VL 9 (Elke Dittmann) Flashcards
Explain Photosynthesis
Co2 + H2O + sunlight –> C6H12O6 + O2
Photosynthesis captures light energy and uses it to power chemical reactions that
convert carbon dioxide and water into oxygen and energy-rich carbohydrates
- releases oxygen to the atmosphere and “removes” carbon dioxide
- Light reactions of photosynthesis use energy from photons to generate “high-energy electrons”
What are phototrophic organisms?
- plants
- algae
- protists
- cyanobacteria
- anoxygenic phototrophs
What is the diversity of pigments ?
main pigments:
* chlorophyll + bacteriochlorophyll
* substituted, circular tetrapyrroles (prophytins)
* additional reduced pyrrole ring
* central atom Mg2+
* lipophilic C20 phytol, esterified to an acid chain, membrane bound
* alternating dingle and double bound (polyene)
* absorb light at different wavelengths
accessory pigments
* light harvesting and anergy transfer to chlorophylls
* photoprotection
* carotenoids
* phycobilins
Light Absoption and photoinduces charge seperation
- The peak molar absorption coefficient of chlorophyll a is 105 M-1 cm-1, among the highest observed for organic compounds
- Chlorophylls absorb in the visible light range where the solar output is maximal
- light energy excites an electron from its ground energy level to an excited energy level
- excited electron can simply return to the ground state, energy is converted to heat or
- move from the original molecule to a nearby acceptor
- results in „photoinduced charge separation“: positive charge at the initial molecule and a negative charge at the acceptor molecule
- electrons can be transported that way through a chain of molecules
Why is photosynthesis named photosynthesis?
named after first stable electron acceptor:
–> quinone (type II), Photosystem II
–> FeS (type I), Photosystem I
flow from PSII to PSI in oxygenic phototrophs, electrons from H2O
–> plants, algae, cyanobacteria
Photosynthetic membranes and evolutions of chloroplasts
- electrons “energized” by photons are used directly to reduce NADP+ to NADPH
- electrons pass through an electron transport chain, indirectly generating a proton motive force (pmf) across a membrane thus driving the synthesis of ATP
- in the “dark reactions”, NADPH and ATP drive the reduction of CO2 to organic compounds
- photosynthesis takes place at thylakoid membranes, located in specialized organelles, the chloroplasts
Two photosystems generate a proton gradient and NADPH in oxygenic photosynthesis
- Plants, algae and cyanobacteria perform an
oxygenic photosynthesis - realized by two photosystems (PSII and PSI) responding to light of different wavelengths
- normally, electrons flow from PS II,
to cytochrome bf and then to PS I - electrons derived from water,
2 H2O are oxidized to form O2, four electrons
sent through the electron transport chain - electrons finally reduce NADP+ to NADPH
- pmf generated drives formation of ATP
Explain photosystem II of oxygenic phototrophs and its reactions
- PSII core formed by D1 and D2 (red and blue) proteins, homologs of L + M of purple bacteria
- unlike the bacterial system, PS II contains a large number of additional subunits
- bind additional chlorophylls, increase efficiency of light energy absorption
Overall reaction:
2Q + 2 H2O –> O2 + 2 QH2
Q: Plastoquinone, QH: Plastoquinole
Proton gradient direction
- PS II spans the thylakoid membrane such that the site of Q reduction is on the side of the stroma
- Mn center/water oxidation in the thylakoid lumen
- 2 protons taken up Plastoquinone reduction
come from the stroma - 4 protons released by water oxidation are
released in the thylakoid lumen
–> results in proton gradient across the thylakoidmembrane
* reverse proton gradient compared to oxidative phosphorylation in mitochondria
* reverse orientation of ATP synthase
cytochorme bf links photosystem II to Photosystem I
- plastoquinol (QH2) from PS II contributes 2 electrons to the electron chain, terminating at PSI
- electrons transferred to plastocyanin (PC), a soluble copper protein in the thylakoid lumen
- cytochrome bf:
–> 23 kDa cytochrome b, 2 b-type hemes
–> 20 kDa Rieske-type Fe-S protein
–> 33 kDa cytochrome f, 1 cytochrome C
–> 17 kDa chain
Q cycle 1st half
* electrons from QH2 flow through the Fe-S protein to reduce plastocyanin
Q cycle 2nd half
* Cytochrome bf reduces second Q and reoxidizes QH2 to release 2H+ to the lumen further contributing to the pH gradient
QH2 + 2PC(Cu2+) –> Q+ 2PC(Cu+) + 2H
in thylakoid lumen
Explain Photosystem I and electron flow from PSI to Ferredoxin
- catalyzes final stage of the light reactions
- core: pair of homologous subunits PsaA (83 kDa, red) and PsaB (82 kDa, blue).
- chlorophyll special pair P700 at the center
- absorbs at 700 nm, initiating charge separation
Electron flow PSI –> Ferredoxin
* electron transferred via a chlorophyll at A0 site and quinone at A1 site to a set of Fe-S clusters to ferredoxin
ferredoxin: soluble stroma protein with a 2Fe- 2S cluster coordinated to four Cys residues, very strong reductant
* positive charge of P700+ neutralized by the transfer of an electron from reduced
plastocyanin
overall reaction
PC(Cu+) + Fdox à PC(Cu2+) + Fdred
- standard free enthalpy is +79,1 kJ mol-1
- uphill reaction, driven by the absorption of a 700 nm photon
What is the role of the Ferredoxi-NADP+ Reductase?
Ferredoxin-NADP+ Reductase converts NADP+ into NADP+
- Ferredoxin carries only 1 available
electron. - NADPH, a two electron reductant is widely used in biosynthetic processes.
–> The reaction is catalyzed by ferredoxin- NADP+ reductase, a flavoprotein
photosynthesis and oxidative phosphorylation
- Photophosphorylation closely resembles oxidative phosphorylation in mitochondria
- Proton-motive force (Dp ) is sum of two
components:
–> charge gradient and chemical
gradient - chloroplasts: nearly all Dp derives from DpH (chemical gradient)
- mitochondria: larger contribution from membrane potential (charge gradient)
- electrical neutrality in chloroplasts maintained by transfer of Cl- and Mg2+ in exchange for H+
- ATP synthase complex CF0-CF1 closely resembles F0-F1 of mitochondria, amino acid sequences conserved
–> CF0 conducts protons across thylakoid membrane
–> CF1 catalyzes formation of ATP
What is cyclic photophosphorylation
- electrons from P700 of PSI can be transferred back to cytochrome bf complex rather than NADP+
- reverse flow to reduce plastocyanin which can be re-oxidized by P700+ to complete a cycle
- in cyclic photophosphorylation, ATP is
generated without formation of NADPH - PSII not involved, no O2 is released
- takes place at high NADPH:NADP+ ratios
- contributes to versatility of photosynthesis
