LIGHT DEPENDENT REACTIONS OF PHOTOSYNTHESIS

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LIGHT DEPENDENT REACTIONS OF PHOTOSYNTHESIS
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• (1) As light falls upon the pigments of the
  chloroplast, energy causes the electrons in
  photosystem II (p680) to leave the chlorophyll a.

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• (2) At the same time, light is absorbed by
  photosystem I (p700) and electrons are
  transferred to carrier proteins. Electrons that
  leave photosystem II, replace electrons leaving
  photosystem I.

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• ( They move along a series of molecules called
  cytochromes called an electron transport chain. )
• Plastiquinone transfers electrons from
  photosystem II to the cytochrome chain)

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• (3) Electrons from photosystem I move through
  another transport chain, and travel to the
  stroma side of the thylakoid membrane.

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• There, the electrons combine with protons and
  NADP. The NADP becomes NADPH, a proton
  electron carrier which is needed for the Calvin
  Cycle.

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• Note Moving electrons during the light
  dependent reactions is really about a series of
  reduction-oxidation (redox) reactions in which
  compounds lose and gain electrons to maintain the
  electric current.

Here is a redox reaction in which hydrogen
reacts with fluorine. Electrons leave hydrogen
for the fluorine. Fluorine is reduced in charge
and hydrogen is oxidized. H2 becomes H ions.
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• 4) Electrons must replace those in photosystem
  II. This is accomplished as water decomposes, or
  splits (with the aid of an enzyme).

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(No Transcript)
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• 2H2O 4H 4e- O2
• (For every 2 water molecules split, four
  electrons are produced 2 ATPs are produced)
• Here is one reason that plants need water.

water splitting
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• 5) The passage of electrons from photosystem
  II to photosystem I causes protons to be actively
  transported (against the concentration gradient)
  into the interior of the thylakoid. This is why
  the energy from absorbed from Photosystem II is
  responsible for ATP production. (Plastiquinone
  transports the protons)

H
H
H
H
H
H
H
H
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• Photophosphorylation Light energy used to
  make ATP molecules (phosphorylate adding
  phosphate to ADP)
• PHOSPHORYLATION...   adding a phosphate to
  ADP
•                  ADP      P    ------gt    ATP

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• 6) Protons move back to the stroma through ATP
  synthase. Since this occurs with the
  concentration gradient, energy is released, and
  is used to make ATP. This is called chemiosmosis.

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ATP Synthase ATP production
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• ALSO
• A recirculation of electrons from
  photosystem I can provide energy to produce ATP
  molecules. This is called cyclic
  phosphorylation. It is present in some plants and
  some bacterial photosynthesis. (Photosystem II is
  not involved)

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Summary