The Light-Independent Reactions / Calvin Cycle / Calvin-Benson Cycle / Dark Reaction of Photosynthesis

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The Light-Independent Reactions / Calvin Cycle /
Calvin-Benson Cycle / Dark Reaction of
Photosynthesis
Biochem. Animation
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• The ATP and NADPH made in the light-dependent
  reaction combined with CO2 begins the process of
  making glucose (and other good stuff)
• Glucose is actually synthesized outside the
  thylakoid membrane in the stroma.
• 6CO2 12NADPH 12H 18ATP ----gt C6H12O6
  12NADP 18ADP 18Pi 6H2O
• All of this happens in a circular pathway called
  the Calvin cycle
• Biochem. Animation

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Main categories of the Calvin Cycle (see Fig.
7.15 in text

• 1. Carboxylation/Carbon fixation
• CO2 is added to ribulose bisphosphate (a 5-carbon
  molecule) by the enzyme ribulose bisphosphate
  carboxylase (rubisco) to make two
  phosphoglycerates (a 3 carbon molecule)
• 2. Phosphorylation
• The two phosphoglycerates are phosphorylated by
  ATP to make two biphosphoglycerates

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• 3. Reduction
• The two biphosphoglycerates are reduced by NADPH
  to form two glyceraldehyde-3-phosphates
• 4. Regeneration or Yield
• Yield - Some (only 1 out of 6) of the
  glyceraldehyde-3-phosphates leave the chloroplast
  and enter the cytoplasm to form glucose (or other
  pathways leading to starches, fatty acids,
  glycerol, or amino acids)
• Regeneration most (5 out of 6)
  glyceraldehyde-3-phosphates are phosphorylated by
  ATP to make the original ribulose bisphosphate (a
  5-carbon molecule)

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enzyme
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Limiting Factors in Photosynthesis

• The factor/step that is lowest to its minimum and
  prevents the system from working is called the
  limiting factor or rate-limiting step
• Three factors that can determine the rate of
  photosynthesis--

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• Light intensity
• at low light there is a shortage of products made
  by the light-dependent reactions (NADPH and ATP)
  and therefore the Calvin cycle is limited
• not the limiting factor very often

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• CO2 concentration
• at low CO2 concentrations, the fixation process
  in the Calvin cycle is limited and rubisco and
  NADPH accumulate
• this is often the limiting factor because
  normally only .04 of earths atmosphere is CO2
• this can cause photorespiration to occur where
  oxygen binds with the enzyme rubisco instead of
  CO2

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• Temperature
• at low temperatures, all of the enzymes in the
  Calvin cycle work very slowly and NADPH
  accumulates
• at high temperatures, ribulose bisphosphate
  carboxylase (rubisco) does not work effectively
  and NADPH accumulates.

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