Photosynthesis

1

• Lecture 12
• Photosynthesis
• Carbon Fixation
• Calvin Cycle

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Light and Carbon reactions of Photosynthesis

• Light Absorption Electrons are pulled from
  water, and O2 is evolved (light reaction)
• 2. Electron Transport NADPH is formed (light
  reaction)
• 3. Generation of ATP (light reaction)
• 4. Conversion of CO2 into Carbohydrates (carbon
  reaction)

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Overview of the Calvin Cycle 3 Stages
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The CO2 fixation reaction by RUBISCO
RUBISCO
Ribulose-1,5-bisphosphate carboxylase-oxygenase, t
he most abundant enzyme on earth!
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RUBISCO Protein
RUBISCO 8 large subunits (green, blue) 8 small
subunits (red)
500 kDa
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The Calvin Cycle C3 Carbon Fixation
Carboxylation
Reduction
Regeneration
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Calvin Cycle
Carboxylation
Reduction
First stable intermediate !
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Calvin Cycle
Regeneration
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Net reactions of the Calvin Cycle
6 CO2 18 ATP 12 NADPH 6 H2O C6H12O6
18 ADP 18 Pi 12 NADP 12 H 6O2
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Regulation of the Calvin Cycle
Light, CO2, increase in pH and Mg2 concentration
activate RUBISCO. Light regulates key enzymes in
Calvin Cycle turned on in light turned off in
dark (via Fd-thioredoxin system). Light-dependent
ion movements regulate Calvin Cycle
enzymes. Synthesis of enzymes in Calvin
Cycle. gene ? mRNA ? protein
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Light-dependent increase in pH and Mg2
concentration activates RUBISCO
Ready to bind CO2 or O2
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The ferredoxin-thioredoxin system reduces
specific enzymes in the light
Calvin Cycle enzymes modulated by the
Fd-thioredoxin system RUBISCO Fructose-1,6-bisph
osphate phosphatase Sedoheptulose-1,7-bisphosphat
e phosphatase Ribulose-5-phosphate
kinase NADP-glyceraldehyde-3-phosphate
dehydrogenase
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Light-dependent ion movements regulate Calvin
Cycle enzymes
Upon illumination ? flow of protons from stroma
into thylakoid lumen is coupled to release of
Mg2 from thylakoid lumen into stroma In
light ? pH in stroma raises from 7 to 8 ? Mg2
concentration increases in stroma In dark These
changes are reversed. ? several Calvin cycle
enzymes are more active at pH 8 than at pH 7
and require Mg2 as a cofactor
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Other CO2 fixation mechanisms
1. C3 pathway to fix CO2 to make sugars C3
reduction or Calvin cycle CO2 ? C3 ? C6 2.
Regulation of key enzymes by light 3. Many
plants have photorespiration 4. C4 plants
maximize CO2 fixation using a C4 pathway that
increases CO2 concentration 5. CAM plants
minimize water loss by fixing CO2 at night
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Photorespiration
A process where O2 is consumed and CO2 is given
off in light by photosynthetic tissues. Photosynt
hetic CO2 fixation and photorespiratory
oxygenation are competing with each other ? loss
of CO2 This competition leads to decrease in the
efficiency of photosynthesis.
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The main reactions of the photorespiratory cycle
No ATP production Circuit of C is connected with
circuit of N and O2 C4 plants suppress
photorespiration Biological function is unclear