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Quantum Yield (QY)
Amount of photochemical produced per photon.
phi = (O2 made)/(photon absorbed)
Quantum Requirement (QR)
Reciprocal of QY
Quantum Requirement (theoretical and actual)
- MIN = (8 photons)/(O2 produced)
- MAX = (0.125 O2)/(photon produced)
Actual = 8-10 photons per O2, so some energy not used efficiently.
Test to fine QY and QR
Cyclic electron flow
Used to change ATP/NADPH ratio and occurs when no NADP for PS1 to reduce
cyclic electron flow pathways
- 1. PS1 stimulated
- 2. Ferredoxin reduced and oxidized
- 3. Cyt b6f reduced and oxidized
- 4. Plastocyanin reduced.
- 1. NADPH oxidized by NAD dehydrogenase
- 2. Plastoquinone reduced and oxidized.
- 3. Cyt b6f reduced.
Asymmetric Distribution of complexes
PSII and Cyt bf enriched in grana thylakoid (stacks)
PSI and ATP synthase enriched in stromal thylakoid
Unequal excitation of RCI and RCII
1. Short term response
2. Long term response
1) State transition - State 1 LHCII associated with RCII. State 2 PSII excitation > PSI so LHCII migrates to PSI.
2) Change in amount of antennas and RCs
How does LHCII migrate to RCI
Phosphorylation by a kinase that attaches to LHCII compound. Negative charge on P is relevant. Phosphatase removes P and pushes LHCII back.
Excess photon flux density causes too much light absorption that biochemical pathways that utilize NADPH and ATP are not quick enough use the excess made by the light reactions.
Light reactions don't have enough electron acceptors and # of functional PS reduced.
When does excess light occur?
- 1) Cold temperature
- 2) Mid-day sun
- 3) Shaded plants transferred to sun
- 4) Conditions that reduce the utilization of NADPH, ATP, Fd (i.e. pathogens)
3 types of ways plants deal with excess light
- 1) Avoidance
- 2) Dissipate
- 3) Repair
Ways plants avoid excess light
- 1) Leaf loss (expensive)
- 2) Greater reflective surface (fuzzy hairs, wax)
- 3) Change angle of leaf
- 4) Chloroplast movement
- 5) Reduce chlorophyll/antenna size
- 6) UV/light absorbing pigments
Ways plants dissipate excess light
After absorption, eliminates excess light energy to prevent production of toxic compounds
Toxic compounds from excess light
Reactive Oxygen Species
- 1. Singlet Oxygen
- 2. Superoxide (HO2)
- 3. Hydrogen peroxide (H2O2)
- 4. Hydroxyl Radical (OH)
P680+ oxidant (@ PSII)
Acceptor Side Damage
RCII - PSII no acceptor of electron from P680 or Pheo so they are transfer they electron back to form Chlorophyll a* --> Chla(t (triplet)).
This reacts with O2 to form singlet O2 which reacts with amino acid side chains adjacent to P680 and reacts with P680 itself.
Donor Side Damage
If electron not delivered fast enough to P680+.
P680+ is a strong oxidant and oxidizes RCII components.
Damage at LHCII
Long-lived Chl* becomes a triplet and causes damage to itself or produces singlet O2.
RCI Damage (Mehler reaction)
Electron flow continues but to different acceptors.
FeSx- + O2 --> FeSx + -O2 (superoxide)
Superoxide adds oxygen to unsaturated lipids and oxidizes met, trp, his, cyz, and metals.
Converts superoxide to hydrogen peroxide (not as bad).
However hydrogen peroxide can lead to hydroxyl radical which oxidizes everything.
Majority comes from RCII (RCI has water to water reaction)
Measures RCII damage (measures chlorophyll triplet form)
Quenching by using biochemical oxidation-reduction reactions
- i. Cyclic electron flow
- ii. RCII cyclic flow
- iii. water to water cycle at RCI
Process that compete with fluorescence but do not lead to electron flow.
- 1. Carotene
- 2. Xanthophyll cycle
These molecules accept energy and lose it through heat and molecular motion.
Cyclic flow in RCII
Mechanism to reduce P680+ to P680
Electron transferred from Ph- --> P680+ --> P680 (prevents long lived oxidant)
Accepts excitation energy from singlet O2 or Chl(t)
Zeaxanthin accepts energy from singlet excited chl* and dissipates it.
Bad at energy transfer so only synthesize when needed (high light)
High pH when electron flow maximal which activates enzymes that make Zeaxanthin
Water to water cycle
Prevents block to electron flow and deals with toxic compounds produced at RCI/PSI.
H2O2 + 2 Ascorbate ---> 2 monodehydroascorbate (MDA) +2 H20
enzyme - Ascorbate peroxidase
MDA is ascorbate's -OH group ionized.
- Monodehydroascorbate reductase
- 1. 2 MDA + NADPH --> 2 Ascorbate + NADP+
- 2. MDA + Fdred --> ascorbate _ Fdox
RCII Damage and Repair
Resulting from P680+ oxidant. Damages D1 protein.
- 1. Remove old RCII and D1 (protease)
- 2. Synthesis of new D1 protein
- 3. Insertion in membrane and assembly of new RCII
- 4. Maturation of proteins
- 5. Assembly of OEC