Photosynthesis 2

Question 1

Cyclic electron transport in Chloroplasts

Answer 1

Used to increase ATP synthesis

• Occurs when reducing power is high (lots of NADPH) and ATP is limiting
• Involves diverting electrons from PSI back through Cyt b6f complex

Question 2

Light harvesting

Answer 2

Photosynthetic organisms may have light harvesting/antenna pigments to:

• Take advantage of different light wavelengths in different environments
• Provide protection against oxidative damage (could normally happen from partial water oxidation creating ROSs)

Question 3

Light harvesting: Carotenoids

Answer 3

Absorb light @ 400-500nm and transfer energy to chlorophyll

• Isoprenoid compounds derived from geranyl geranyl PP
• Found in LCH-II light harvesting protein associated with PSII (each polypeptide binds 12 chlorophyll a, b and several carotenoid molecules (chlorophyll b only found in antenna complex). 3x proteins form trimer

Question 4

Carotenoid protective function

Answer 4

Under high light intensity protect from oxidative damage:

• Converts Vio to Zea to dissipate excess light energy
• Non-Photochemical Quenching (NPQ)

Question 5

Passing light energy from Antenna complex to Reaction Centre

Answer 5

Energy transferred by Resonance Energy Transfer (no electron transfer, just electromagnetic interactions through space)

• doesn’t require absorption/re-emission of photons
• If donor and acceptor in close proximity, transfer takes picoseconds at 99% efficiency

Question 6

Heterogenous Thylakoid membrane

Answer 6

Stacked granum and stroma-exposed parts

• PS1 and ATPase primarily in unstacked region
• PSII primarily in stacked region
• b6f complex in both stacked and unstacked

Question 7

Association of LHCII with PSII controlled by redox state

Answer 7

Both photosystems SHOULD receive the same amount of sunlight but doesn’t always happen:

If PSII gets more light than PSI, Plastoquinone (PQ) is predominantly reduced: PQH2 > PQ

• Ratio imbalance activates LHCII kinase to phosphorylate LHCII promoting membrane unstacking and migration away from PSII (receives less light)
• PQH2/PQ ratio falls promoting activation of phosphatase to reverse process

Question 8

Phycobilins as accessory pigments in some algae and cyanobacteria

Answer 8

Different pigments to that of plants:
- Phycobilins = linear tetrapryolle molecules linked to proteins, absorb mainly yellow-green light (little red/blue penetrate a meter of water)

Question 9

Phycobilisome

Answer 9

Equivalent to light harvesting complex found within Phycoerythrin

• lay on surface of thylakoid
• Light harvesting phycoerythrin transfers energy to phycocyanin which transfers energy to core (bound to thylakoid membrane, associates with PSII)