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.