Introduction to Photosynthesis
Photosynthetic process converts light energy into chemical energy.
Plants, algae and some types of bacteria use photosynthesis to make
organic compounds.
The photosynthetic process is carried out by a set of complex protein
molecules that are located in and around a membrane. Energy drives a
series of reactions transforming light energy into chemical energy.
Oxygenic photosynthesis produces O2, but not anoxygenic
photosynthesis.
Photosynthetic organisms remove 1e15 grams of C y-1 (or 4e18 kJ)
(Houghton and Woodwell, 1990).
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Ref: THE PHOTOSYNTHETIC PROCESS
In: "Concepts in Photobiology: Photosynthesis and Photomorphogenesis",
Edited by GS Singhal, G Renger, SK Sopory, K-D Irrgang and Govindjee,
Narosa Publishers/New Delhi; and Kluwer Academic/Dordrecht, pp. 11-51.
John Whitmarsh
Photosynthesis Research Unit, Agricultural Research Service/USDA
Department of Plant Biology and Center of Biophysics and
Computational Biology, Uiversity of Illinois at Urbana-Champaign
Govindjee
Department of Plant Biology and Center of Biophysics and
Computational Biology, University of Illinois at Urbana-Champaign
Site: life.uiuc.edu/govindjee/paper/gov.html
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Photosynthetic studies
1640: Jan van Helmont - a Belgian philosopher, chemist and physician –
grew a 5 lb willow tree in 200 lbs of soil, and watered regularly with rain
water. Within 5 years the tree weighed 169 lbs and the soil 199 lbs.
Plants got nutrients from water and somewhere.
1772: Joseph Priestly, a british scientist showed plant shoots produce
oxygen. Plants restore oxygen. A few years later, Jan Ingenhousz
demonstrated that light is required to produce oxygen. Jean Senebier, a
Swiss botanist and naturalist, discovered that CO2 is required for
photosynthetic growth and Nicolas- Théodore de Saussure, a Swiss
chemist and plant physiologist, showed that water is required.
1800s: F.F. Blackman showed a two-step process for photosynthesis.
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Photosynthetic studies – cont.
1845 that Julius Robert von Mayer, a German physician and physicist,
proposed that photosynthetic organisms convert light energy into
chemical energy.
Middle of 19th century: A chemical reaction was proposed,
CO2 + 2H2O + Light Energy  [CH2O] + O2 + H2O
Where does O2 comes from, CO2 or H2O?
Hill and Scarisbrick (1940) demonstrated oxygen evolution in the
absence of CO2 in illuminated chloroplasts and by Ruben et al. (1941)
who used 18O enriched water
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Photosynthetic studies – cont.
Carbon reduction can occur in the dark and involves a series of
biochemical reactions that were elucidated by Melvin Calvin, Andrew
Benson and James Bassham in the late 1940s and 1950s. Using the
radioisotope 14C, most of the intermediate steps that result in the
production of carbohydrate were identified. Calvin was awarded the
Nobel Prize for Chemistry in 1961 for this work.
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Plant pigments and light absorption
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CHO in Chlorophyll b
Chlorophyll
Chlorophyll, a pigment, absorbs sun light.
Antenna contains chlorophyll molecules to
collect light.
Antenna absorb quanta and transfer the
reaction center.
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Energy in light reactions
Excite electrons of antenna
chlorophyll anchored to proteins
Electron transfer chemical
reactions in reaction center
Proton and electron transfer
(redox) reactions (Photosystems I
and II)
Produce high-energy molecules
NADP+ + e  NADPH
ADP –electrochemical rxn ATP
Reduction of CO2
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Photosystem II
PSII is composed of
polypeptides P680 and redox
components (chlorophyll,
pheophytin, plastoquinone,
tyrosine, Mn, Fe, cytochrome
b559, carotenoid and histidine)
light-induced electron transfer
drive the oxidation of water
and the reduction of
plastoquinone PQ
Cytochrome b559 must be
present, but its involvement in
reaction is unknown.
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Photo-energy in PSII
The energy aspect of the
various intermediate
species in photosynthesis
and the products in PSII
O2 from H2O
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The Cytochrome bf Complex
The cytochrome bf complex removes the electrons from reduced plastoquinone (PQ or
PQH2) and facilitates the release of the protons into the inner aqueous space.
The electrons are eventually
transferred to the PS I
reaction center.
Electron transfer from the cytochrome bf
complex to photosystem I is mediated by a
small Cu-protein, plastocyanin (PC).
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Photosystem I
PS I is composed of a
heterodimer of proteins that act
as ligands for most of the electron
carriers
Antenna system (200 mainly
chlorophyll a P700 molecules)
serve PS I.
PS I catalyzes the oxidation of
plastocyanin, a small soluble Cuprotein, and the reduction of
ferredoxin, a small FeS protein
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Energy aspects in PS I
PS I reduces, Fd, ferredoxin,
a small FeS protein
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The ATP synthesis
ATP Synthase complex is
composed CF0 and CF1
CF1
CF0 is a channel for H+
CF1 has several protein subunits
for the reaction
ADP + Pi + H+  ATP+ H2O
CF0
ATP is the energy molecule of
life.
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Photosynthesis
The protons released into the inner aqueous solution results in a
difference in pH and electrochemical potential across the
membrane.
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h
Light and Dark Reactions
H2O
CO2
NADPH
ATP
O2
Sugar
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The Calvin Cycle for Dark Reactions
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Final exam date
Chem218 Dec. 15, 19-22 pm, MC 4040
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