Light-Independent Reaction:
Calvin Cycle
The light-independent Calvin cycle, also (misleadingly) known as the "dark reaction" or
"dark stage", uses the energy from short-lived electronically-excited carriers to convert
carbon dioxide and water into organic compounds that can be used by the organism (and
by animals which feed on it). This set of reactions is also called carbon fixation. The key
enzyme of the cycle is called RuBisCO. RuBisCO is thought to be the most abundant
protein on earth. It stands for ribulose bisphosphate carboxylase oxygenase.
The sum of reactions in the Calvin cycle is the following:
6 CO2 + 12 NADPH + 12 H+ + 18 ATP → C6H12O6 + 6 H2O + 12 NADP+ + 18 ADP + 18 Pi
The Calvin Cycle can be broken up into three phases: carbon fixation, reduction, and
regeneration of ribulose.
Steps in the Calvin Cycle
1) The enzyme RuBisCO catalyses the carboxylation of Ribulose-1,5-bisphosphate, a
5-carbon compound, by carbon dioxide resulting in a 6 carbon molecule.
2) This 6 carbon compound then splits apart into 2 molecules of 3-phosphoglycerate (PGA).
RuBisCO
Ribulose-1,5-bisphosphate + CO2 
2 (3-PGA)
3) Each PGA molecule is then phosphorylated by ATP and then reduced by NADPH to
form glyceraldehyde 3-phosphate or 3-phosphoglyceraldehyde (PGAL or G3P). PGAL
or G3P serves as the starting material for synthesis of glucose and fructose molecules.
Note that two PGAs are produced for every CO2 that enters the cycle, so this step
happens twice.
phosphoglycerate kinase
3-PGA + ATP

1,3 bisphosphoglycerate (1,3 BPGA)+ ADP
G3P dehydrogenase
1.3 BPGA + NADPH

Glyceraldehyde 3-phosphate (G3P) + NADP+
For every 6 carbons that enter the Calvin cycle, two molecules of G3P exit the cycle after
reduction occurs. The two G3P molecules which have exited the cycle are used to make
carbohydrates. In simplified versions of the Calvin cycle they may be converted to F6P or
F5P after exit, but this conversion is also part of the cycle. Hexose isomerase converts
about half of the F6P molecules into glucose-6-phosphate. These are dephosphorylated
and the glucose can be used to form starch, which is stored in, for example, potatoes, or
cellulose used to build up cell walls. Other glucose molecules combine with fructose to
form sucrose, the plant sugar.
4) The remaining 30 carbons (we began with 36 carbons and 6 have exited at this point)
are used to regenerate Ribulose-1,5-bisphosphate. Six of these molecules were used in
carboxylation, and so six are regenerated in this phase of the cycle. To accomplish this it
requires a total of six more ATP molecules.
The Calvin Cycle
The cycle spends ATP as an energy source and consumes NADPH as reducing power for
adding high energy electrons to make the sugar. There are three phases of the cycle. In
phase 1 (Carbon Fixation), CO2 is incorporated into a five-carbon sugar named ribulose
bisphosphate (RuBP). The enzyme which catalyzes this first step is rubisco. The product
of the reaction is a six-carbon intermediate which immediately splits in half to form two
molecules of 3-phosphoglycerate. In phase 2 (Reduction), ATP and NADPH from the
light reactions are used to convert 3-phosphoglycerate to glyceraldehyde 3-phosphate, the
three-carbon carbohydrate precursor to glucose and other sugars. In phase 3
(Regeneration), more ATP is used to convert some of the pool of glyceraldehyde 3phosphate back to RuBP, the acceptor for CO2, thereby completing the cycle.
For every three molecules of CO2 that enter the cycle, the net output is one molecule of
glyceraldehyde 3-phosphate (G3P). For each G3P synthesized, the cycle spends nine
molecules of ATP and six molecules of NADPH. The light reactions sustain the Calvin
cycle by regenerating the ATP and NADPH. In total, 6 molecules of CO2 enter the cycle
consuming 18 ATP and 12 NADPH.
hexose isomerase
2G3P 


fructose  glucose
dehydration synthesis
then
glucose + fructose

sucrose
Glucose and fructose make the disaccharide sucrose, which travels in solution to
other parts of the plant (e.g., fruit, roots).
Glucose is also the monomer used in the synthesis of the polysaccharides starch
and cellulose.
Check out the animation on this websites:
http://faculty.nl.edu/jste/calvin_cycle.htm
http://www.science.smith.edu/departments/Biology/Bio231/calvin.html