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Photosynthesis 1
Light Reactions and
Photosynthetic Phosphorylation
Lecture 31
Key Concepts
• Overview of photosynthesis and carbon fixation
• Chlorophyll molecules convert light energy to redox
energy
• The Z scheme of photosynthetic electron transport
• Oxidative phosphorylation and photosynthesis are
related processes
How do photosynthetic reaction centers convert light
energy into redox energy?
In what ways are photosynthesis and oxidative
phosphorylation similar? Different?
Together, the photosynthetic
electron transport system and
Calvin cycle (carbon fixation)
convert sunlight energy into
chemical energy (ATP,
NADPH, triose phosphate),
and in the process, oxidize H2O
to form O2.
The photosynthetic electron
transport system is often
referred to as the light
reactions of photosynthesis,
whereas, the Calvin cycle has
been called the dark
reactions.
However, the term "dark
reactions" can be misleading
because the Calvin cycle is
most active in the light when
ATP and NADPH levels are
high.
Oxygenic Photosynthesis
H2 O
O2
CO2 CH2O
Buchanon, et. al., Fig.12.2
Overview of Photosynthesis
Light excitation of Photosystems I and II results in oxygen evolution from
the splitting H2O, and the generation of chemical energy in the form of
ATP and NADPH. Plants use this chemical energy (ATP and NADPH)
to convert CO2 into sugars via the Calvin cycle which takes place in the
stroma.
Net reaction of photosynthesis and carbon fixation
H2O + CO2 ─(light energy)→ (CH2O) + O2
Although written as a balanced reaction, O2 generation is the result of
H2O oxidation, whereas, the CO2 is used to synthesize carbohydrate
(CH2O)
It takes 2 H2O to make an O2 and six CO2 molecules are required for
the synthesis of each molecule of glucose, therefore:
6 H2O + 6 CO2 ─(light energy)→ C6H12O6 + 6 O2
∆Gº' for this reaction is +2868 kJ/mol!
This is overcome by the energy potential stored in the products of
photosynthetic electron transport, namely, ATP and NADPH.
General equation for photosynthesis:
2H2A (red. e- donor) + CO2
(CH2O) + H2O + 2A (ox. e- donor)
In “oxygenic PS” A= O
Diverse photosynthetic organisms use electron donors other
than H2O for photosynthesis:
Green sulfur bacteria:
light
2H2S + CO2
(CH2O) + H2O + 2S
Other PS bacteria:
light
2 lactate + CO2
(CH2O) + H2O + 2 pyruvate
Joseph Priestly’s Experiment (200 yrs ago!)
Live plants and respiring animals
could coexist in a closed system for
a limited period of time as long as
H2O and light were provided
It was later shown that chloroplasts
contain light gathering pigments
called chlorophyll that work with
proteins to convert light energy into
chemical energy and in the process
release O2.
A modern Priestly experiment
Biosphere 2 outside of Tucson, built in the
1990s by a private company called Space
Biospheres Ventures.
This experiment did not
actually work very well
because CO2 levels
built up inside the
sealed environment and
periodic CO2 removal
was required.
Metabolic pathway questions related to the
photosynthetic electron transport system and
the Calvin cycle
1. What do the photosynthetic electron transport system
and Calvin cycle accomplish for the cell?
•
The photosynthetic electron transport system converts light
energy into redox energy which is used to generate ATP by
chemiosmosis and reduce NADP+ to form NADPH.
•
Calvin cycle enzymes use energy available from ATP and NADPH
to reduce CO2 to form glyceraldehyde-3-P, a three carbon
carbohydrate used to synthesize glucose.
•
Photosynthetic cells use the carbohydrate produced by the Calvin
cycle as a chemical energy source for mitochondrial respiration in
the dark.
Photosynthetic organisms are autotrophs because they derive energy
from light rather than from organic materials (as food).
Questions: Do plants use oxidative phosphorylation?
Do plants have mitochondria?
2. What are the overall net reactions of photosynthetic
electron transport system and the Calvin cycle?
Photosynthetic electron transport system
(production of ATP and O2):
2 H2O + 8 photons + 2 NADP+ + ~3 ADP + ~3 Pi →
O2 + 2 NADPH + ~3 ATP
Calvin cycle (six turns of cycle yields glucose):
6 CO2 + 12 NADPH + 18 ATP + 12 H2O →
Glucose + 12 NADP+ + 18 ADP + 18 Pi
3. What are the key enzymes in the photosynthetic electron
transport system and the Calvin cycle?
