biochemistry lect 9 2011 colour 2 slides per page.pdf
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LECTURE 9: Conversion of Light Energy to Chemical
Energy - Photosynthesis
Key Concepts (the big picture):Key Concepts (the big picture):
Photosynthesis takes place in the chloroplasts.
Photoreceptive compounds (eg. chlorophyll) within thechloroplast are grouped together with other compounds, to
form photosystems, and it is these complexes which
absorb light.
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Key Concepts (the big picture) cont:Key Concepts (the big picture) cont:
The processes within photosystems are called the
light reactions. These reactions include:
- splitting of water to produce O2 and H+
- NADP+ reduction to produce NADPH
+-
WHAT YOU NEED TO KNOW!! You should:WHAT YOU NEED TO KNOW!! You should:
describe the two hotos nthetic sta es and the
overall reaction of photosynthesis
explain the structure of the chloroplast and the
molecules involved in light absorption
understand the reactions taking place during the
light reactions
discuss how the absorption of light is coupled to ATP
synthesis
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10 x 1017 kcal/year of energy is stored by photosynthesis
= 10 x 1011 tons/year carbon fixed(~5 x 109 tons petroleum consumed in 2005)
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0
500
1000
Multicellular algae
Cambrian explosion
O2 @ 5-18% present
O2 @ 100% present +
2000
2500
3000
Cyanobacteria?
Stromatolite fossils
First eukaryotes?
Sterols
Mitochondria
3500
4000
4500
Meteors stop
Carbon signatures
Greenland
2
x million years
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Jean Baptist van Helmont
1579-1644
Grew 2.5 kg tree in 91
kg soil. Five years later,only water added 76.5 kg
soil only 56 g lighter
Priestly Lavoisier
Scheele Ingenhouz
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Mint leaves in chamber sustained mouse,
That the air would neither extinguish a candle, nor was it all
inconvenient to a mouse which I put into it
Priestly
Phlogistron?
Evil air
http://s.ngeo.com/wpf/media-live/photos/000/002/cache/jar-mouse_247_600x450.jpg
IngenhouzRepeated Priestly's work
Showed that light cleaned the air
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Lavoisier's work revealed the
Lavoisier
conservat on o matter
Very precise measurements
combustion
Reactants mass = products
Priestley's theory of phlogistation
Scheele
Unfortunate habits
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Antoine Lavoisier
And lost his head for issues with his taxes
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Senebier & Saussure
Showed that the increase in plant
mass was due to the uptake of
CO2 and H2O
Julius Mayer 1814-1878
German sur eon/ h sician
Discovered that light powered reactions
Showed the conservation of energy
1st law of thermodynamics
Light transferred to chemical energy
But where was the Oxygen coming from?
H2O or CO2?
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Cornelius van Niel (1897-1985)
Showed that H2O was split not CO2
Sulphur Bacteria CO2 + 2H2S (CH2O) + 2S + H2O
Plants CO2 + 2H2O (CH2O) + O2 + H2O
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Using labeled oxygen isotopes 18O
in CO2 or H2O
van e s owe
CO2 + H218O (CH2O) +
18O2
6CO2 + 6H2O (C6H12O6) + 6O2
The site of photosynthesis in a plant
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The thylokoid membrane is impermeable to most ions and molecules
(not Mg2+ and Cl- however)
Fig 9.1 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle.
