1. “Life is Bottled Sunshine”
Photosynthesis
“Life is Bottled Sunshine”
Wynwood Reade, Martyrdom of Man, 1872
Taiz & Zeiger: Chapters 7 & 8
Mo: Light reactions
Wd: Light reactions/carbon reactions
Fri: Carbon reactions
Eva Farre, s150

2. Topics
Overview of photosynthesis
Key early photosynthesis experiments
Photosynthesis: light reactions

3. Clicker question: For the last 4 billion of years has been the oxygen concentration in the atmosphere constant?
A. Yes, more of less.
B. It fluctuated as did the Earth temperature.
C. It started rising ~3 billion years ago.
D. It has decreased.
E. It started rising since the origin of earth.

4. Oxygenic Photosynthesis
Clicker question:
A. CH2
B. ATP
C. O2
D. NADPH
E. H2O
F. CO2
Light Reactions
Carbon Reactions ?
? ?
? ?

5. Oxygenic Photosynthesis
Light Reactions
Carbon Reactions
ATP
NADPH
H O2
CO CH20

6. Clicker question:
What is the difference between the carbon in CO2 and the carbon in CH20?
A. In CO2 the carbon is more oxidized than in CH2O.
B. In CO2 the carbon is more reduced than in CH2O.

7. Photosynthesis is a redox reaction
Where does the oxygen come from ?
H20 + CO2
CH20 + O2
Van Niel (1920s) analyzed the photosynthesis of purple bacteria:
nCO nH2O + light  (CH2O)n + nO2 + nH2O
nCO nH2S + light  (CH2O)n + 2nS + nH2O
Hill equation (1937): H20 + A + light  ½ O2 + H2A
Van Niel (1920s) inferred correctly that CO2 was not the source of the evolved O2,
or alternatively said, the initial oxidation-reduction reactions did not involve CO2.
In other words, the conversion of light energy to biosynthesis using CO is not obligatory.
Hill equation: Robert Hill in 1937 detected that isolated chloroplasts give off oxygen in the presence
of unnatural reducing agents like iron oxalate, ferricyanide or benzoquinone after exposure to light.
The reaction went down in literature as the HILL-reaction.
(Ziong et al., 2000, Science)
“Purple bacteria are the earliest emerging photosynthetic lineage”
OXYGENIC / ANOXYGENIC photosynthesis
LIGHT/ DARK or CARBON-LINKED reactions

8. So, what happened before photosynthesis evolved?
William Martin*, John Baross‡, Deborah Kelley‡ and Michael J. Russell (Nat. Rev. Microbiology, 2008)
The chemistry of life is the chemistry of reduced organic compounds, and therefore all theories for the origin of life must offer testable hypotheses to account for the source of these compounds.
So, what happened before photosynthesis evolved?
From Martin et al., 2008: Sources of reduced carbon (remember, energy is needed)
1. Chemical reactions catalyzed by the minerals in hydrothermal (energy) vents might lead to the production of formate and acetate
2. pH gradients generated at the hydrothermal vents could be harnessed to generate chemiosmotic coupling.
Others.

9. H20 + CO2
CH20 + O2

10. Topics
Overview of photosynthesis
Key early photosynthesis experiments
Photosynthesis: light reactions

11. History: From air to starch
Stephen Hales ( ): Plants assimilated “air”
Joseph Priestley (1733–1804) : plants can “purify” air
Jan Ingen-Housz ( ): this was dependent on light
Jean Senebier (1742–1809) :”fixed air” (CO2) is essential for photosynthesis
Pierre Joseph Pelletier (1788– 1842) and Joseph Bienaimé Caventou (1795–1877): chlorophyll
Hugo von Mohl (1805–1872) discovered chloroplasts in plant cells
Julius Robert Mayer (1814–1878): plants convert light energy into chemical energy
Julius von Sachs (1832–1897 ): starch granules are product of photosynthesis
Stephen Hales ( ): He found a decrease in the volume of air when he grew plants in a closed environment
Charles Bonnet ( ) : illuminated leaves produced bubbles under water
Joseph Priestley (1733–1804) : plants can “purify” air
Jan Ingen-Housz ( ), he found that Priestley experiment was dependent on light (1779)
Jean Senebier (1742–1809) :”fixed air” (CO2) is essential for photosynthesis
Pierre Joseph Pelletier (1788– 1842) and Joseph Bienaimé Caventou (1795–1877)
named the green plant pigment chlorophyll
Hugo von Mohl (1805–1872) dis- covered chloroplasts in plant cells
Julius Robert Mayer (1814–1878) pro- posed ‘the law of conservation of energy,’ known
also as the First Law of Thermodynamics. He clearly stated that ‘plants convert light energy into chem-
ical energy’ during photosynthesis
Julius von Sachs (1832–1897 ): starch granules are product of photosynthesis

12. Topics
Overview of photosynthesis
Key early photosynthesis experiments
Photosynthesis: light reactions

13. Oxygenic Photosynthesis
Light Reactions
Carbon Reactions
ATP
NADPH
H O2
CO CH20

14. Chloroplast structure

15. Photosynthesis Light Reactions
(Simplified)
e-acceptor (NADP)
ADP + Pi e-
Reduced e-acceptor (NADPH)
ATP
Stroma
Thylakoid
Lumen
What does this scheme resemble to: respiration in mitochondria e- H+
e-Donor (H20) H+
Oxidized-e-Donor (O2)

16. Absorbance of light: Photochemistry
There are chlorphylls per reaction center in higher plants.
Some photosynthetic bacteria have chlorophylls
Some types of algae and bacteria can have up to a few thousands per reaction center
The molecular structures of antenna pigments are quite diverse.

17. Light absorption and emission of chlorophyll
E = h (c/λ)
H= Planck’s constant
C= light velocity
λ= wavelength
c/λ = frequency

19. Engelmann Experiments
(1843 – 1909)
In essence more sophisticated modern technologies to study photosynthesis rely on the same base as Engelmann Experiments: Particular parts of the photosynthesis system absorb light of a particular wave length. When their status changes (get reduced or oxidized) their absorbance spectrum changes, which allows to follow these reactions.
Spyrogyra up to ~200 um long

20. Photosynthetic Pigments Absorption Spectra
Bacteriochlorophyll a
Cholorphyll a
Chlorophyll b
Phycoerythrobilin (bilin pigment, are open chain tetrapyrroles found in antenna structures known as phycobilisomes that occur in cyanobacteria and red algae)
B-carotene

21. Photosynthetic Pigments : Structure
Chlorophyll and billin pigments originate from glutamate
Carotenoids are tetraterpene molecules derived from eight isoprene units (isoprenes derive from Acetyl-CoA)

22. The Electron Transfer Process
Energy
what happens to this "excited" electron?
The “activated” electron in the donor orbital has two choices, to transfer to another orbital of another molecule as shown here of “come down” to its original orbital.
The decision is made by the different kinetics of these two options: The faster reaction occurs.
The photosynthesis have evolved to optimize this reaction.

23. The Electron Transfer Process
Clicker question: What happens to this "excited" electron?
1. Excited chlorophyll re-emits a photon and returns to its ground state (fluorescence).
2. Excited chlorophyll returns to its ground state by directly converts its excitation energy to heat?
3. Excited chlorophyll transfers its energy to another pigment.
4. Energy is used to catalyze a chemical reaction.
A: 1, 3
B: 1, 4
C: all
C: 1, 2
D: 2, 3, 4
The “activated” electron in the donor orbital has two choices, to transfer to another orbital of another molecule as shown here of “come down” to its original orbital.
The decision is made by the different kinetics of these two options: The faster reaction occurs.
The photosynthesis have evolved to optimize this reaction.

24. Important photosynthesis concepts
Photochemical quantum yield/ efficiency:
Φ = Number of photochemical products
Total number of quanta absorbed (energy of a photon)
Energy efficiency:
= Notal number of quanta absorbed
Photosynthesis products (ATP/NADPH)
Quantum yield of photochemistry is very high, close to perfect 1 (little energy loss) in chloroplasts kept in dim light and the quantum yield of fluorescence is 0.05 or lower.
Energy efficiency is about ~30% (modern solar panels have an efficiency of ~30%)

25. The Electron Transfer Process
2 H+ + ½ O2
Energy
The “activated” electron in the donor orbital has two choices, to transfer to another orbital of another molecule as shown here of “come down” to its original orbital.
The decision is made by the different kinetics of these two options: The faster reaction occurs.
The photosynthesis have evolved to optimize this reaction.

26. Red light enhancement effect (~1950s) :
2 photochemical complexes (~1960)
The rate of photosynthesis when red and far-red light are given together is greater than the sum of the rates when they are given apart.

27. Z- Scheme of photosynthesis

28. Photosynthesis: Light Reactions
The electrons from photosystem II are transferred to Pheophytin.
Pheophytin is a chloprhyll in which the central magnesium atom has been replaced by two hydrogen atoms. This gives pheophytin chemical and spectral properties that are slightly different from those of chlorophyll.
Two plastoquinones PQA and PQB are bound to the reaction center and receive electrons from pheophytin in a sequential fashion. PQB2- takes then protons from the stroma to form fully reduced plastohydroquinone.The reduced plastohydroquinone (PQH2) dissociates from the reaction center complex and and entersthe hydrocarbon portion of the membrane, where it in turn transferes its electrons to the cytochrome b6f complex.

29. Z-Scheme: Electron transfers (redox reactions)

30. Photosynthesis: Light Reactions
The electrons from photosystem II are transferred to Pheophytin.
Pheophytin is a chloprhyll in which the central magnesium atom has been replaced by two hydrogen atoms. This gives pheophytin chemical and spectral properties that are slightly different from those of chlorophyll.
Two plastoquinones PQA and PQB are bound to the reaction center and receive electrons from pheophytin in a sequential fashion. PQB2- takes then protons from the stroma to form fully reduced plastohydroquinone.The reduced plastohydroquinone (PQH2) dissociates from the reaction center complex and and entersthe hydrocarbon portion of the membrane, where it in turn transferes its electrons to the cytochrome b6f complex.

31. 7.31 Summary of the experiment carried out by Jagendorf and coworkers: the chemiosmotic mechanism
pp4e-fig jpg

32. Topics
Overview of photosynthesis
Key early photosynthesis experiments
Photosynthesis: light reactions

33. What are the main open questions about the origin of oxygenic photosynthesis?
Which are the current hypothesis to explain the beginning of oxygenic photosynthesis?

34. Next lecture:
-Genetics and Evolution of Photosynthetic Systems
-Regulation of the Photosynthetic Machinery
-Carbon reactions: Calvin-Benson cycle

35. Monday’s Topics
Overview of photosynthesis
Key early photosynthesis experiments
Photosynthesis: light reactions