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Photosynthesis I: The Light Reactions

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This half-filled orbital will be filled with an electron taken away from water. ... Water-spitting areas stick out the membrane, and are shown in peach/tan. ... – PowerPoint PPT presentation

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Title: Photosynthesis I: The Light Reactions


1
  • Photosynthesis I The Light Reactions
  • 1. PSII--An excited chlorophyll at the
    reaction center reduces (gives its electron away
    to) an acceptor molecule.
  • a. The excited photoreactive chlorophyll has
    one electron jump to a higher energy level.
  • This electron is Figure 7.23 passed to
    pheophytin w/in
  • PSII.
  • This half-filled orbital will be
    filled with an electron taken away
    from water.

2
  • Photosynthesis I The Light Reactions
  • b. The PSII reaction center absorbs maximally
    at 680 nm. Figures 7.24, 7.25
  • The reaction center is a dimer of tetramers,
    surrounded by antenna complexes. Water-spitting
    areas stick out the membrane, and are shown in
    peach/tan.
  • c. A cluster of Manganese atoms associated
    with the reaction center is responsible for
    the reaction that splits water. Figure 7.25
  • d. A tyrosine residue w/in the D1 protein is
    thought
  • to accept the electrons from water one at a
    time
  • each time forming a radical which is the YZ
    molecule
  • that carries the electron to replace the
    electron that
  • chlorophyll donates to the ETS. Figure 7.26

3
  • Photosynthesis I The Light Reactions
  • e. Pheophytin accepts electrons from
  • the photoreactive
  • chlorophyll.
  • 2. Plastoquinones-- 2 identical
  • lipophillic molecules that are Figure 7.27
  • reduced to plastohydroquinones
  • in order to carry electrons to the
  • cytochrome b6f complex and move
  • hydrogen ions into the thylakoid lumen.

4
  • Photosynthesis I The Light Reactions
  • but were not quite done with the plastoquinones
    yet.
  • 3. The cytochrome b6f complex--made up of
    proteins with one c-type heme group (a.k.a. the
    f heme), two b- type heme groups and an
    FeS-containing protein.
  • Figure 7.28
  • What happens here is a bit of a mystery, but
    there is one explanation that seems to come
    closest to covering it.
  • a. Plastoquinone donates two electrons to the
    complex one goes to the FeS protein, and the
    other to one of the cyt b heme rings. Figure
    7.29
  • b. The FeS protein transfers its electron to
    the c-type heme ring (cyt f) which passes it on
    to a carrier molecule.
  • c. The other electron is passed to another
    cyt b ring,
  • which then reduces a plastoquinone to a
    plastosemiquinone.

5
  • Photosynthesis I The Light Reactions
  • d. The next time a QH2 donates its
    electrons, the one
  • that is given to the cyt b ring will
    eventually be used to reduce the
    plastosemiquinone to a full QH2.
  • This cyclic process is not fully understood
    (seems like theres
  • got to be a non-spontaneous reaction in there
    somewhere) but it
  • its usefulness is not questioned. What would
    this do?
  • 4. Plastocyanin (a soluble copper-containing
    protein found in the thylakoid lumen) or some
    other carrier takes electrons to the
    photoreactive chlorophyll in PSI.
  • 5. PSI is a large, multisubunit protein
    complex. Fig. 7.30
  • a. The P700 chlorophyll takes the electron
    from plastocyanin for the same reason that the
    P680 chlorophyll
  • took it away from the water-splitting complex.

6
  • Photosynthesis I The Light Reactions
  • b. Electrons pass from the chlorophyll
    through a set of unidentified electron carriers
    to a series of FeS-containing proteins, called
    FeS centers.
  • They are then passed to a soluble protein called
    ferredoxin, which also has an FeS group.
  • From here, the electrons can go either of two
    ways

7
  • Photosynthesis I The Light Reactions
  • d. Electrons can be passed to an enzyme
    called Ferredoxin-NADP reductase
  • which reduces NADP() and
  • H into NADPH.

8
  • Photosynthesis I The Light Reactions
  • e. Or the can be cycled back to the
    cytochrome b6f complex to be used to pump more
    H into the thylakoid lumen, to generate more
    ATP.
  • D. ATP synthase is powered by a pH gradient
    component of a proton motive force established by
    the electron transport chain, allowing H ions to
    move back out into the stroma in exchange for
    energy storage as ATP.
  • ATP synthase is madeup of a transmembrane part,
    called CF0,
  • and a part that sticks into the stroma, called
    CF1.
  • 1. H ions enter from the lumen side, through
    the CF0.
  • Figure 7.33

9
  • Photosynthesis I The Light Reactions
  • c. They enter between the rotor, which is
    made of 14 c-subunits, and a stator, which is
    made one a subunit.
  • This causes the rotor to spin within the membrane

10
  • Photosynthesis I The Light Reactions
  • d. The spinning motion is generated by an
    attraction between an arginine residue within the
    stator, and aspartate
  • residues within each subunit of the rotor. It
    is at these aspartate residues that the H ion
    binds when it enters the complex. The spin all
    the way around the rotor, and exit
  • at the other side of the stator.

11
  • Photosynthesis I The Light Reactions
  • e. But the spin motion is pushing against a
    force (the resistance caused by the reaction
    going on in the CF1 part, more on this later).
    From where does the energy come to push against
    this force?
  • 2. The force they are pushing against is the
    resistance due to the ADP Pi -----gt ATP
    reaction. This reaction occurs in the CF1
    part of the complex, which is made of 9
    subunits one each of ?, ?, and ? which serve
    as the internal connection between the CF0 and
    CF1, and three subunits each of ? and ?, which
    catalyze the ATP reactions.

12
  • Photosynthesis I The Light Reactions
  • a. The current model has the reaction
    proceeding as the binding sites rotate around.

b. The opposite reaction can be achieved if ATP
is supplied. This can actually be visualized
with the right technique, using a fluorescent
dye-tagged filament attached to the ? subunit.
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