Photosynthesis 1 Flashcards
Photosynthetic organisms
Bacteria, plants, algae
Can be oxygenic or non-oxygenic depending on electron donor
steps of photosynthesis
1) Light harvesting
2) Photochemical charge separation
3) Electron transport
4) Oxidation of electron donor
5) Proton pumping
6) Reduction of electron acceptor
Principle Light Harvesting molecules (e.g. in plants)
Chlorophylls in plants, Bacteriochlorophylls in bacteria
Chlorophyll a is abundant in plants:
- A cyclic tetrapyrrole with a conjugated double bond system to facilitate absorption of light (most in visible spectrum)
- green wavelength (500-600nm) reflected
- Has central magnesium atom
Light Absorption through electron excitation
charge separation
Light energy excites electron from ground state to excited state
1) electron can fall back to ground state
- releases heat or light at different wavelength (fluorescence)
2) Charge separation from electron transfer
- excited electron moves to another nearby acceptor molecule and reduces it… charge separation!
Photosynthetic bacteria: Rhodopseudomonas viridis
Charge separation photochemistry occurs in reaction centres made of 4 polypeptides and a prosthetic group:
- Heme
- Special Pair (Bacteriochlorophyll)
- Bacteriopheophytic (chlorophyll w/out Mg)
- Quinone (electron/proton mediator)
Stages of converting light to chemical energy in bacteria
1) light photon arrives at special pair (P960) in reaction centre
2) Excited P960 undergoes charge separation, becoming oxidised as neighbouring Bacteriopheophytin (BPh) is reduced
3) Oxidised P960 = strong oxidising agent and is quickly reduced by heme in cytochrome subunit
4) Reduced BHp quickly donates electron to a quinone (bound to Qa)
5) Electron transfer from Qa to Qb which can diffuse in/out of photosystem complex where it picks up a proton (semi-quinone intermediate formed)
6) 2nd light photon excites P960 reaction centre and repeat stages
7) 2nd round of excitation fully reduces Qb into QH2 which can diffuse out of complex into quinone pool
Using the QH2 produced from charge separation in generating energy
QH2 is re-oxidised by complex 3 of respiratory chain as it reduces cytochrome via cytochome c2
Cyclic process of electron movement also produces a proton gradient anaerobically that can be used for ATP synthesis
- The 2H+ used to make QH2 in reaction centre from cytoplasm are released into periplasm in complex 3
Converting light to chemical energy in Chloroplast
Linear system with 2 photosystems each absorbing light, linked by a cytochrome bf complex (similar to complex 3)
PS2: Absorbs light @680nm, splits water (electron donor)
Cytochrome bf: translocates protons from stromal side of membrane to thylakoid lumen
PS1: Absorbs light @700nm, reduces NADP into NADPH
2H2O + 2NADP —> O2 + 2NADPH + 2H
PSII structure
Contains 2 core proteins (D1&D2) which contain the special pair.
It also contains an oxygen evolving complex for water splitting (contains a manganese centre)
Found within thylakoid membreane
PSII action
1) Photon of light excites P680 reaction centre causing it to undergo charge separation and reduce Pheophytin
2) Oxidised P680 re-reduced by oxygen evolving complex’s manganese (electron from water hydrolysis)
3) Reduced pheophytin donates electron to plastoquinone (Qa)
4) Qa donates electron to exchangeable plastoquinone (Qb)
Oxygen evolving complex
4 photons needed per 2 H2O
- complex contains 4x Manganese ions that are oxidised by P680 one step at a time
S0 –> S1 –> S2 –> S3 –> S4
Cytochrome b6f complex
homologous to bc1 complex in mitochondria, also employs a Q cycle:
1) Qb binds at the Qp site
2) 1 of 2 electrons goes to Rieske FeS centre, passing it onto Cyt f and then onto plastocyanin (small mobile electron carrier)
3) 2nd of 2 electrons moves via b-type cytochromes to second quinone bound at Qn site (only semi-reduced)
4) 2nd cycle fully reduces quinone which picks up 2 protons to become plastoquinone (PQH2) which can diffuse away
Proton pump: protons consumed on stromal side, released on lumenal side
PS1 structure
Core complex contains 2 core proteins, the special pair (P700), additional units for chlorophyll binding
On stromal side have Iron-Sulphide clusters which accept electrons passing
PS1 action
1) Photon excited P700 reaction centre causing charge separation to reduce nearby A0
2) Oxidised P700 re-reduced by Plastocyanin (comes from Cytochrome b6f)
3) A0 donates electron to quinone (A1)
4) Further redox movement of electrons iron-sulphide clusters (4Fe-4S) before finally reducing Ferredoxin (a small 2Fe-2S containing protein - redox active centre)
5) Ferredoxin donates electrons to Ferredoxin-NADP+ Reductase which converts NADP to NADPH
NADPH produced can be used for Calvin Cycle and other biosynthetic reactions
