Light reaction of photosynthesis

1. Ele-ctron Transport

2. Transfer of excitation is accompanied by loss of energy as heat

3. Concept of energy transfer during photosynthesis

4. The Z-scheme, showing Em values of electron carriers
Strong oxidant
Weak oxidant
Weak reductant
Strong reductant

5. 2H20 O2 + 4H+ + 4e
_
Light-dependent oxidative splitting of water (photolysis) in PSII

6. Joseph Priestly discovered that plants produce oxygen, and Lavoisier, his
contemporary in France were among the first to study oxygen consumption by
living organisms, a process known today as respiration.
MILESTONES IN RESEARCH ON PHOTOSYNTHETIC O2 EVOLUTION
Priestley (1770's) Discovers that plants produce O2
Lavoisier (1780's) Carries out first experiment on O2 consumption (respiration)*
Kamen/Ruben (1930's) Use isotopes for first time in photosynthesis
experiments
Hill (1930's-60's) Reconstitutes O2 evolution in isolated chloroplasts; proposes
existence oftwo light reactions (photosystem II, photosystem I)
Kok/Joliot (1960's) show that O2 evolution oscillates, propose a model for the
reaction
*Later becomes an administrator; guillotined in 1793
Oxygen evolution

7. Martin Kamen and Sam Ruben carried out the first biological
isotope experiments as a way to study photosynthesis. They
identified the source of oxygen, and confirmed the fate of
carbon dioxide. Today, isotopes are widely used in biological
research, but these were the very first experiments to use this
technique .
+ H2o+ (CH2O)

8. 2H2O O2 + 4H+ + 4e
_
Light-dependent oxidative splitting of water (photolysis) in PSII

9. Kinetics of the PSII electron carriers
Photooxidation of P680 results to formation of P680+ and Pheo-:
That is charge separation

10. Nicotinamide adenine
dinucleotide phosphate

11. Kinetics of the PSI electron carriers

12. The zigzag scheme (Z scheme)

13. 2 4H+
2H20 O2 + 4H+ + 4e
_
Structural model of PSII system

14. Oxygen evolving complex (OEC)
Four Mn atoms are ligated to amino-acid residues of PSII protein D1, as well as
to O2, Ca and Cl.

15. Oxygen evolution
Each excitation of P680 is followed by withdrawal of one electron from
manganese cluster, that stores the residual positive charge. When 4+charges
are accumulated, the complex oxidizes 2 H2O molecules and releases O2.
Photo-oxidation of water, called the S state mechanism

16. Cyclic electron transport in chloroplasts

17. Cyclic electron flow generates ATP and not NADPH

19. Quantum Yield Of Oxygen evolution
Φ=
Number of products formed photochemically
Number of quanta absorbed
Quantum yield determines the efficiency of photochemistry.
Hence under optimal condition Φ= 1.
Quantum yield for oxygen evolution or photosynthetic efficiency of O2
is= 1/8
That is 0.125 molecules of O2 evolved/photon absorbed**.
** Quantum yield will vary depending on the environmental conditions to
which plants are exposed.

20. Electrons and protons are transferred Vectorially

21. Herbicides block electron transport in plants