10/20/10
Photosynthesis
From inorganic C to organic C
and
Light energy to Chemical Energy
Photosynthesis BioFlix
Photosynthesis: an overview I
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Photosynthesis is the process that converts
solar energy into chemical energy.
Autotrophs sustain themselves without
eating anything derived from other organisms
Almost all plants are photoautotrophs, using
the energy of sunlight to make organic
molecules from H2O and CO2
These organisms feed not only themselves
but also most of the living world
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Photosynthesis: an overview II
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Redox process
H2O is split, e- (along w/ H+)
are transferred to CO2, reducing
it to sugar
2 major steps:
• light reactions (“photo”)
√ NADP+ (electron
acceptor) to NADPH
√Photophosphorylation:
ADP ---> ATP
• Calvin cycle (“synthesis”)
√ Carbon fixation:
carbon into organics
The chloroplast
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Sites of photosynthesis
Pigment: chlorophyll
Plant cell: mesophyll
Gas exchange:
stomata
Double membrane
Thylakoids, grana,
stroma
Why are leaves green?
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Photosystems
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Light harvesting units of the
thylakoid membrane
Composed mainly of protein
and pigment antenna
complexes
Antenna pigment molecules
are struck by photons
Energy is passed to reaction
centers (redox location)
Excited e- from chlorophyll is
trapped by a primary eacceptor
Linear electron flow I
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Photosystem II (P680):
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√ photons excite chlorophyll e- to an acceptor
√ e- are replaced by splitting of H2O (release of O2)
√ e-’s travel to Photosystem I down an electron transport chain
(Pq~cytochromes~Pc)
√ as e- fall, ADP ---> ATP (noncyclic photophosphorylation)
Linear electron flow II
√ photons excite chlorophyll e- to an acceptor
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Linear electron flow III
√ e- are replaced by splitting of H2O (release of O2)
Linear electron flow IV
√ e-’s travel to Photosystem I down an electron transport chain
√ as e- fall, ADP ---> ATP
(noncyclic photophosphorylation)
Linear electron flow V
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Photosystem I (P700):
Light hits PSI, and excites an
electron just as in PSII
√ ‘fallen’ e- PH II replaces excited
e- which jumps to another primary
e- acceptor
√ 2nd ETC ( Fd~NADP+
reductase) transfers e- to NADP+
---> NADPH (...to Calvin cycle…)
These photosystems produce equal
amounts of ATP and NADPH
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Linear electron flow VI
‘fallen’ e- PH
II replaces
excited ewhich jumps
to another
primary eacceptor
Light hits PSI
Linear electron flow VII
√ 2nd ETC
( Fd~NADP+
reductase)
transfers eto NADP+ to
NADPH (...to
Calvin
cycle…)
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Linear electron flow IX
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ATP and NADPH
are produced on
the side facing
the stroma,
where the Calvin
cycle takes place.
Protons are
pumped into the
thylakoid space
and drive ATP
synthesis as they
diffuse back into
the stroma.
Light reactions
generate ATP
and increase the
potential energy
of electrons by
moving them
from H2O to
NADPH.
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Light Reactions
The Calvin cycle I
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3 molecules of CO2 are
‘fixed’ into glyceraldehyde 3phosphate (G3P)
Phases:
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1- Carbon fixation~ each
CO2 is attached to RuBP
(rubisco enzyme)
2- Reduction~ electrons
from NADPH reduces to
G3P; ATP used up
3- Regeneration~ G3P
rearranged to RuBP; ATP
used; cycle continues
The Calvin cycle II
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The Calvin cycle III
The Calvin cycle IV
Calvin Cycle, net synthesis
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For each G3P (and for 3 CO2)…….
Consumption of 9 ATP’s & 6 NADPH
(light reactions regenerate these
molecules)
G3P can then be used by the plant to
make glucose and other organic
compounds
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Calvin Cycle
Cyclic electron flow
Alternative cycle when ATP is
deficient
Photosystem I used but not
II; produces ATP but no
NADPH. (Cyclic electron flow
generates surplus ATP,
satisfying the higher demand
in the Calvin cycle)
Why? The Calvin cycle
consumes more ATP than
NADPH…….
Cyclic photophosphorylation
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Alternative carbon fixation methods,
I
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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 a seemingly wasteful
process called photorespiration
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Alternative carbon fixation methods,
II
In most plants (C3 plants), initial fixation of
CO2, via rubisco, forms a three-carbon
compound
In photorespiration, rubisco adds O2
instead of CO2 in the Calvin cycle
Photorespiration consumes O2 and organic
fuel and releases CO2 without producing ATP
or sugar
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C4 Plants
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C4 plants minimize the cost
of photorespiration by
incorporating CO2 into fourcarbon 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 bundlesheath cells, where they
release CO2 that is then used
in the Calvin cycle,
CAM Plants
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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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C4 v. CAM: Side by side
A review of photosynthesis
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