THE LIGHT REACTIONS
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2)
3)
Begin when photons strike the photosynthetic
membrane. The process can be divided into three
parts.
Photoexcitation: absorption of a photon by an
electron of chlorophyll
Electron transport: transfer of the excited
electron through a series of membrane-bound
electron carriers, resulting in the pumping of
proton through the photosynthetic membrane,
which creates a H+ reservoir and eventually
reduces an electron acceptor
Chemiosmosis: the movement of protons through
ATPase complexes to drive the phosphorylation
of ADP to ATP
We recall that in an atom
electrons want to occupy
the lowest energy level,
or its ground state.
 When it gains energy and
rises to a higher energy
level, excitation takes
place.
 When it returns to its
original level, heat and
light (photon) are
emitted.
 fluorescence
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Chlorophyll in isolation, when bombarded with white light,
emits flourescence: due to the when electrons fall from higher
energy to lower energy.
but when it is associated with the photosynthetic membrane,
the excited electron is immediately captured by the primary
electron acceptor.
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Excited electron is captured by the primary
electron acceptor.
Redox reaction: chlorophyll is _______________
and primary acceptor is __________________.
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Light is absorbed
by chlorophyll or
accessory pigment
molecules that are
associated with
proteins in clusters.
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Is composed of a number of
chlorophyll molecules and
accessory pigments set in a
protein matrix in the
thylakoid membrane.
The photon energy of the
ant. pigment molecules
transfer from pigment to
pigment (resonance) until it
reaches a chlorophyll a
molecule in an area called
the reaction centre.
The excited electron of the
chlorophyll a is captured by
the primary electron
acceptor.
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Both are embedded in the thylakoid
membranes.
Both contain the exact same chlorphyll a
molecules structurally, but PH. I has
chlorophyll P700 and PH. II has chlorophyll
P680. Why?
They differ in the wavelengths they best
absorb, 700 nm and 680 nm respectively.
It’s caused by the different proteins
associated with chlorophyll a in each
photosystem.
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The process in which photon-energized
electrons flow from water to NADP+ through
electron transport chains in thylakoid
membranes, producing NADPH by reduction
and ATP by chemiosmosis.
Photon strikes photosystem II
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Electron of chlorophyll P680 are excited
Electron captured by primary electron acceptor:
pheophytin
Series of redox reactions
Electron transferred to plastoquinone, PQ.
Z protein, associated with PSII
Splits water into oxygen, H+, and etwo of these e- is used to replace the missing
electrons in _________________________.
H+ remaining in the thylakoid lumen
Oxygen leaves the cell: ___________________________
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initial e-, now in _____, goes through an
electron transport chain similar to that in
________________________.
THIS PROCESS OCCURS TWICE: ____ e- pass
through the ETC.
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The e- that leave PSII pass through the Q
cycle
◦ This causes protons to be transported from the
stroma INTO the thylakoid lumen.
 4 H+ for each pair of electrons
◦ Difference from cellular respiration?
________________________________________________
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CREATES A H+ GRADIENT FOR CHEMIOSMOSIS.
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The 2e- move through plastocyanin, Pc and other
components of the ETC until they reach PSI.
◦ PSI also continually undergoes the same electron
excitation process (struck by photons) as PSII (therefore,
loses 2 electrons)
◦ The 2e- originating from PSII replace the displaced e- in
PSI.
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Electrons from PSI pass through another ETC
containing an iron-containing protein called
ferredoxin (Fd).
Move to the NADP reductase that uses the two
electrons and H+ ions from stroma:
◦ NADP+ + 2e- + H+  NADPH
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What about the electrochemical gradient
produced by the Z protein?
◦ Remember: protons are in the thylakoid lumen
◦ H+ moves through the ATPase from lumen to the
stroma  ATP is formed!
◦ Ratio: four H+ per ATP.
◦ PHOTOPHOSPHORYLATION: light-dependant
formation of ATP by chemiosmosis in
photosynthesis.
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In some cases, excited electrons take a cyclic
pathway
Uses PSI only
Electron is passed to Fd Q cycle 
cytochrome chain (b6-f complex)  back to
chlorophyll P700.
Generates an H+ gradient for chemiosmotic
ATP synthesis
Does not release electrons to generate
NADPH
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The overall goal :
◦ Energy of light is transferred to ATP and NADPH.
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Both of these substances play a critical role in
carbon fixation, the next step.
Page 166
#1, 2, 3, 4, 5, 6, 7.
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