Transcript Electron Transport System

Photosynthesis:
Overview of photosynthesis, conversion of light
energy to redox energy, and the Z scheme
Bioc 460 Spring 2008 - Lecture 31 (Miesfeld)
Paraquat
Paraquat is an inhibitor of
PSI that was used to kill
marijuana plants
Agricultural crops are the
primary means for converting
solar energy into chemical
energy for life on earth
Solar power can provide a
sustainable energy source
for some areas in the world
Key Concepts in Photosynthesis
• The energy from sunlight is used to initiate photooxidation reactions in
light-absorbing pigments that convert light energy into chemical energy.
• The light reactions of photosynthesis are similar to the electron transport
system in that they require a proton impermeable membrane and a
series of linked redox reactions to generate proton motive force used
for ATP synthesis.
• Photooxidation uses the oxidation of H2O to produce O2 in a process
that provides electrons for photophosphorylation and the reduction of
NADP+ to produce NADPH.
• Chemical energy in the form of ATP and NADPH is used to convert CO2
to glyceraldehyde-3-P using enzymes in the Calvin Cycle pathway.
Plants use sunlight during the day and aerobic respiration at night.
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.
Net reaction of photosynthesis and carbon fixation
H2O + CO2 ─(light energy)→ (CH2O) + O2
The 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
ΔGº' 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.
The Biosphere Experiment, then and now!
Biosphere 2 - 1991
Joseph Priestly - 1772
Biosphere 2 did not actually work very well because CO2 levels
built up inside the sealed environment and periodic CO2 removal
was required. The first “Biospherian” came out after ~1 year.
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).
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.
PSI
PSII
Overview of Photosynthesis
The photosynthetic electron transport system and photophosphorylation work
togethe to generate ATP and NADPH for sugar synthesis by the Calvin Cycle
4e-
Light energy is used to oxidize 2 H2O which releases 4e- and 4H+
Redox reactions are used to translocate an additional 8H+
Pay attention to the
compartmentalization
inside and outside of
the chloroplast.
The product of the
carbon fixation is
glyceraldehyde-3P
(GAP) which is
converted to hexose
sugars for use as
chemical energy at
night.
What pathway
has GAP as a
central
intermediate,
does this make
sense here?
Peter Mitchell’s
chemiosmotic theory
was actually first
proven using
photosynthetic
systems in which an
artificial proton
gradient was
established using
buffered solutions at
different pH values.
1
2
3
What would happen to
the ATP* if the buffer
was now switched back
to pH4?
5
4
The chloroplast ATP synthase is structurally and functionally similar to the
mitochondrial ATP synthase except 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).
Chloroplast Structure
Inner
envelope
Outer Stroma
envelope
Granal
thylakoids
Stroma
thylakoids
Light energy is absorbed
by numerous accessory
pigments which can
transfer the absorbed
energy to nearby reaction
centers containing
specialized chlorophyll
molecules.
These accessory pigments
function as light harvesting
antenna.
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.
Plant pigments cover a broad absorption spectra
Organization of Photosynthetic pigments
• Light absorbing pigments are organized in functional arrays called
photosystems of which there are two; PSII and PSI in plants.
• 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.
Light harvesting pigment molecules transfer energy to nearby
molecules which eventually results in photooxidation
Top view of antenna chlorophylls. Note that
both PSI and PSII are reaction centers.
Z Scheme of Photosynthetic Electron Transport
Light energy absorbance “kicks” electrons into higher energy states which is used
to convert solar energy into redox energy when the electrons are passed one at a
time to electron carriers with increasingly positive standard reduction potentials.
The redox energy is captured in the form of chemical energy (ATP, NADPH.
The oxidation of 2 H2O to produce one mole of O2 requires the
absorbance of 8 photons to move the 4 e- through the entire
photosynthetic electron transport system.
Energy input at both the PSII and PSI reaction centers
Light harvesting involves
resonance energy
transfer, whereas,
photooxidation involves
e- transfer from
chlorophyll to the electron
acceptor 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.
Light absorption
kicks an e- into a
higher orbital,
what happens
next is either
energy transfer,
photooxidation,
or fluorescence.
Now all we have to do is follow the photons,
electrons, and protons starting with PSII
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.
Functional organization of
the PSII complex
The electron that was
transferred from the P680
chlorophyll reaction center
needs to be replaced, this
replacement electron comes
from the oxidation of H2O within
the oxygen evolving complex.
The tricky part is that the
oxidation of H2O releases 4 e-,
however, photooxidation only
transfers one e- at a time to
pheophytin.
Therefore, the Mn atoms must
be able to “store” electrons and
release them one at a time.
Functional organization of the
cytochrome b6f complex
The same deal
here as in
complex III of
ETS, we need to
convert the 2 ecarrier PQBH2
into a 1 e- carrier
in PC. The
answer is the Q
cycle, duh!
The light-driven Q cycle is responsible for
translocation of 8 H+ from the stroma to the lumen
ATP synthase
complex
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
Functional organization of
the PSI complex
Ferrodoxin NADP+
reductase plays a
crucial role in
converting redox
energy into a
useable form for
the Calvin Cycle
by generating
NADPH.
Since e- arrive in
PSI one at a time,
the FAD
coenzyme must
store on e- in a
semiquinone
chemical
structure.
Paraquat was once used extensively as an aerial herbicide to
destroy illegal fields of marijuana and coca plants in North and South
America. However its use was discontinued because smoking
paraquat-contaminated plants causes lung damage.