Chemical reactions that convert carbon dioxide and other compounds into glucose.
Occurs in stroma-fluid filled area outside thylakoid membrane of chloroplast.
3 phases of the reaction are:
• Carbon fixation
• Reduction reactions
• Ribulose-1,5-bisphosphate(RuBP) regeneration
These are collectively called Calvin cycle.
Despite its name, this process occurs only when light is available.
Plants do not carry out Calvin cycle by night.
Calvin cycle is a series of biochemical redox reactions that take place in the stroma of
chloroplasts without light during photosynthesis and carbon fixation.
It is also known as light-independent reactions, used for carbon fixation.
OTHER NAMES FOR CALVIN CYCLE
• Dark reactions
• C3 cycle
• Calvin-Benson-Bassham (CBB) cycle
• Reductive pentose phosphate cycle
The cycle was discovered by Melvin Calvin, James Bassham and Andrew Benson in
1950 at the University of California, Berkeley.
They use radioactive carbon-14 to trace the path of carbon atoms in carbon fixtion.
OVERVIEW OF CALVIN CYCLE
Calvin cycle, a part of photosynthesis, occurs in 2 stages.
1st stage –light dependent chemical reactions capture the energy of light and use it
to make the energy-storage and transport molecules ATP and NADPH.
2nd stage – light independent Calvin cycle uses the energy from short-lived
electronically excited carriers to convert CO2 and H2O into organic molecules
This set of reactions is also called carbon fixation.
The key enzyme of the cycle is called RuBisCO.
Although called “dark reactions", these reactions don’t occur in dark or night time.
Reactions require reduced NADP(from light dependent reaction).
The sum of reactions in the Calvin cycle:
3 CO2 + 6 NADPH + 5 H2O + 9 ATP → glyceraldehyde-3-phosphate(G3P) + 2H+ + 6NADP+
+ 9 ADP + 8 Pi
Product of Calvin cycle : 3-carbon sugar phosphate molecule, glyceraldehyde-3-
Calvin cycle consists of:
• Carbon fixation
• Reduction reactions
• Ribulose-1,5-bisphosphate(RuBP) regeneration.
In 1st stage, a CO2 molecule is incorporated into one of two three-carbon molecules
(glyceraldehyde 3-phosphate or G3P), where it uses up 2 molecules of ATP and
NADPH, produced in the light-dependent stage.
The 3 steps involved are:
RuBisCO catalyses carboxylation of ribulose-1,5-bisphosphate[RuBP : a 5-carbon
compound] by carbon dioxide.
• Product is enediol-enzyme complex that can capture CO2 or O2 , which is the real
• CO2 captured by enediol in 2nd step produces 6-carbon intermediate that splits
into 2 molecules of 3-phosphoglycerate[3-PGA: a 3-carbon compound]
Phosphoglycerate kinase catalyses phosphorylation of 3-PGA by ATP (produced in light-
• Products are 1,3-Bisphosphoglycerate (1,3-BPGA)and ADP
[Two 3-PGAs produced for every CO2 entering the cycle, so this step utilizes 2 ATP
per CO2 fixed].
Glyceraldehyde 3-phosphate dehydrogenase catalyses reduction of 1,3BPGA by NADPH
(product of light-dependent stage).
• Glyceraldehyde 3-phosphate is produced, and the NADPH itself oxidized and
• 2 NADPH are utilized per CO2 fixed.
The next stage is to regenerate RuBP.
5 G3P molecules produce 3 RuBP molecules, using 3 ATP.
Since each CO2 produces 2 G3P molecules, 3 CO2 produce 6 G3P molecules, of which
5 are used to regenerate RuBP, leaving net gain of 1 G3P molecule per 3 CO2
The regeneration stage can be broken down into steps:
Triose phosphate isomerase converts all G3P reversibly into
dihydroxyacetone phosphate (DHAP).
Aldolase and fructose-1,6-bisphosphatase convert a G3P and a DHAP into
fructose 6-phosphate.( A phosphate ion is lost into solution).
Fixation of another CO2 generates 2 more G3P.
F6P has 2 carbons removed by transketolase, giving erythrose-4-
The 2 carbons on transketolase are added to a G3P, giving xylulose-5-
E4P and a DHAP are converted into sedoheptulose-1,7-bisphosphate by
Sedoheptulose-1,7-bisphosphatase (1 of only 3 enzymes of Calvin cycle
unique to plants) cleaves sedoheptulose-1,7-bisphosphate into
sedoheptulose-7-phosphate, releasing an inorganic phosphate ion into
Fixation of 3rd CO2 generates 2 more G3P.
