosystem ILight
THYLAKOID SPACE(High H+ concentration)
STROMA(Low H+ concentration)
Thylakoidmembrane
ATPsynthase
PqPc
Fd
NADP+
reductase
NADPH + H+
NADP+ + 2H+
ToCalvincycle
ADP
PATP
3
H+
2 H++2 H+
2 H+
NADPH and O2 Not produce. Does not go toCalvin Cycle
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Calvin cycle
• Uses ATP and NADPH to convert CO2 to sugar
• Similar to the citric acid cycle
• Occurs in the stroma
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Calvin Cycle Happens in the Stroma
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The “Other” Calvin Cycle
• Calvin cycle
(G3P)
Input(Entering one
at a time)CO2
3
Rubisco
Short-livedintermediate
3 P P
3 P P
Ribulose bisphosphate(RuBP)
P
3-Phosphoglycerate
P6 P
6
1,3-Bisphoglycerate
6 NADPH
6 NADPH+
6 P
P6
Glyceraldehyde-3-phosphate(G3P)
6 ATP
3 ATP
3 ADP CALVINCYCLE
P5
P1
G3P(a sugar)Output
LightH2O CO2
LIGHTREACTION
ATP
NADPH
NADP+
ADP
[CH2O] (sugar)
CALVINCYCLE
O2
6 ADP
Glucose andother organiccompounds
Phase 1: Carbon fixation
Phase 2:Reduction
Phase 3:Regeneration ofthe CO2 acceptor(RuBP)
From Light
Reactions
From Light
Reactions
Glyceraldehyde-3-PCan go to sugars, amino acids,fatty acids
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• Alternative mechanisms of carbon fixation have evolved in hot, arid climates
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• On hot, dry days, plants close their stomata
– Conserving water but limiting access to CO2
– Causing O2 to build up photorespiration
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Photorespiration: An Evolutionary Relic?
Photosynthetic rate is reduced
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C4 Plants (e.g. corn)
• Minimize photorespiration
– Incorporate CO2 into four carbon compounds in mesophyll cells
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• 4 carbon compounds in bundle sheath cells
release CO2 CO2 Calvin cycle
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• C4 leaf anatomy and the C4 pathway
CO2
Mesophyll cell
Bundle-sheathcell
Vein(vascular tissue)
Photosyntheticcells of C4 plantleaf
Stoma
Mesophyllcell
C4 leaf anatomy
PEP carboxylase
Oxaloacetate (4 C) PEP (3 C)
Malate (4 C)
ADP
ATP
Bundle-Sheathcell CO2
Pyruate (3 C)
CALVINCYCLE
Sugar
Vasculartissue
Figure 10.19
CO2
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CAM Plants (e.g. pineapple)
• Open their stomata at night, CO2 organic acids
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• During the day, stomata close
– CO2 is released from the organic acids for use in the Calvin cycle
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• CAM pathway is similar to the C4 pathway
Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in differenttypes of cells.
(a) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cellsat different times.
(b)
PineappleSugarcane
Bundle-sheath cell
Mesophyll Cell
Organic acid
CALVINCYCLE
Sugar
CO2 CO2
Organic acid
CALVINCYCLE
Sugar
C4 CAM
CO2 incorporatedinto four-carbonorganic acids(carbon fixation)
Night
Day
1
2 Organic acidsrelease CO2 toCalvin cycle
Figure 10.20
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• Review
Light reactions:• Are carried out by molecules in the thylakoid membranes• Convert light energy to the chemical energy of ATP and NADPH• Split H2O and release O2 to the atmosphere
Calvin cycle reactions:• Take place in the stroma• Use ATP and NADPH to convert CO2 to the sugar G3P• Return ADP, inorganic phosphate, and NADP+ to the light reactions
O2
CO2H2O
Light
Light reaction Calvin cycle
NADP+
ADP
ATP
NADPH
+ P 1
RuBP 3-Phosphoglycerate
Amino acidsFatty acids
Starch(storage)
Sucrose (export)
G3P
Photosystem IIElectron transport chain
Photosystem I
Chloroplast
Figure 10.21
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• Organic compounds produced by photosynthesis
– Provide the E and building material for ecosystems
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Extra stuff: Light energy causes the removal of an electron from a molecule of P680 that is part of Photosystem II. The P680 requires an electron, which is taken from a water molecule, breaking the water into H+ ions and O-2 ions. These O-2 ions combine to form the diatomic O2 that is released.
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