Download Week 8 Lecture 3 (Photosynthesis: light reactions)

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 CO2 + 6 H2O ------------->
C6H12O6 + 6 O2
• The fixation of carbon is an endergonic reaction
• Estimates indicate that 1011 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 O2
• O2 is reduced to H2O
• 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 CO2, 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
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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
O2)
• 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 CO2 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 H2O 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 H2O
The evolution of this water splitting process allows oxygenic photosynthesis
and is responsible for the high level of O2 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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see figure 14-35 in text
Reaction center:
• acts as a trap for quanta of energy captured by the
pigment molecules in the antennae
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