Photosynthesis is a TwoStep Process

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Photosynthesis is a Two-Step Process

• Light dependent rxns change light S into chemical
  S (ATP, NADPH)
• occurs on thylakoid membrane
• Light independent rxns use S in ATP and NADPH to
  reduce CO2 to glucose
• occurs in stroma

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Light-Dependent Reactions

• Light S from sun converted to chemical S
  initially as excited e- then in chemical bonds of
  ATP and NADPH
• chlorophyll absorbs photon in blue or red part of
  visible spectrum
• S txf to e- in chlorophyll (excited state)
• e- txf to special chlorophyll molecule (rxn
  center)
• chlorophyll located in thylakoid membrane

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Light-Dependent Reactions

• 2 types of rxn centers that consist of special
  chlorophyll a molecules supported by antenna
  complexes
• photosystem II (PSII)
• stimulated by 680 nm (red)
• splits water
• produces ATP
• photosystem I (PSI)
• stimulated by 700 nm (far-red)
• produces NADPH
• Ps runs at peak efficiency when both PSI and II
  are active
• enhancement effect Ps greater w/ red and
  far-red light than the combined rates w/ each
  separate

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Photosystem II

• Antenna complex transmits S
  to rxn center
  (P680)
• P680 special chlorophyll a
• Photon absorbed by P680
• e- in P680 gets excited (P680)
• e- passed from P680 to pheophytin
• P680 oxidized as looses e- to pheophytin
• PSII splits water to replace lost e- (thus P680
  re-reduced)
• 4 photons 2H2O ? 4 H 4e O2
  (photolysis)
• occurs in lumen (contributes to PMF across
  thylakoid membrane)
• e- passed from pheophytin to ETC (series of redox
  rxns)

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Photosystem II

• ETC contains several quinones and cytochromes
• plastoquinone (PQ) shuttles e- from pheophytin to
  cytochrome
• as PQ passes e-, it pumps H into thylakoid lumen
  thus PMF
• ATP synthase syn. ATP (similar to ETC in R)
• production of ATP by this process
  photophosphorylation
• e- passed from PQ to cytochrome b6f complex then
  to plastocyanin (PC)
• e- passed from PC to P700 (PS II)

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Photosystem I

• e- passed to P700 (reduced) from PC in PS II
• Photon absorbed by reduced P700
• e- in P700 gets excited (P700)
• e- passed from P700 through ETC (iron- and
  sulfur-containing proteins) to ferredoxin (Fd)
• NADPreductase txf 2e- and
  H to
  NADP to form NADPH
• NADPH used to reduce
  
  CO2 in Calvin Cycle
• occurs in stroma

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Z-Scheme and Photosystems I and II

• Z-scheme used to describe how photosystems I and
  II interact
• plastocyanin (Pc) carry electrons from
  photosystem I to II
• e- flow H2O ? PS II ? PS I ? NADP (Z scheme)
• Called noncyclic photophosphorylation

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Z-Scheme and Photosystems I and II Summary

• Photon absorbed by P680 in PSII and excites e-
• Excited e- passed to pheophytin
• Pheophytin donates e- to ETC
• Using S released by redox reactions occurring in
  ETC, PQ pumps H into thylakoid lumen
• ATP synthase uses PMF created by PQ to generate
  ATP
• PC accepts e- from ETC, transports e- across
  thylakoid membrane and txf it to P700 in PSI
• When photon absorbed by P700 in PSI, excited e-
  passed to ETC then Fd
• NADP reductase txf e- and H to NADP to form
  NADPH

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Cyclic Photophosphorylation

• Cyclic photophosphorylation occurs when extra ATP
  is needed
• PSI donates e- to ETC of PSII thus additional ATP
  made and no NADPH
• electrons transferred via PQ

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Light-Independent Reactions (Calvin Cycle)

• PSI and II produce ATP and NADPH only in light
• Rxns that produce sugar from CO2 are light
  independent
• require ATP and NADPH from light dependent rxns
• occur in stroma
• Carbon reduction (CO2 to sugar) (a.k.a. Calvin
  cycle) has 3 phases
• fixation, reduction, regeneration
• see next slide

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Light-Independent Reactions (Calvin Cycle)

• fixation
• CO2 (1C) reacts w/ ribulose bisphosphate (RuBP)
  (5C), producing two 3-phosphoglycerate (3PG) (3C)
• attachment of CO2 to organic cmpd carbon
  fixation
• catalyzed by RUBISCO (ribulose-1,5-bisphosphate
  carboxylase/oxygenase)
• most abundant enzyme on earth
• reduction
• 3PG phosphorylated by ATP and reduced by NADPH to
  produce glyceraldehyde 3-phosphate (G3P)
• some G3P used to make glucose
• regeneration
• remaining G3P used to regenerate RuBP

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Light-Independent Reactions (Calvin Cycle)

• RUBISCO
• slow, inefficient enzyme that catalyzes 3
  rxns/active site/sec
• plants make up for lack of speed by syn. large
  amnts of RUBSICO
• catalyzes 2 different rxns
• ? CO2 RUBISCO combines CO2 and RuBP to
  produce 3PG
• ? O2 RUBISCO combines O2 and RuBP to
  phosphoglycolate
• phosphoglycolate is toxic (neutralized by
  lysosomes)
• called photorespiration and reverses Ps (makes
  CO2 and uses ATP)

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The Big Dilemma

• How do plants maximize CO2 while minimizing O2
  availability to RUBISCO?
• stomata
• leaves have guard cells that open and close
  stomata
• CO2 diffuses into leaves along conc. gradient
• water also diffuses along conc. gradient

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The Big Dilemma

• Open stomata CO2 moves in, water moves out
• when dry and hot, lots of water will evaporate
  from open stomata
• thus, many plants close stomata to prevent water
  loss
• if stomata stay closed, CO2 ?, O2 ? and
  photorespiration ?
• thus, Ps ? in hot, dry conditions

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Solutions to Photorespiration Alternative
Carbon Fixation Systems

• Keep RUBISCO in ? O2 environment to ? oxygenase
  activity
• ? photorespiration by keeping CO2 ? in cells
  w/ Rubisco
• C4 plants - live in hot, dry climates
• grasses, sedges
• spatial isolation w/ 2 types of cells
• mesophyll - CO2 PEP (3C) ? OAA (4C) which is
  exported to ..
• bundle sheath CO2 released and enters Calvin
  cycle where RUBISCO fixes CO2 to RuBP
• more efficient at high temp.

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Solutions to Photorespiration Alternative
Carbon Fixation Systems

• CAM plants (crassulacean acid metabolism) dry
  climates
• cacti
• spatial and temporal isolation (same cells,
  different times)
• night stomata open
• CO2 PEP (3C) ? malate (4C) (in cytosol), malate
  exported to vacuole
• day stomata closed
• malate exported to chloroplast,
  CO2 released and enters Calvin cycle where
  RUBISCO fixes CO2 to RuBP

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Fate of Products Produced in Calvin Cycle

• G3P produced in Calvin cycle can be used to make
  glucose, fructose, sucrose and starch
• when sucrose is abundant, glucose stored as
  starch in chloroplast
• Sucrose, starch and
  intermediates can
  be
  used in glycolysis and
  
  Krebs to make ATP