Photosynthesis

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Photosynthesis

• AGRI 6203

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Pathway of Electron Flow the Zig Zag Scheme

• Raise energy level of electrons derived from
  water to the energy level required to reduce
  NADP to NADPH.
• Each electron must be boosted twice
• ATP production is coupled with electron flow

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Z-Scheme
Source Kimballs Biology Pages
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Excitation of Electrons
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Redox Potentials (E)

• Measure (volts) of a an affinity of a substance
  for electrons - its electronegativity- compared
  with hydrogen (which is set a 0)
• positive redox potentials
• capable of oxidizing
• substance more electronegative than H (E)
• negative redox potentials
• capable of reducing
• substances less electronegative than H (-E)

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Electronegativity
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Source Lehninger p.657
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Downhill flow of electrons

• Free energy is released
• -?G

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Uphill flow of electrons

• Input of free energy
• ? G

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? E

• Greater the ? E between two substances, the
  greater the vigor with which electrons will flow
  spontaneously from the less positive to the more
  positive substance

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Available free energy

• ? G -n(23.062 kcal) (? E)
• n number of electrons transferred
• 23.0672 amount of energy released when one
  electron passes through a potential drop of 1 volt

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Synthesis of glucose by Photosynthesis

• 24 electrons must be removed from water
• water redox potential 0.82v
• must be pumped uphill to
• carbon atoms which they partially reduce to
  carbohydrate with a redox potential -0.42v
• difference is 1.24 v (? E), so
• ? G -24(23.062 kcal) (1.24) 686 kcal

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Light energy causes electrons to flow uphill

• Chlorophyll absorbs light and the electron in its
  structure is boosted
• excited chlorophyll migrates to the reaction
  center of the photosystem
• causes an electron to acquire a large amount of
  energy
• hot electron is expelled from the reaction
  center and accepted by the first electron carrier

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• First electron carrier becomes reduced and the
  reaction center has become __________
• oxidized reaction center now has an electron
  hole
• energy-rich electron has high reducing pressure
• passes to NADP
• must be sufficiently negative to reduce NADP

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Filling the electron holes

• PS I electron hole is filled by PSII
• PSII electron hole is filled by water
• 2H2O -----gt 4H 4e- O2
• electron flow from water to NADP
• 2H2O 2 _______ ---light---gt O2 _____ 2H

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Energy Profile of Photosynthetic Electron
Transport

• Transfer processes occurring the upward direction
  require energy
• downward flow of electrons proceed with a loss of
  free energy
• electrons flow from water (0.82 V) to NADP
  (-0.32 V).

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Photosynthetic Phosphorylation

• ADP to ATP is coupled with the energy released as
  high-energy electrons flow down the
  photosynthetic electron-transport chain from
  excited photosystem ____ to the electron holes in
  photosystem ___.
• One ATP is formed per pair of electrons

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ATP Synthesis
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Cyclic Phosphorylation

• Involves only PSI, cyclic because
• electron boosted to P430 by illumination of PSI,
  instead of passing to NADP, flows back into the
  electron hole of PSI
• overall reaction
• Pi ADP light energy ---gt ATP H20
• Why?
• light reactions 11 ATPNADPH formed
• dark reactions 32 ATPNADPH consumed

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Cyclic Phosphorylation
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Photosynthetic formation of glucose

• Generation of glucose and other carbs
• from ____and _____,
• at the expense of _______ and ________ produced
  in the light reactions
• overall equation
• What prevents animals from being able to carry
  out net conversion of CO2 to glucose?

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CO2 Fixation
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Calvin-Benson Cycle
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Calvin Cycle
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Synthesis of plant carbohydrates form
glucose-6-phosphate
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C4 Plants

• Tropics
• all plant ultimately use the C3 pathway
• C3 pathway is preceded by preliminary fixation of
  CO2 into oxaloacetate
• phosphoenopyruvate caboxylase
• OAA produced

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C4 Pathway
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C4 Pathway
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Photorespiration
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Sources of Images
Kimballs Biology Pages
Principles of Biochemistry, 1982, Lehninger,