5.21.08 Photosynthesis: Light reactions
Reading Assignment: Chapter 14
Nice tutorial on photosynthesis http://bioweb.wku.edu/courses/Biol120/images/Photosynthesis.asp
Another decent site on photosynthesis http://www.biology.arizona.edu/biochemistry/problem_sets/photosynthesis_1/photosynthesis_1.html
animation of electron transport in thylakoid membrane http://instruct1.cit.cornell.edu/Courses/biomi290/MOVIES/OXYGENIC.HTML
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• The vast majority of the energy consumed by living organisms stems from solar energy captured by the process of photosynthesis
• Only chemoautotrophs (aka chemolithotrophs) are independent of this energy source
Of the sunlight that reaches the earth each day:
• 1% is absorbed by photosynthetic organisms and transduced into chemical energy
• of the remaining 99%: 2/3 is absorbed by the earth and oceans
(heating the planet) and 1/3 is lost as light reflected back into space
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Often referred to as carbon dioxide fixation: light energy
6 CO
2
+ 6 H
2
O -------------> C
6
H
12
O
6
+ 6 O
2
• The fixation of carbon is an endergonic reaction
• Estimates indicate that 10 11 tons of carbon dioxide are fixed globally per year
• 1/3 is fixed in the oceans, primarily by photosynthetic marine microorganisms
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• cyanobacteria
• other photosynthetic bacteria
• algae
• plants
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Two stages of photosynthesis in a green plant or cyanobacteria
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NADPH = NADH plus an extra phosphate
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Respiration as a series of energy transformations:
• electrons released from carbohydrates(and other foodstuffs) are transferred by a circuitous route to O
2
• O
2
is reduced to H
2
O
• the free energy released as electrons flow from a high energy to a low energy state is transformed into a proton gradient
• the potential energy in this proton gradient is then converted into chemical energy in the form of ATP
Aspects of photosynthesis can be described in similar terms:
• Photosynthesis also involves a series of oxidation-reduction events
• Flow of electrons is from water to CO
2
, reducing it to a carbohydrate
• light provides the energy required to move electrons from a lower to a higher free energy state
• energy interconversions carried out by chemiosmotic mechanisms
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Whether photosynthesis occurs in a cyanobacteria or a giant sequoia, some generalitites apply:
• photosynthesis is associated with membranes
• eukaryotic cells, photosynthesis occurs in the chloroplast
• in prokaryotes, photosynthesis is associated with the plasma membrane
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Thylakoids
• flattened membrane-bound sacs inside the chloroplast
• location of chlorophyll
Chloroplasts possess three membrane bound aqueous compartments
• the intermembrane space
• the stroma -- inside the inner membrane and outside the thylakoid sacs (like the mt matrix)
• thylakoid space -- inside the thylkoid sacs

Comparison of membrane components of the chloroplast and mt
Similar:
• electron transport chains: entire set of proteins and small molecules involved in the orderly sequence of electron transfers
• ATP synthase
• inner membrane very impermeable and narrow intermembrane space
Different
• photosystems: site where light energy is captured and harnessed to drive the transfer of electrons
• in chloroplasts the electron transport chains and ATP synthase are located in the thylakoid membrane, not the inner membrane
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The many reactions that occur during photosynthesis can be grouped into two broad categories:
Photosynthetic electron-transfer reactions
• AKA light reactions
• energy derived from sunlight “energizes” an electron in chlorophyll
• chlorophyll obtains its electrons from water (generating
O
2
)
• the energized electrons move along an electron transport chain in the thylakoid membrane (analogous to mt electron transport)
• an H + gradient is generated and drives the production of
ATP
• NADP + is reduced to NADPH
Carbon Fixation Reactions
• also called Calvin cycle or dark reactions
• the ATP and NADPH produced by the light reactions serve as the source of energy and reducing power to drive the conversion of CO
2
to carbohydrate
• reactions take begin in the stroma of the chloroplast and end in the cytosol
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Nice tutorial on photosynthesis http://bioweb.wku.edu/courses/Biol120/images/Photosynthesis.asp
ATP synthase: see link on 205 home page
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Change in the redox potential during photosynthesis. The redox potential for each molecule is indicated by its position along the vertical axis. Transferring electron “up the scale” from H
2
O requires energy
• The net electron flow through the two photosystems is from water to NADP + to form NADPH.
• In the process of electron transfer a H + gradient is generated across the thylakoid membrane. ATP is synthesized by membrane bound ATP synthase -- same process as in the mitochondria
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http://www.sumanasinc.com/webcontent/animations/content/harvestinglight.html
Link to light harvesting animation
Two different photosystems:
Photosystem II:
• chlorophyll molecules of the reaction center are a form of chlorophyll a, known as P680 (for its optimal absorption)
• includes a “water splitting enzyme” which removes electrons one at a time from H
2
O
The evolution of this water splitting process allows oxygenic photosynthesis and is responsible for the high level of O
2
in our atomosphere
Photosystem I:
• chlorophyll molecules of the reaction center are a form of chlorophyll a, known as P700
• the P stands for pigment and the 700 designates the optimal absorption maximum in nanometers (wavelength = 700 nm)
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Figure 10.6 Interactions of light with matter in a chloroplast. The pigments of chloroplasts absorb blue and red light, the colors most effective in photosynthesis. The pigments reflect or transmit green light, which is why leaves appear green.
Maximal absorption by chlorophylls between 400-500 and 600-700 nm antenna complex on left
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Photosystems consist of two closely linked components:
Photochemical reaction center:
• complex of proteins and chlorophyll (transmembrane protein-pigment complex)
• converts light energy to chemical energy
Figure 14-34
The antenna complex is a collector of light energy in the form of excited electrons. The energy of the excited electrons is funneled, through a series of resonance energy transfers, to a special pair of chlorophyll molecules in the photochemical reaction center. The reaction center then produces a high-energy electron that can be passed rapidly to the electrontransport chain in the thylakoid membrane, via a quinone.
Antenna complex:
• pigment molecules that capture light energy and feed it to the reaction center
• cluster of several hundred chlorophyll molecules and accessory pigments (collect light of other wavelengths)
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• When a chlorophyll molecule in the antenna complex is excited, energy is rapidly transferred from one molecule to the next until it reaches chlorophyll molecules in the reaction center
• These chlorophyll molecules transfer the excited electrons to the electron transport chain
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