Calvin Cycle

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Calvin Cycle

• Uses ATP and NADPH to convert CO2 to glucose or
  other sugars
• Glyceraldehyde-3-phosphate is final product of
  Calvin cycle
• G3P can easily be converted to glucose or other
  sugars
• Cycle because CO2 acceptor is regenerated during
  the process
• 3 CO2 molecules (and 3 cycles) are needed to
  produce on net G3P molecule

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Calvin cycle
3
Calvin cycle-Carbon fixation

• Carbon from CO2 is added to organic acceptor and
  can then be used for synthesis
• Ribulose biphosphate (RuBP) is the acceptor
• 5 carbon sugar
• Rubisco (Ribulose biphosphate carboxylase) is the
  enzyme that adds the new carbon to RuBP
• Most common enzyme on earth
• Results in a 6 carbon molecule that immediately
  degrades into two three carbon molecules-3
  phosphoglycerate s
• has carboxyl group

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Calvin Cycle-Reduction

• ATP is used to phosphorylate 3-Phosphoglycerate
• New molecule is 1,3-Bisphosphoglycerate
• Activated intermediate
• Electrons from NADPH are used to reduce
  1,3-Bisphosphoglycerate
• Results in a carbonyl group
• Sugars have carbonyl group
• G3P is produced and it is a 3 carbon sugar
• For every 6 G3P molecules produced-5 are recycled
  and one can be used for anabolic rxns

5
Calvin Cycle-Regeneration

• Uses 5 molecules of G3P to produce 3 molecules of
  RuBP
• Requires ATP to perform this task
• This is why more ATP is needed than NADPH
• Without regeneration of G3P, this would not be a
  cyclical reaction
• RuBP is ready to act as Carbon acceptor for
  fixation stage

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Where does CO2 come from

• Plants get CO2 from air via stomata
• Have guard cells that open and close stomata
• Also guard against loss of water vapor

7
Photorespiration

• Happens in many plants during hot daytime
  conditions-soybeanss
• Guard cells close stomata to prevent water loss
• Unable to take up new CO2 molecules
• OxygenCO2 ration shifts
• Rubisco uses O2 instead of CO2
• Eventually yields CO2 for Calvin cycle but so
  much effort was expended it is a net loss for
  plant

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C4 Plants

• Way to avoid photorespiration
• Takes place in grasses like corn
• Two types of tissue for photosynthesis
• First acts like antechamber-Mesophyll cell
• PEP carboxylase has higher affinity for CO2 than
  Rubisco
• Adds Carbon from CO2 onto PEP to form
  Oxaloacetate
• Eventually CO2 is released into adjacent Bundle
  Sheath cell where Calvin takes place
• Maintains high CO2 to O2 ratio in BSC
• Rubisco can still use CO2

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C4 Plants
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CAM Plants

• Pineapples, cacti and other succulents
• Similar to C4
• Way to get around photorespiration
• Stomata open during night to take in CO2
• Carbon is stored in an organic acid molecule
• During daylight (when light cylce produces ATP
  and NADPH) organic acids degrade releasing CO2
  into the Calvin Cycle

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Overview