More tomorrow on this
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Photosynthesis
b has -CHO
Chloroplast thylakoid
membranes have chlorophyll a
and b
Chlorophyll
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Absorbance of Chlorophyll a & bnote lack of absorbance around 500-600 nm
Fig. 9.2
Excitedstate
ron
e
ea
PhotonGround
Photon(fluorescence)
Energy
ofelect
(a) Excitation o f isolated chlorophyll molecule (b) Fluorescence
statemolecule
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Fig. 9.3 How a photosystem harvests light
1
Photosystem II
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Near perfect quantum yield (PSI, 1.0
PSII not so good at 0.85)
Nature Reviews 2004 Volume 5
Note that there are two photosystems II and I
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e
e
e
ATP
Fig. 9.4 A
mechanical
analogy for
the light
reactions:
Mill
makes
ATP
NADPH
e
e
e
Photosystem II Photosystem I
e
P680*
Photosystem 2
Pheophytin chlorophyll minus Mg2+
e
Electron transfer and proton translocation
Plasto uinone/ol
Q QH2 Q QH2
PCQ
e
P680
Cytbf
complex
2H+ in to thylakoid space adds to proton gradient
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P680*
Photosystem 2 P680
e
Splitting of water and O2release
H2O
Mn Mn
Mn Mn
O
e
e
P680
H+ in Thylakoid space - Adds to proton gradient
H2
Bacterial Photosystem IIScience 303, 1831, 2004
Photosystem 11 (Nature 2004 Vol 5)
Splitting O2 involves some of the nastiest reactions in
the biosphere
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e
e
e
ATP
Fig. 9.4 A
mechanical
analogy for
the light
reactions:
Mill
makes
ATP
NADPH
e
e
e
Photosystem II Photosystem I
e
Cytochrome-b6f
Homology to Complex III
of mitochondria
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e
e
e
ATP
Fig. 9.4 A
mechanical
analogy for
the light
reactions:
Mill
makes
ATP
NADPH
e
e
e
Photosystem II Photosystem I
e
P700*
Photosystem 1 P700
A0
Acceptor chlorophylls
Ae
PC
Fe-S
Ferredoxin
Ferredoxin
reductase
NADP+NADPH
e
P700
Note that NADPH removes 2H+ removed from thylakoid space
Cytbf
complex e
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Photosystem II
Fig. 9.5 How non-cyclic electron flow during the light reactions
generates ATP and NADPH
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Mitochondrion ChloroplastFig. 9.6Comparison of
chemiosmosis in
mitochondria
and
chloroplasts
CHLOROPLAST
STRUCTURE
MITOCHONDRION
STRUCTURE
Intermembrane
space
Inner
membrane
Electrontransport
chain
H+ DiffusionThylakoid
space
Thylakoid
membrane
KeyMatrix
Higher [H+]
Lower [H+]
Stroma
ATP
synthase
ADP + Pi
H+ATP
Chloroplast ATPase
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Difference between mitochondria and
chloroplastsChloro lasts are more reliant on H roton radient
Chloroplast inner membranes permeable to Cl- and Mg2+
So mitochondria use net charge potential and pH gradient
Larger pH difference required in chloroplasts.
Difference between mitochondria andchloroplasts
In c orop asts PSII removes H+ rom stroma, an t e
cytochrome complex translocates into thylakoid space
In mitochondria Complex I, and IV pump and Complex
III translocates protons out of matrix
Mitochondria brown purple!
Chloroplasts green!
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Light
Fd
Cytochromecomplex
STROMA(low H+ concentration)
Photosystem II Photosystem I
4 H+Light
NADP+
reductase
NADP+ + H+3
Fig. 9.7 A tentative model for the organization of the thylakoid membrane
ToCalvinCycle
THYLAKOID SPACE(high H+ concentration) 4 H+
Pq
Pc
NADPH
+2 H+
H2OO2
e
e
1/21
2
ADP+
iH+
ATPP
ATPsynthase
STROMA
(low H+
concentration)
Thylakoidmembrane
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Photosystem II and I separated?
Cyclic electron flow?
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Fig. 9.8 Cyclic electron flow
Why have cyclic electronflow?
-This produces ATP only
-Some bacteria only have PSI
-Mutant plants without cyclic flow
still grow, but not in bright light?
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P700*
Photosystem 1 P700
A0
A1
Cyclic electron flow
e
PC
Fe-S
Ferredoxin
Cyt
bf
e
P700
comp ex
H+Calvin Cycle appears to require more ATP than first
thought! PS1 can make extra ATP
Tomorrow Melvin Calvin
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