L6 - Excited Leaves and Photochemistry

Question 1

Give the 4 main stages of light harvesting

Answer 1

1) LHCs collect and pass light to RCs
2) RCs containing special pair of chlorophylls start photochemistry
3) Electrons transferred from series of donors to acceptors to PSI RC
4) Proton gradient formed across thylakoids, enabling ATP synthesis

Question 2

Name the pigments used in LHCs and sketch their rough absorption spectra in ether.

Answer 2

• Chlorophyll a and b
• Carotenoids e.g alpha-carotene and xanthophyll. used where Chl absorption down. Yellow/orange.
• See diagrams on L6 pg 11 (triple banded in carotenoids)

Question 3

Roughly sketch the leaf absorption spectrum. Explain how it differs to the combined chlorophyll - carotenoid spectra in ether. Then sketch the quantum yield (define this).

Answer 3

• More absorption in leaf in 500 - 600nm range where absorption very low in ether.
• Due to lipid environment, proteins binding pigments and path lengthening due to internal reflection.
• Quantum yield is mol O2/photons absorbed.
• Dip at 480nm due to inefficient energy transfer due to carotenoids being primary absorbers (not Chls)

Question 4

Outline the general structure of chlorophyll and the function of each part.

Give the differences between Chl a and Chl b and the effect.

Give the difference between bacteriochlorophyll and chlorophyll and the effect.

Answer 4

Chlorophyll:
1) Porphyrin Head - Tetrapyrrole ring plus Mg. Delocalised e- in ring used fo absorption of light.
2) Phytol chain - Anchors Chl among lipid membrane for correct positioning.

• Chl b has formyl group, chl a has methyl group. Formyl group increases absorption in blue.
• Bacteriochlorophyll has acetyl group and fewer double bonds. Absorbs at longer wavelengths.

Question 5

Outline structure of carotenoids and function.

Answer 5

• Alternating double and single bonds delocalise electrons, used in absorption.

Question 6

Outline phycobilins pigment structure, absorption range and where they’re found.

Answer 6

• Linear tetrapyrrole ring gives delocalised e- for absorption.
• Absorb 500 - 600nm.
• Found in some cyanobacteria.

Question 7

Give the 4 steps in light capture and transfer by LHCs.

Answer 7

1) Absorption of photon by pigment raises e- to excited state S1 or S2.
2) S1 more excited and unstable, e- returns to S2 emitting energy as heat.
3) S2 more stable, can be transferred to proximal Chl through resonance energy transfer (RET) or return to ground via heat emission or florescence.
4) RET continues until ‘special pair’ in RC.

Question 8

Where are the genes for PSII encoded?

Outline structure of LHCs around PSII.

Answer 8

• Both the nucleus and chloroplast.
• Major distal LHCs, Lhcb 1-3 genes (large, far away)
• Minor distal LHCs, Lhcb 4-6 genes (small, far away)
• Inner antenna complex, CP47 and CP43 chl binding proteins.

Question 9

Detail the structure of the different Lhcb units around PSII

Answer 9

Lhcb 1,2,3:

• Multiple Lhcs 1-3s
• Either strongly (S), moderately (M) or loosely (L) bound
• Trimeric
• Each monomer binds 14 Chl
• Bind 60% of Chl

Lhcb 4,5,6:

-Monomeric
-Bind 8-10 Chl each

CP43 + CP47
- Bind 50 Chl each

Question 10

Detail the structure of the RC in PSII and draw sketch.

Answer 10

• Dimeric
• D1 and D2 proteins hold P680 (‘special pair’) and cytochrome b-599 (used in e- chain)
• P680 (and P700 in PSI) photochemically active pigment

Question 11

Outline how an electron is released and then transferred in the RC in 5 steps.

Answer 11

1) Light induced charge separation converts P680 to P680*

2) e- then trapped by pheophytin on D1-D2 heterodimer (P680* to P680+)

3) e- passed to strongly bound plastoquinone at Qa site.

4) P680+ oxidises tyrosine 161 (Zy)

5) Zy reduced by Manganese Cluster. Repeat

Question 12

Detail the structure of the Oxygen Evolving Complex (OEC)

Answer 12

• Made of 4 Mn and 1 Ca atom in cubane shape
• Mn cluster surrounded by polypeptides (holds it in place) e.g. 33kDa

Question 13

Outline the cyclic model for oxygen evolution and the experiment used. Sketch the cycle.

Answer 13

• Algae used with precise flashes of light.
• Usually 4 oxidations (flashes) of S needed for O2 evolution (S0-S4)
• 3rd flash gives max. O2
• Initially after dark rest S1 state assumed so only 3 oxidations needed

Question 14

Give the next steps in the electron chain after Qa has accepted the electron. Draw diagram.

Answer 14

• Qa accepts on e- at a time.
• Qb (a mobile plastoquinone) accepts 2e- from Qa and 2H+ from stroma, forming a plastoquinol.
• Plastoquinol diffuses in lipid bilayer (membrane) to cytochrome bf complex (cytb6f)

Two routes for e- transport at cytb6f:
- via Fe2S2 of Rieske protein and onto soluble blue, copper protein, plastocyanin (carrier to PSI).
- via cytb563 protein where plastoquinone binds on stromal side, picking up 2 H+ ions and e-, forming plastoquinol again (Q cycle)
- Extra H+ ions moved to lumen in Q cycle.