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

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

C4 Plants

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

Overview