BIOL 204 Lecture Notes - Lecture 41: Ferredoxin, Chloroplast, Photon
26 May 2018
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BIOL204 – Chemiosmotic Synthesis
Linear Electron Transport
- Noncyclic electron pathway
o 1) Oxidation of P680 – absorption of light by photosystem II forms
P680* and is oxidized by pheophytin
o 2) Oxidation-reduction of the plastoquinone pool – electrons transfer
from primary acceptor to PQ, migrates through lipid bilayer and acts
as link between PSII and cytochrome complex, PQ also releases
protons
o 3) Electron transfer from the cytochrome complex and shuttling by
plastocyanin – from cytochrome complex electrons pass to
plastocyanin which shuttles electrons from cytochrome to PSI
o 4) Oxidation-reduction of P700 – absorption of light by PSI results in
formation of P700*, oxidized to P700+ then reduced back to P700
o 5) Electron transfer to NADP+ by ferredoxin – electron from P700 is
transferred to ferredoxin (iron-sulphur protein) which transfers
electrons to NADP+ which is the final acceptor of noncyclic pathway
▪ NADP+ reduced to NADPH by enzyme NADP+ reductase
Chemiosmotic Synthesis of ATP
- Proton gradient across the thylakoid membrane (photosynthetic electron
transport) has 3 processes:
o 1) protons translocated into lumen by cyclic reduction and oxidation
of plastoquinone
o 2) gradient enhanced by addition of 2 protons for each water oxidized
o 3) removal of 1 proton for each NADPH synthesized enhances
gradient
- Proton motive force used to synthesize ATP by chemiosmosis using
chloroplast ATP synthase
- Photosystem II is most important development in evolution because it allows
the organism to harvest energy from Sun and extract electrons from water,
producing O2
o O2 accumulated in atmosphere and formed aerobic respiration (18X
more energy extracted)
o Photosystem II known as the engine of life
o Splitting water is most energetically demanding reaction carried out
by P680 found in PSII and bound to D1
o PSII absorbs light, P680 converted into P680+ (strongest known bio
oxidant) which breaks H2O
o PSII constantly being damaged at D1 found in core which takes
20minutes then there is a D1 repair system
Light Actually Used For
- mitochondrial respiration flow is downhill from NADH to O2
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