The Calvin Cycle
• The Calvin Cycle is much like the Citric Acid
Cycle in that the circular process regenerates
the starting molecule at the end of the cycle.
It differs in that it is an anabolic process
consuming energy to produce a sugar
molecule whereas the Citric Acid Cycle is
catabolic producing energy as it consumes
sugar
Calvin Cycle
• Calvin Cycle - 3 cycles are required to produce
1 3 carbon sugar (G3P)
• Carbon fixation
– 3 CO2 molecules are incorporated 1 at a time
by the enzyme rubisco to ribulose biphosphate
(RuBP, 5 carbon sugar)
– the product immediately splits into two 3
carbon sugars, 3-phosphoglycerate
• Reduction
– ATP is used to attach an additional PO4 group
to 3-phosphoglycerate
– product is now 1,3-biphosphoglycerate
– NADPH donates 2 e- to 1,3-biphosphoglycerate
to reduce it to form glyceride-3-phosphate
(G3P)
– G3P is the same molecule produced in the 1st
stage of gylcolysis
• Regeneration of RuBP
– 5 G3P molecules (from the 3 cycles) are
converted back to 3 RuBP
Summary
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Summary
Consumed (used)
– 9 ATP
– 6 NADPH
– 3 CO2
Produced
– 1 G3P (through 3 cycles)
•
DG = 114kcal/mol
•
6CO2 + 18ATP + 12 NADPH + 12H2O g
C6H12O6 + 18ADP + 18Pi + 12NADP + + 12H+ + 6O2 + 6H2O
•
Note that the product is not glucose, fructose, or sucrose.
Another step is required to fuse 2 G3P to form a 6 carbon
sugar.
– This process takes 6 turns of the cycle!
– Glucose is formed from the fusion of 2 molecules of G3P
Alternate Pathways
•
C3 Plants (Rice, soy beans, grass)
– 1st product is 3-phosphoglycerate
– as stomata close on hot dry days the Calvin cycle fixes O2 instead of CO2 to produce CO2
•
called photorespiration
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produces no sugar
rxn has a net consumption of ATP
decreases photosynthetic output by robbing Calvin cycle of organic material
C4 Plants (Sugar cane, corn)
– have specialized tissues arranged around the veins
– mesophyll cells
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•
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store excess CO2 in organic molecules (oxaloacetate) by PEP carboxylase
can later release the bound CO2 in dry hot conditions when the stomata are closed
process minimizes photorespiration
– Bundle-sheath cells
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place where Calvin Cycle takes place
arranged around the veins to increase storage of sugar product
CAM Plants (Succulent)
– Stomata are closed during the day to minimize water loss
– Uptake of CO2 at night and incorporation into organic acids
– Calvin cycle operates during the day when the light cycle can produce ATP & NADHP for the
Calvin cycle