Undergraduate level notes
C3 Carbon Fixation
 C3 carbon fixation refers to the biochemical process in
which CO2 is fixed initially as a 3C compound by
RuBisCO.
 This is the first step of the CBB cycle, which shall be
detailed in the rest of this slide show.
 It is important to note that C3 Carbon fixation does
occur in C4 and CAM plants – how carbon fixation
differs in these plants is explained in the resources
dedicated to these processes.
C3 Carbon Fixation
1. CO2 from the air is fixed
by RuBisCO, which
catalyses the reaction
between the CO2 and
RuPB (ribulose bisphosphate), a 5C
compound.
C3 Carbon Fixation
2. This forms a shortlived 6C intermediate
which rapidly
dissociates into two
3C molecules of 3-PGA
(3-phosphoglycerate).
C3 Carbon Fixation
3. The hydrolysis of 1
molecule of ATP per 3PGA results in the
formation of 1 molecule
of (again 3C) 1,3-bisPGA
(note that this is 2
molecules of 3-PGA, 2
molecules of ATP and two
molecules of 1,3-bisPGA
per CO2 molecule).
C3 Carbon Fixation
4. Each 1,3-bisPGA molecule
is reduced by NADPH and
undergoes
dephosphorylation, to
produce glyceraldehyde3-phosphate (Ga3P) – this
is what is called a triose
phosphate, on the basis of
being a 3 carbon sugar.
C3 Carbon Fixation
5. At this point in the cycle we must pause and consider
the stoichiometry of these reactions, as we have
reached a branch point at which a certain fraction of
the Ga3P that have been generated are used to
regenerate RuBP, while a smaller fraction contributes
to the sugar yield of the cycle.
Let’s work backwards...
C3 Carbon Fixation

For each molecule of triose phosphate produced that
can be considered the yield or product of the cycle
(i.e. can “leave” the cycle and contribute to other
biochemical pathways, such as the conversion to
sucrose), 5 more must be used to regenerate RuPB:
C3 Carbon Fixation
C3 Carbon Fixation

This means that to gain 1 triose phosphate, the cycle
must generate 6 triose phosphates in total.

Remember that there was a 1:1 relationship between
3-PGA, 1,3-bisPGA and Ga3P, so working backwards,
we need 6 3-PGA to be produced if we are to obtain
one triose phosphate.
C3 Carbon Fixation

2 3-PGA are produced per CO2 fixed, thus to produce
6 3-PGA and so 6 triose phosphates, of which one is
yielded, requires 3CO2.

To take this a step further, if we wish to obtain the
equivalent of one hexose sugar (e.g. Glucose), we
need 2 triose phosphates, thus all the above numbers
need to be doubled, and we require 6CO2.

Note that sucrose (glucose + fructose) is actually the
primary product of C3, not glucose.
C3 Carbon Fixation

We must also consider the input of energy and
reducing power from ATP and NADPH respectively.

Producing 2 triose phosphates equivalent to 1 hexose
sugar requires the phosphorylation of 12 3-PGA to
1,3-bisPGA, thus requires the hydrolysis of 12 ATP.

Additionally, the 5Ga3P per triose phosphate require
3ATP to regenerate 3RuBP, thus 6ATP are required to
regenerate 6RuBP from the 10Ga3P produced per 2
Ga3P yielded.
C3 Carbon Fixation

Finally, 12 NADPH are required to reduce the 12 1,3bisPGA to 12 Ga3P, to yield 2Ga3P, equivalent to 1
glucose.

Crunching all the above together then, it can be seen
that the production of the equivalent of one hexose
sugar requires 6CO2, 18ATP and 12NADPH.

These ATP and NADPH are produced in the lightdependent reactions of photosynthesis.