Photosynthesis: The Carbon Reactions

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Photosynthesis The Carbon Reactions
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Intro

• Ps process by which plants convert sunlight
  into chemical energy, how energy enters biosphere
• chemical energy used to convert water and CO2
  into sugars, O2 is produced as byproduct
• 6 CO2 6 H2O ? C6H2O6 6 O2
• Consists of light-dependent rxns and
  light-independent rxns

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Intro

• 200 billion tons CO2
  converted into biomass per yr
• 40 phytoplankton
• Light-independent rxns dark rxns, carbon
  fixation rxns, stroma rxns, Calvin cycle
• dont require light, occur in chloroplast stroma
• use ATP, NADPH produced in light-dependent rxns
• produce reduced carbon cmpds (i.e. glucose) from
  CO2

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Calvin Cycle

• Calvin cycle
  (a.k.a. C3
  pathway)
• Common to all
  photosynthetic eukaryotes
• 3 stages
• carboxylation
• CO2 linked to RuBP
• reducation
• carbohydrate formed using
  ATP and NADPH from
  light-dependent rxns
• regeneration
• RuBP regenerated
• enzyme for every step

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Carboxylation

• Carboxylation (addition of CO2) of RuBP
  (ribulose-1,5-bisphosphate) catalyzed by RUBISCO
  (ribulose bisphosphate carboxylase/oxygenase) (1C
  5C 6C)
• Transient, unstable 6C molecule splits to form 2
  PGA (a.k.a. 3PG) (3-phosphoglycerate) (2 x 3C
  6C)

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Reduction

• PGA phosphorylated by ATP to 1,3-bisphosphoglyerat
  e
• 1,3-bisphosphoglyerate reduced by NADPH to PGAL
  (a.k.a. G3P) (gyceraldehyde 3-phosphopahte) (2 x
  3C)

Reduction
Carboxylation
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Regeneration

• RuBP must be regenerated
• 3 5C RuBP regenerated from 5 3C triose phosphates
• Each turn 1 CO2 fixed and reduced, RuBP
  regenerated
• 3 turns PGAL (phosphorylated C3H6O6)
• 3CO2 9ATP 6 NADPH 6H ?
• PGAL 9ATP NADP 3H2O
• PGAL exported to cytosol where converted to
• sucrose for transport
• starch for storage in chloroplast

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3 CO2
1. CO2 fixed
3 P
P
6 P
3PG a.k.a. PGA
RuBP
CALVIN CYCLE
6 ATP
3. RuBP regeneratedfrom PGAL a.k.a. G3P
2. PGA reduced to PGAL
6 ADP
3 ADP
3 ATP
6 NADPH
6 NADP 6 Pi
5 G3P
6 P
G
1 PGAL a.k.a. G3P
Glucose
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Photosynthetic Efficiency

• 5/6 triose phosphates used to regenerate RuBP
• 1/6 exported for sucrose/starch synthesis
• S input for 1 6C sugar 6 CO2, 18ATP, 12 NADPH
• thus 3 ATP, 2 NADPH per CO2
• red light (680 nm) 175 kJ (42 kcal)/quantum
  mole of photons
• gt 8 photons required per CO2
• thus minimum S to convert 6 mole CO2 to fructose
  6
  x 8 x 175 kJ 8400 kJ (2016 kcal)
• however, mole of fructose 2804 kJ (673 kcal)
• 2804/8400 .33 thus 33 efficiency for entire
  Ps
• inefficiency comes from light-dependent rxns

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Photosynthetic Efficiency

• Efficiency of Calvin cycle
• ATP hydrolysis 29 kJ (7 kcal)/mole
• NADPH oxidation 217 kJ (52 kcal)/mole
• 1 fructose-6-phosphate 6 CO2 12 NADPH 18
  ATP (12 x 217) (18 x 29) 3126
  kJ (750 kcal)
• 2804/3126 .9 thus 90 efficiency for Calvin
  cycle
• 83 S comes from NADPH oxidation

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Regulation of Calvin Cycle

• 5 enzymes regulated by light, some affected by pH
• RUBISCO
• NADPglyceraldehyde-3-phosphate dehydrogenase
• fructose-1,6-bisphosphatase
• sedoheptulose-1,7-bisphophatase
• ribulose-5-phosphate kinase

