C 3 Carbon Fixation the CalvinBensonBassham CBB Cycle

C 3 Carbon Fixation – the Calvin-Benson-Bassham (CBB) Cycle Undergraduate level notes

C 3 Carbon Fixation �C 3 carbon fixation refers to the biochemical process in which CO 2 is fixed initially as a 3 C compound by Ru. Bis. CO. �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 C 3 Carbon fixation does occur in C 4 and CAM plants – how carbon fixation differs in these plants is explained in the resources dedicated to these processes.

C 3 Carbon Fixation 1. CO 2 from the air is fixed by Ru. Bis. CO, which catalyses the reaction between the CO 2 and Ru. PB (ribulose bisphosphate), a 5 C compound.

C 3 Carbon Fixation 2. This forms a shortlived 6 C intermediate which rapidly dissociates into two 3 C molecules of 3 -PGA (3 -phosphoglycerate).

C 3 Carbon Fixation 3. The hydrolysis of 1 molecule of ATP per 3 PGA results in the formation of 1 molecule of (again 3 C) 1, 3 -bis. PGA (note that this is 2 molecules of 3 -PGA, 2 molecules of ATP and two molecules of 1, 3 -bis. PGA per CO 2 molecule).

C 3 Carbon Fixation 4. Each 1, 3 -bis. PGA molecule is reduced by NADPH and undergoes dephosphorylation, to produce glyceraldehyde-3 -phosphate (Ga 3 P) – this is what is called a triose phosphate, on the basis of being a 3 carbon sugar.

C 3 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 Ga 3 P that have been generated are used to regenerate Ru. BP, while a smaller fraction contributes to the sugar yield of the cycle. Let’s work backwards. . .

C 3 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 Ru. PB:

C 3 Carbon Fixation

C 3 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 -bis. PGA and Ga 3 P, so working backwards, we need 6 3 -PGA to be produced if we are to obtain one triose phosphate.

C 3 Carbon Fixation � 2 3 -PGA are produced per CO 2 fixed, thus to produce 6 3 -PGA and so 6 triose phosphates, of which one is yielded, requires 3 CO 2. � 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 6 CO 2. � Note that sucrose (glucose + fructose) is actually the primary product of C 3, not glucose.

C 3 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 -bis. PGA, thus requires the hydrolysis of 12 ATP. � Additionally, the 5 Ga 3 P per triose phosphate require 3 ATP to regenerate 3 Ru. BP, thus 6 ATP are required to regenerate 6 Ru. BP from the 10 Ga 3 P produced per 2 Ga 3 P yielded.

C 3 Carbon Fixation � Finally, 12 NADPH are required to reduce the 12 1, 3 bis. PGA to 12 Ga 3 P, to yield 2 Ga 3 P, 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 6 CO 2, 18 ATP and 12 NADPH. � These ATP and NADPH are produced in the lightdependent reactions of photosynthesis.