Phases of Photosynthesis

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Phases of Photosynthesis

• Photosynthesis occurs in 2 phases, which include
  3 main goals
• A. The Light Reactions
• 1. Capturing light energy
• 2. Using the light energy to create chemical
  energy (ATP and NADPH (electron carrier in
  plants))
• B. The Light Independent Reactions
• 3. Using the free energy from ATP and reducing
  power of NADPH to synthesize organic compounds
  from atmospheric carbon.

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Where They Occur

• The Light Reactions require chlorophyll and occur
  on the thylakoid membrane.
• The Light Independent reactions (aka The Calvin
  Cycle) take place in the stroma.

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Light Reactions

• When light strikes the thylakoid membrane, the
  photosystems absorb a photon and the energy is
  passed from pigment to pigment until it reaches
  the reaction center (P680 or P700).
• The light energy excites the electrons in the
  chlorophyll a molecules.

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Excitation of Electrons

• Electrons in a chlorophyll molecule are normally
  at their ground state or lowest potential energy
  level
• Excitation The photon hits the chlorophyll and
  the electrons are energized to a higher energy
  level.
• Outside a living organism excited electrons want
  to return to their ground state and lose
  potential energy as heat and light (fluoresce).

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Non-Cyclic Electron Flow

• In a cell the excited electron is captured by a
  special molecule called the primary electron
  acceptor (no fluorescing).
• The energy of the electrons is used to produce
  ATP and NADPH in a two-part process called
  non-cyclic electron flow.

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Steps 1. Photoexcitation

• A photon of light strikes the antenna complex of
  PII and PI
• The energy is passed around the antenna pigments
  until it excites an electron of P680 / P700.

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• Noncyclic flow begins at PII
• A redox reaction transfers the excited electrons
  from P680 to the primary electron acceptor
  (pheophytin)
• Therefore Chlorophyll a is oxidized and the
  primary acceptor is reduced (redox!)

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2. Electron Transport i.

• .

• From the primary electron acceptor (pheophytin),
  the electrons travel by redox through a series of
  membrane bound electron carriers (cytochrome
  complexes, aka an ETC)

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Electron Transport ii.

• They power a proton pump called the Q cycle.
• Protons are pumped from the stroma into the
  thylakoid lumen creating a proton gradient.
• (High concentration inside thylakoid and low
  outside).

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Electron Transport iii.

• At the end of the ETC the electrons from PII are
  transferred to PI (to replace the electrons it
  loses...next)

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iv.

• At PI
• The excited electrons from P700 pass to an
  electron acceptor (ferredoxin (Fd)).
• From Fd they move through a short ETC to the
  enzyme NADP reductase.
• The enzyme uses two electrons and two protons
  from the stroma to reduce NADP to NADPH.

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v.

• The PII electrons are also replaced
• A Z protein splits water into oxygen, 2 hydrogen
  ions and 2 electrons.
• The electrons replace the missing electrons from
  P680.
• Oxygen leaves as a waste product
• Protons stay in the thylakoid lumen

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3. Photophosphorylation

• A proton gradient was set up in PII. There is a
  high concentration of H in the lumen.
• Protons move through an ATPase along the proton
  gradient from the thylakoid lumen into the
  stroma. ATP is generated.
• This process is called photophosphorylation since
  light creates the proton gradient.

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Summary of the Light Reactions
The ATP and NADPH go on to the Light Independent
reactions...the Calvin Cycle
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CYCLIC ELECTRON FLOW

• In some cases (low O2, low levels of NADP, low
  ATP to NADPH ratio) electrons take a cyclic
  pathway instead to increase ATP production.
• Only PI is used. P700 loses its electron pair to
  ferredoxin (Fd) but then electrons go through the
  Q cycle (ETC of PII) and back to P700.
• This generates a proton gradient for ATP
  synthesis, but does not release electrons to
  generate NADPH.

• Plants cycle back and forth through cyclic and
  non-cyclic to maintain adequate levels of both
  ATP and NADPH.