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RUBISCO

• RUBISCO
• slow, inefficient enzyme that catalyzes 3
  rxns/active site/sec
• compensate by synthesizing large amounts
• 40 soluble protein in lvs,
• most abundant enzyme on earth
• catalyzes two competing reactions (carboxylase vs
  oxygenase)
• hi CO2 CO2 RuBP to produce PGAL
• equal CO2 and O2 CO2 fixed 80x faster
• in air CO2 favored over O2 by 2.5 - 3 to 1
• hi O2 O2 RuBP to produce toxic product
• toxin converted to CO2 but uses S, releases CO2
• process called photorespiration, reverses Ps
• current atmosphere 21 O2, 0.036 CO2

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Photorespiration

• Rubisco binds CO2 2 PGA (3-phosphoglycerate)
• Rubisco binds O2 PGA 2-phosphoglycolate
• 2-phosphoglycolate (bad), PGA (good)
• C2 oxidative photosynthetic C cycle salvage
  process of fixed C from phosphoglycolate
• Occurs in C3

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Photorespiration

• Oxygenase activity salvage photorespiration
• Salvage occurs in chloroplast/peroxisome/mitochond
  ria
• RuBP O2 ? PGA phosphoglycolate
• occurs in chloroplast
• phosphoglycolate ? glycolate
• glycolate exported to peroxisome
• glycolate ? glyoxylate H2O2 (destroyed later)
• glutamate glyoxylate ? glycine
  ?-ketoglutarate
• glycine exported to mito. matrix
• 2 glycines 1 NAD ? 1 serine 1 NADH NH4
  CO2
• NH4 exported to chloropast where converted to
  AA
• serine exported to peroxisome where N removed to
  become hydroxypyruvate
• hydroxypyruvate reduced to glycerate
• imported to chloroplast where phosphorylated to
  PGA (enters Calvin)

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Photorespiration

• Conclusions
• 2 phosphoglycolate (2C each 4C) ? 1 PGA (3C)
  CO2 lost
• thus oxygenase activity ? efficiency of Ps
• 75 (¾) fixed C lost to oxygenase activity
  recovered and sent to Calvin cycle
• 25 (¼) lost (bye-bye!)
• 100 N retained
• Mother natures 75 solution to RUBISCO problem
• evolutionary remnant - CO2 higher in previous
  times
• N needs in C3 plants

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CO2 Pumps in PM

• Some plants evolved mechanisms to avoid
  photorespiration
• alternative carbon fixation systems
• new anatomy and biochemistry
• mechanisms to concentrate CO2 w/ RUBISCO (thus
  RUBISC)
• C4, CAM (crassulacean acid metabolism), CO2 pumps
  in PM
• CO2 pumps in PM aquatic
• cyanobacteria, algae
• CO2 and HCO3- pumps at PM
• pump requires ATP
• induced by ? CO2

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C4
C4 monocot

• Hatch-Slack
• C4 plants - hot climates, grasses/sedges
• specialized anatomy
• CO2 sequestered in bundle sheath cells
• ? photorespiration by keeping CO2 ? in
  cells w/ RUBISCO
• CO2 stored in
  mesophpyll
  cell
• CO2 used in BS cell
• both cell types contain
  chloroplasts

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C4

• C4 b/c malate (4C)
• 4 stages
• CO2 (1C) PEP (3C) ? malate (4C) in mesophyll
  cell
• malate exported to BS cell where decarboxylated
• CO2 used in Calvin, pyruvate (3C) sent back to
  mesophyll

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C4

• Efficiency
• 2 ATP consumed per CO2 (5 ATP and 2 NADPH
  consumed for total CO2 fixation via C4 and C3)
• higher cost but no photorespiration
• PEPs affinity for HCO3- means O2 not competitor
• thus can close stomata to conserve water and
  still fix CO2 at rates greater than C3 plants
• ½ N in C3 plants is RUBISCO
• C4 need less N in b/c ? RUBISCO in mesophyll
• thus ? N fertilizer, ? nutritious, ? insects
• bottomline more efficient at high temperatures

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CAM

• CAM plants - dry climates, cacti
• night initial CO2 fixation, plants open stomata
  and not risk excessive water loss
• daytime stomata close to conserve water
• CO2 sequestered at night, used for Ps during day
• thus improves WUE
• CAM 150-100
• C4 1250-300
• C3 1400-500
• competitive advantage
  in dry
  environments

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CAM

• Spatial and temporal isolation
• Night
• CO2 (1C) PEP (3C) ? malate (4C)
  in cytosol
• malate exported to vacuole
• Day
• malate in vacuole exported to chloroplast
• malate decarboxylated
• CO2 used in Calvin
• pyruvate (3C) sent back to chloroplast
• Facultative CAM plants
• C3 when not stressed
• CAM under stress
• due to gene expression

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Starch and Sucrose Synthesis

• Sucrose used for translocation
• syn. in cytosol
• Starch used for storage
• syn. in chloroplast

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