C3 Cycle

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C 3 Cycle

• Paper 1 Metabolomics
• Unit 4 Plant metabolism

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C 3 cycle

• Also called as Calvin cycle as it is named after
  Melvin Calvin.
• Calvin-Benson-Bassham Cycle.
• Light independent reactions of photosynthesis.
• Happens in the stroma of chloroplast.
• Uses ATP and NADPH from Light dependent
  reactions.
• Reductive conversion of CO2 into carbohydrates.
  Therefore, also called as the photosynthetic
  carbon reduction cycle.
• Produces glucose and other sugars using products
  from the light energy 

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C 3 cycle

• Stage 1 Carbon Fixation / Carboxylation
• Carboxylation of RuBP 2 mol. of
  3-PGA
• Stage 2 Reduction Phase
• 3-PGA G3P (glyceraldehye
  3-phosphate) using ATP and NADPH. 
• Stage 3 Regeneration Phase
• Ribulose 5-phosphate RuBP
• 5 GAP 3 ATP 3 RuBP

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Stage 1 Carbon fixation / Carboxylation

• Gaseous CO2 and the 5-carbon sugar ribulose 1,5-
  bisphosphate form 2 molecules of
  3-phosphoglycerate.
• Enzyme that catalyzes this reaction is RuBisCo.

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RuBisCo

• Ribulose-1,5-bisphosphate carboxylase /
• oxygenase
• Unique to Calvin cycle
• Most abundant soluble protein in
• chloroplast and in the biosphere.
• 8 large subunits (56-KDa) --- L
• 8 small subunits (14-KDa) --- S
• L --- Chloroplast genome
• S --- Nuclear genome
• Also functions as Oxygenase.

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Regulation of the RuBisCo Reaction

• RuBisCo cycles between an active form (in the
  light) and an inactive form (in the dark)
• Activation requires light, CO2, Mg2? and correct
  stromal pH
• At night 2-carboxyarabinitol 1-phosphate (CA1P)
  inhibits RuBisCo

Inactive
active
Carbamate
Carbamate -Mg2? complex
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Stage 3 Regeneration Phase
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3
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Calvin Cycle Summary
Fixation
Reduction
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Regulation of the Calvin Cycle

• 1. Stromal Environment Increases in Mg2?, NADPH,
  Fdred, and a decrease in H
• 2. Activation of RUBISCO by light addition of
  CO2 to lysine (201), is favored by alkaline pH
  and increased Mg2? ions in the stroma.

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3. Regulation of enzyme activities via
thioredoxin One key protein in these regulatory
processes thioredoxin, a 12-kd protein containing
neighboring cysteine residues
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Products of C3 cycle

• The immediate products of one turn of the Calvin
  cycle are 2 glyceraldehyde-3-phosphate (G3P)
  molecules, 3 ADP, and 2 NADP .
• G3P requires 3 carbons, and therefore 3 turns of
  the Calvin cycle.
• To make one glucose molecule would require 18 ATP
  and 12 NADPH2 and 6 turns of the Calvin cycle.

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References

• Molecular biology by Buchanan
• http//www.ncbi.nlm.nih.gov/books/NBK22376
• Google images

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Thank you