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-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
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
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
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