Photosynthesis
“Life is Bottled Sunshine”
Wynwood Reade, Martyrdom of Man, 1872
Eva Farre, s150
Topics
1.Overview of photosynthesis
2.Key early photosynthesis experiments
3. Photosynthesis: light reactions
Where does the oxygen in the atmosphere come from?
Was the oxygen concentration constant?
What are the necessary components of photosynthesis?
What is the overall reaction of photosynthesis?
Which is the order of events?
ATMOSPHERE COMPOSITION
CONSTITUENT
Nitrogen
Oxygen
Argon
Carbon Dioxide
Others
CHEMICAL SYMBOL
N2
O2
Ar
CO2
MOLE PERCENT
78.084
20.947
0.934
0.035
http://www.globalchange.umich.edu/
H20 + CO2
CH20 + O2
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
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
(Des Marais, 2000. Science)
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
Organism
Chlorophyll
a b c d
Plants
+ +
Green algae
+ + Diatoms
+ Dinoflagelates
+ Brown and yellow algae + Red algae
+ Cyanobacteria
+ -
+
+
+
-
Carotinoids Phycobilins
-
+
+
+
-
-
+
+
+
+
+
+
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)
Red light enhancement effect (~1950s) :
2 photochemical complexes (~1960)
Z- Scheme of photosynthesis
Topics
1.Overview of photosynthesis
2.Key early photosynthesis experiments
3. Photosynthesis: light reactions
Next lecture:
-Electron Transport Details (check respiration)
-Regulation of the Photosynthetic Machinery
-Genetics and Evolution of Photosynthetic Systems