C3 Carbon Fixation – the Calvin-Benson-Bassham (CBB) Cycle

Undergraduate level notes

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

C3 Carbon Fixation

1. CO2 from the air is fixed by RuBisCO, which catalyses the reaction between the CO2 and RuPB (ribulose bis-phosphate), a 5C compound.

C3 Carbon Fixation

2. This forms a short-lived 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 3-PGA 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 glyceraldehyde-3-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

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

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 1,3-bisPGA 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 light-dependent reactions of photosynthesis.