Protein components of the photosynthetic electron transport system
– three protein complexes are required for the oxidation of H2O and
reduction of NADP+; photosystem II (P680 reaction center), cytochrome
b6f (proton pump) and photosystem I (P700 reaction center).
Chloroplast ATP synthase – enzyme responsible for the process of
photophosphorylation which converts proton-motive force into net
ATP synthesis; this enzyme is very similar to mitochondrial ATP
synthase.
Ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) - is
responsible for CO2 fixation in the first step of the Calvin cycle.
Rubisco activity is maximal in the light when stromal pH is ~8 and
Mg2+ levels are elevated due to proton pumping.
4. What are examples of the photosynthetic electron
transport system and Calvin cycle in real life?
DCMU (dichlorophenyl dimethylurea) is
a broad spectrum herbicide that
functions by blocking electron flow
through photosystem II and is used to
reduce weeds in non-crop areas.
Another herbicide, paraquat, prevents
reduction of NADP+ by accepting
electrons from intermediate reductants
in photosystem I.
Chloroplast Structure
Inner
envelope
Chloroplast Structure
Outer Stroma
envelope
Stroma
Granal
thylakoids
thylakoids
Buchanon et al., Fig.12.1
Light energy is absorbed by
numerous accessory
pigments which can transfer
the absorbed energy to
nearby reaction centers
containing specialized
chlorphyll molecules.
These accessory pigments
function as light harvesting
antenna.
Chlorophyll (and Other Pigments)
Grab hold of those photons!
Chlorophylls are the primary light gathering pigments.
They have a heterocyclic ring system that constitutes an
extended polyene structure, which typically has strong
absorption in visible light.
Buchanon et al., Fig.12.4
Chlorophyll absorption spectra
Buchanon et al., Fig.12.6
Other Photosynthesis Pigments
Buchanon et al., Fig.12.6
Expression of
carotenoid
biosynthetic
genes in E. coli
Buchanon et at., Fig.12.8
Light Energy:
In photosynthesis, light energy is converted to chemical
energy, therefore it is important to understand the energy
content of light.
Light energy is expressed as:
Einsteins = moles of photons
Light energy is wavelength dependent:
E = hν
h = Planck’s constant (6.626 x 10-34 J sec)
ν = light frequency = c/λ (speed of light/wavelength)
How is the energy of light related to light wavelength?
Energy in the
electromagnetic
spectrum
Buchanon et al., Fig. 19-36:
1) Light energy is wavelength dependent:
E = hn = c/λ
h = Planck’s constant (6.626 x 10-34 J sec)
n = light frequency = c/λ (speed of light/wavelength)
2) Standard Free Energy change of redox reactions:
∆G°’ = - nF·∆E°’
∆E°’= E°’ acceptor - E°’ donor
3) Free Energy of PMF:
∆G = RT·ln(C2/C1) + ZF·∆Ψ
∆G = 2.3 RT·∆pH + F·∆Ψ
Chlorophyll energy levels
Buchanon et al., Fig.12.3
Organization of Photosynthetic pigments
•
Light absorbing pigments are organized in functional arrays called
Photosystems.
•
“Light harvesting” or “antenna” pigment molecules are specialized to
absorb light and transfer the energy to neighboring pigment
molecules.
•
“Photochemical reaction center” pigment molecules are specialized
to transduce light energy into chemical energy.
•
Several hundred light harvesting pigment molecules funnel light to
one reaction center molecule.
Steps in Photochemistry
Or, How Reaction Centers Work
1) Light harvesting chlorophyll (LHC)
electron is excited by light.
2) Energy of LHC electron is passed to
successive LHC chlorophyll
molecules by resonance energy
transfer.
3) LHCs near reaction center (RC)
transfer energy to RC chlorophyll.
4) Excited RC chlorophyll donates
electron to an electron acceptor in the
RC.
5) RC regains electron from electron
donor creating “charge separation” in
the RC.
Chl* = excited chlorophyll
In the chloroplast PSII reaction
center, the electron acceptor is a
molecule called pheophytin
which becomes negatively
charge as denoted by •Pheo-.
Importantly, the oxidized
chlorophyll molecule (now
positively charged, Chl+) returns
to the ground state by accepting
an electron through a coupled
redox reaction involving the
oxidation of H2O.
This process of O2 evolution
takes place in the manganese
center present in the thylakoid
membrane and is ultimately the
source of electrons needed for
the photosynthetic electron
transport system.
The Z Scheme of Photosynthetic Electron Transport
Light energy captured by chlorophyll molecules actually involves the
interplay of two reaction centers called photosystem I and photosystem II
that are linked together by redox reactions.
