of another group of plants, where the first stable product of CO fixation was again an organic acid, but one which had carbon atoms in it. This acid was identified to be oxaloacetic acid or OAA. Since then CO assimilation during photosynthesis was said to be of two main types: those plants in which the first product of CO fixation is a C acid (PGA), i.e., the C pathway, and those in which the first product was a C acid (OAA), i.e., the C pathway . These two groups of plants showed other associated characteristics that we will discuss later.
. . The Primary Acceptor of CO Let us now ask ourselves a question that was asked by the scientists who were struggling to understand the ