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
Topics
1.Overview of photosynthesis
2.Key early photosynthesis experiments
3. Photosynthesis: light reactions
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.
Clicker question:
A. CH2
B. ATP
C. O2
D. NADPH
E. H2O
F. CO2
Oxygenic Photosynthesis
?
Light Reactions
?
?
Carbon
Reactions
?
?
?
Oxygenic Photosynthesis
ATP
Light Reactions
H2 0
O2
Carbon
Reactions
NADPH
CO2
CH20
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.
Photosynthesis is a redox reaction
Where does the oxygen come from ?
H20 + CO2
CH20 + O2
Van Niel (1920s) analyzed the photosynthesis of purple bacteria:
nCO2 + 2 nH2O + light  (CH2O)n + nO2 + nH2O
nCO2 + 2nH2S + light  (CH2O)n + 2nS + nH2O
Hill equation (1937):
H20 + A + light  ½ O2 + H2A
“Purple bacteria are the earliest emerging photosynthetic lineage”
(Ziong et al., 2000, Science)
OXYGENIC / ANOXYGENIC photosynthesis
LIGHT/ DARK or CARBON-LINKED reactions
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?
H20 + CO2
CH20 + O2
Topics
1.Overview of photosynthesis
2.Key early photosynthesis experiments
3. Photosynthesis: light reactions
History: From air to starch
Stephen Hales (1677-1761): Plants assimilated “air”
Joseph Priestley (1733–1804) : plants can “purify” air
Jan Ingen-Housz (1730-1799): 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
Topics
1.Overview of photosynthesis
2.Key early photosynthesis experiments
3. Photosynthesis: light reactions
Oxygenic Photosynthesis
ATP
Light Reactions
H2 0
O2
Carbon
Reactions
NADPH
CO2
CH20
Chloroplast structure
Photosynthesis Light Reactions
(Simplified)
e-acceptor (NADP)
ADP + Pi
e-
Reduced e-acceptor (NADPH)
ATP
Stroma
e-
H+
e-Donor (H20)
H+
Oxidized-e-Donor (O2)
Thylakoid
Lumen
Absorbance of light: Photochemistry
Light absorption and emission of chlorophyll
E = h (c/λ)
Engelmann Experiments
(1843 – 1909)
Spyrogyra up to ~200 um long
Photosynthetic Pigments Absorption Spectra
Photosynthetic Pigments : Structure
Energy
The Electron Transfer Process
what happens to this
"excited" electron?
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
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)
Energy
The Electron Transfer Process
H20
2 H+ + ½ O2
Red light enhancement effect (~1950s) :
2 photochemical complexes (~1960)
Z- Scheme of photosynthesis
Photosynthesis: Light Reactions
Z-Scheme: Electron transfers (redox reactions)
Photosynthesis: Light Reactions
7.31 Summary of the experiment carried out by Jagendorf
and coworkers: the chemiosmotic mechanism
Topics
1.Overview of photosynthesis
2.Key early photosynthesis experiments
3. Photosynthesis: light reactions
What are the main open questions
about the origin of oxygenic
photosynthesis?
Which are the current hypothesis
to explain the beginning of
oxygenic photosynthesis?
Next lecture:
-Genetics and Evolution of Photosynthetic Systems
-Regulation of the Photosynthetic Machinery
-Carbon reactions: Calvin-Benson cycle
Monday’s Topics
1.Overview of photosynthesis
2.Key early photosynthesis experiments
3. Photosynthesis: light reactions