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6.6
 The energy in ATP and NADPH is used in the
carbon-fixation reactions to ‘fix’ CO2 into a reduced
form and convert it into carbohydrates
 Most CO2 fixation occurs in the light, when ATP
and NADPH are being generated.
 This metabolic pathway occurs in the stroma, or
central region, of the chloroplast
 Calvin cycle: the energy in ATP and NADPH is
used to “fix” CO2 in reduced form in carbohydrates
 Each reaction is catalyzed by a specific enzyme.
 The cycle is composed of three distinct processes.
1.
Fixation of CO2
2.
Reduction of 3PG to form
glyceraldehyde 3-phosphate
3.
Regeneration of the CO2
acceptor, RuBP
5. RuMP is converted
to RuBP in a reaction
requiring ATP. RuBP
is ready to accept
another CO2
4. The remaining fivesixths of the G3P
molecules are
processed in a series
of reactions that
reduce RuMP
1. CO2 combines
with its acceptor,
RuBP, forming
3PG
2. 3PG is reduced
to G3P in a twostep reaction
requiring ATP and
NADPH
3. About one-sixth of the
G3P molecules are used to
make sugars – the output
of the cycle
 TEDEd – Natures Smallest Factory
 Khan Academy – Calvin Cycle
 The initial reaction of the Calvin cyle
adds the one-carbon CO2 to an acceptor
molecule, the five-carbon ribulose 1,5biphosphate (RuBP)
 This 6-carbon molecule quickly splits
into two 3-carbon molecules:
3-phosphoglycerate (3PG)
 Ribulose bisphosphate
carboxylase/oxygenase (rubisco)
catalyzes the reaction.
 Rubisco is a slug, relatively speaking
when talking about enzymes, therefore
plants require a lot of it, which actually
constituents almost half of all the protein
in a leaf
The fate of the
carbon atom in CO2
is followed in red
The enzyme
rubisco catalyzes
the reaction of CO2
with RuBP
The reaction intermediate
splits into two molecules of
3-phosphoglycerate (3PG)

This series of reactions involves
phosphorylation using the high energy P
from ATP made in the light reaction and
reduction from the NADPH also from the
light reaction

The product is 3PG is reduced to form
glyceraldehyde 3-phosphate (G3P).

This is a 3-carbon sugar phosphate, also
known as a triose phosphate

Most of the G3P ends up as ribulose
monophosphate (RuMP), and ATP is
used to covert this compound into
RuBP.

So with every turn of the Calvin cycle,
one CO2 is fixed and the CO2 acceptor
is regenerated.

The CO2 acceptor RuBP is regenerated
from G3P.

What happens to extra G3P made by the Calvin cycle?

It has two fates: depending on the time of day and the
needs of different parts of the plant
 Some of the extra G3P is exported to the cytosol and is
converted to hexoses (glucose and fructose).
 These molecules can then be used as part of cellular respiration;
used as carbon skeletons for synthesis of amino acids and other
molecules; or converted to sucrose, which can be transported out
of the leaf to another part of the plant
 When glucose accumulates, it is linked to form starch, a
storage carbohydrate.
 This storage carbohydrate can be drawn upon at night or
stored in the roots for growth and other cellular activites
 The products are crucial to the biosphere of the Earth
 The C—H bonds generated by the Calvin cycle provide almost all the
energy for life on Earth.
 Photosynthetic organisms (autotrophs) use most of this energy to support
their own growth and reproduction.
 Heterotrophs cannot photosynthesize and depend on autotrophs for
chemical energy.