• The ketose S7P has 2 carbons removed by transketolase, giving ribose-5-
phosphate (R5P), and 2 carbons remaining on transketolase are transferred to
1 of the G3P, giving another Xu5P.
• This leaves 1 G3P as the product of fixation of 3 CO2 , with generation of 3
pentoses that can be converted to Ru5P.
• R5P is converted into ribulose-5-phosphate by phosphopentose isomerase.
• Xu5P is converted into RuP by phosphopentose epimerase.
Finally, phosphoribulokinase (another plant-unique enzyme of the pathway)
phosphorylates RuP into RuBP, ribulose-1,5-bisphosphate, completing the Calvin
• This requires the input of 1 ATP.
Thus, of 6 G3P produced, 5 are used to make 3 RuBP molecules (totaling 15 carbons),
with only 1 G3P available for subsequent conversion to hexose.
This requires 9 ATP and 6 NADPH molecules per 3 CO2 .
The equation of the overall Calvin cycle diagrammatically:
Immediate products of 1 turn of Calvin cycle are 2 glyceraldehyde-3-phosphate (G3P)
molecules, 3 ADP, and 2 NADP+. (ADP and NADP+ are not really "products." They are
regenerated and later used again in the light-dependent reactions).
Each G3P molecule is composed of 3 carbons.
For the Calvin cycle to continue, RuBP (ribulose 1,5-bisphosphate) must be
So, 5 out of 6 carbons from 2 G3P molecules are used for this purpose.
Therefore, only 1 net carbon produced to play with for each turn.
To create 1 surplus G3P requires 3 carbons, and therefore 3 turns of the Calvin cycle.
To make 1 glucose molecule (created from 2 G3P) require 6 turns of the Calvin cycle.
Surplus G3P can also be used to form other carbohydrates such as starch, sucrose, and
cellulose, depending on what the plant needs.
REGULATION OF MELVIN CALVIN CYCLE
There is a light-dependent regulation of the cycle enzymes, as 3rd step requires reduced
NADP (this process would be a waste of energy, as there is no electron flow in the dark).
There are 2 regulation systems at work when the cycle needs to be turned on or off:
THIOREDOXIN/FERREDOXIN ACTIVATION SYSTEM - activates some of the cycle enzymes.
• The thioredoxin/ferredoxin system activates the enzymes glyceraldehyde-3-P
dehydrogenase, glyceraldehyde-3-P phosphatase, fructose-1,6-bisphosphatase,
sedoheptulose-1,7-bisphosphatase, and ribulose-5-phosphatase kinase, which are key
points of the process.
• This happens when light is available, as the ferredoxin protein is reduced in the
photosystem I complex of the thylakoid electron chain when electrons are circulating
• Ferredoxin then binds to and reduces the thioredoxin protein, which activates the cycle
enzymes by severing a cystine bond found in all these enzymes.
• This is a dynamic process as the same bond is formed again by other proteins that
deactivate the enzymes.
• The implications of this process are that the enzymes remain mostly activated by day
and are deactivated in the dark when there is no more reduced ferredoxin available.
RUBISCO ENZYME ACTIVATION-active in the Calvin cycle, which involves its own
• The enzyme Rubisco has its own activation process, which involves a more complex
• It is necessary that a specific lysine amino acid be carbamylated in order to activate
• This lysine binds to RuBP and leads to a non-functional state if left uncarbamylated.
• A specific activase enzyme, called Rubisco activase, helps this carbamylation process
by removing one proton from the lysine and making the binding of the carbon dioxide
• Even then the Rubisco enzyme is not yet functional, as it needs a magnesium ion to
be bound to the lysine in order to function.
• This magnesium ion is released from the thylakoid lumen when the inner PH
drops due to the active pumping of protons from the electron flow.
• Rubisco activase itself is activated by increased concentrations of ATP in the stroma
caused by its phosphorylation.
Chloroplasts given light at 680 and 700 nm simultaneously yield more O2 than
the sum of amounts when each is used alone
PSI (P700) AND PSII (P680)
All chlorophyll is part of either PSI or PSII.
PSI absorbs at 700 nm.
PSII absorbs at 680 nm.
Chloroplasts given light at 680 and 700 nm simultaneously yield more O2
than the sum of amounts when each is used alone.
FUNCTIONS OF PHOTOSYSTEMS
PSI reduces NADP+ .
PSII oxidizes water (termed “photolysis").
ATP is generated by establishment of a proton gradient as electrons
flow from PSII to PSI.