The reaction center complexes are functionally linked by an electron
carrier protein called plastocyanin that shuttles electrons one at a time
from PSII to PSI.
The Z Scheme of Photosynthetic Electron Transport
The oxidation of 2 H2O requires 8 photons to transport 4 e- through the
system, resulting in the accumulation of 12 H+ in the thylakoid space
and generation of 2 NADPH in the stroma.
Z Scheme Described
The photosynthetic electron transport system in plants consists of two
linked electron circuits, each requiring an input of energy from light
absorption at PSII and PSI reaction center complexes to initiate
electron flow. The Z scheme energy diagram showing how photon
absorption by the PS II reaction center complex results in electron flow
from H2O to plastocyanin, providing energy to translocate H+ across the
thylakoid membrane. A second photon absorption event at PS I drives
electron transport from plastocyanin to NADP+.
Electron flow through protein-linked redox reactions involves numerous
electron carriers, including Fe-S centers, the hydrophobic molecule
plastoquinone (Q) which is reduced to form plastoquinol (QH2) and
analogous to ubiquinone/ubiquinol in the mitochondrial electron
transport system. Plastocyanin has the same job in photosynthetic
electron transport as does cytochrome c in mitochondrial electron
transport.
Z Scheme of Photosynthetic Electron Transport
Buchanon et al., Fig.12.22
TIPS 7: 183-185
TIPS 7: 183-185
Photosystem II (PSII)
Photosystem II contains chlorophylls a and b and absorbs light at
680nm. This is a large protein complex that is located in the thylakoid
membrane.
Schematic drawing of electron flow through PS II
The absorption of light energy by PS II results in electron flow leading
to generation of O2 from the splitting of H2O and to the reduction of
plastoquinone (Q) to form plastoquinol (QH2).
Water Oxidation at PSII
2H2O Æ 2H+ + 4e- + O2
A single photon of 680 nm light does not have enough
energy to break the bonds in water.
Instead, the 4 e- are passed one at a time to oxidized
P680 through a Tyr residue in the D1 subunit of PSII.
4 P680+ + 4 Tyr . Æ 4 P680 + 4 Tyr
4 Mn ions store the positive charge and coordinate the
oxygen until oxidation of water is complete.
4 Tyr . + [Mn complex]0 Æ 4Tyr + [Mn complex]4+
[Mn complex]4+ + 2 H2O Æ [Mn complex]0 + 2H+ + O2
Manganese cluster of the PS II Reaction Center
from Synechocystis elongatus Nature 409:739
(2001)
The electron flow resulting from the absorption of 4 photons leads to
the reduction of two molecules of plastoquinone, and in the process,
generates a net increase of 4 H+ inside the thylakoid lumen.
The net reaction from oxidation of QH2, and from proton pumping
through the cytochrome b6f complex, is the addition of 8 H+ to the
thylakoid lumen. This brings to 12 the total number of H+ molecules
accumulated inside the thylakoid lumen for every O2 generated in PS
II.
Photosystem I (PSI)
The final stage of photosynthesis: the absorption of light energy by
PS I is at a maximum of 700 nm. Again 4 photons are absorbed, but
in this case, the energy is used to generate reduced ferredoxin, which
is a powerful reductant.
Structure of PS I complex showing Fe-S clusters
Electron flow diagram of PS I
leading to generation of reduced ferredoxin
Ferrodoxin-NADP+ reductase converts NADP+ to NADPH
through a semiquinone intermediate involving FADH˙
We can put it all together to showing that ATP synthesis results from
protons flowing OUT of the thylakoid lumen back into the stroma.
Note that the chloroplast ATP synthase is structurally and
functionally similar to the mitochondrial ATP synthase we have
already described, with the exception that 4 H+ are required for
every ATP synthesized based on experiments showing that ~3 ATP
are synthesized for every O2 generated (12 H+ transported/O2
generated).
This difference (3 H+/ATP in mitochondria versus 4 H+/ATP in
chloroplasts) could be due to differences in the "gear ratio" of the
complex, or uncertainties in the experimental measurements.
ATP synthesis occurs from protons flowing OUT of the thylakoid
lumen back into the stroma.
Understand the compartmentalization here!
Comparison of
Photosynthesis and Oxidative Phosphorylation
These processes are related.
You should understand how they are related.
The light induced proton motive force generated during
photosynthesis is used to generate ATP in the stroma as a
result of protons moving out of the thylakoid lumen.
In contrast, protons pumped out of the mitochondrial matrix
as a result of NADH oxidation by the electron transport
system, flow back into the matrix to generate ATP inside
the mitochondria.