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
Calvin cycle
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
Calvin Cycle-Reduction

ATP is used to phosphorylate 3-Phosphoglycerate



Electrons from NADPH are used to reduce 1,3Bisphosphoglycerate




New molecule is 1,3-Bisphosphoglycerate
Activated intermediate
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
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
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

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
Oxygen:CO2 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
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
C4 Plants
CAM Plants


Pineapples, cacti and other succulents
Similar to C4


Stomata open during night to take in CO2


Way to get around photorespiration
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
Overview
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