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Photosynthesis 3
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17.0 Photosynthesis
1) 17.1 Overview of photosynthesis2) 17.2 Absorption spectrum of
photosynthesis pigments3) 17.3 Light Dependent Reaction
4) 17.4 Light Independent Reaction/CalvinCycle
5) 17.5 Alternative mechanisms of CarbonFixation : Hatch Slack (C4) andCrassulacean Acid Metabolism (CAM)pathways
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Learning outcomes :
17.4 Light Independent Reaction/
Calvin Cycle
Describe Calvin Cycle involving
carbon fixation, reduction of
PGAL and regeneration of RUBP
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CALVIN CYCLE
Also known as the light independent
reaction
Occurs in stroma of the chloroplast
Results in the reduction of carbon
dioxide to synthesise carbohydrates
The power and energy for the varioussteps are provided by NADPHand ATP
from the light dependent reactions
Learning Outcomes :
17.4 : Describe Calvin cycle involving carbon fixation, reduction of PGAL and regeneration of RuBP
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Phases in Calvin Cycle
1) Carbon fixation
2) Reduction of PGAL3) Regeneration of Ribulose bisphosphate
(RuBP)
Learning Outcomes :
17.4 : Describe Calvin cycle involving carbon fixation, reduction of PGAL and regeneration of RuBP
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Phase 1 : Carbon fixation
Learning Outcomes :
17.4 : Describe Calvin cycle involving carbon fixation, reduction of PGAL and regeneration of RuBP
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Phase 1 : Carbon fixation
Each CO2molecule is
attachedto ribulose
bisphosphate (RuBP),
5C sugar to form 6C
compound which isunstable
Catalyzed by RuBP
carboxylaseor
RuBisCO
Learning Outcomes :
17.4 : Describe Calvin cycle involving carbon fixation, reduction of PGAL and regeneration of RuBP
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The unstable 6C
compound breaks
down to form 2molecules of 3C
called 3-
phosphoglycerate(PGA), for each
CO2
Phase 1 : Carbon dioxide fixation
Learning Outcomes :
17.4 : Describe Calvin cycle involving carbon fixation, reduction of PGAL and regeneration of RuBP
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Phase 2 : Reduction of PGAL
Learning Outcomes :
17.4 : Describe Calvin cycle involving carbon fixation, reduction of PGAL and regeneration of RuBP
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Phase 2 : Reduction of PGAL
Occurs in two steps:
Step 1 : Phosphorylation
Each molecule of 3-phosphoglycerate (PGA)
receives an additional
phosphate group from ATP
to form 1,3-bisphosphoglycerate
Learning Outcomes :
17.4 : Describe Calvin cycle involving carbon fixation, reduction of PGAL and regeneration of RuBP
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Step 2 : Reduction
Electrons donated
fromNADPHreduces 1,3-
bisphosphoglycerate
to Glyceraldehyde-
3-phosphate (G3P)or PGAL
Phase 2 : Reduction of PGAL
Learning Outcomes :
17.4 : Describe Calvin cycle involving carbon fixation, reduction of PGAL and regeneration of RuBP
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The NADP+and ADP
formed in this process
return to thethylakoids to
regenerate NADPH
and ATPin the lightdependent reactions.
Phase 2 : Reduction of PGAL
Learning Outcomes :
17.4 : Describe Calvin cycle involving carbon fixation, reduction of PGAL and regeneration of RuBP
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After fixation and
reduction we would have
six molecules of G3P
One molecule exits thecycle to be used by the
plant
The otherfive molecules
must remainin the cycleto regenerate three
molecule of RuBP.
Phase 2 : Reduction of PGAL
Learning Outcomes :
17.4 : Describe Calvin cycle involving carbon fixation, reduction of PGAL and regeneration of RuBP
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Phase 3 : Regeneration of RuBP
Learning Outcomes :
17.4 : Describe Calvin cycle involving carbon fixation, reduction of PGAL and regeneration of RuBP
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Regeneration of RuBP (the CO2acceptor)
Phase : Regeneration of RuBPLearning Outcomes :
17.4 : Describe Calvin cycle involving carbon fixation, reduction of PGAL and regeneration of RuBP
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Involves a seriesof reactions :
5 molecules of
Glyceraldehyde-3-
phosphate (G3P)are converted to 3
molecules of
Ru5P (ribulose-5-
phosphate), the5C compound
Phase 3 : Regeneration of RuBP
Learning Outcomes :
17.4 : Describe Calvin cycle involving carbon fixation, reduction of PGAL and regeneration of RuBP
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3 molecules of Ru5Pare phosphorylated to
regenerate RuBP
(ribulose
bisphosphate). Requires 3 ATP that
formed in the light
dependent reactions
Phase 3 : Regeneration of RuBP
Learning Outcomes :
17.4 : Describe Calvin cycle involving carbon fixation, reduction of PGAL and regeneration of RuBP
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The remaining
G3P is used to
synthesizeorganic
molecules eg :
glucose
Phase 3 : Regeneration of RuBP
Learning Outcomes :
17.4 : Describe Calvin cycle involving carbon fixation, reduction of PGAL and regeneration of RuBP
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Roles of ATP & NADPH
ATP: used in phosphorylation of PGAto form 1,3-bisphosphoglycerate
NADPH: used in reduction of 1,3-bisphosphoglycerate to form
G3P/PGAL
Learning Outcomes :
17.4 : Describe Calvin cycle involving carbon fixation, reduction of PGAL and regeneration of RuBP
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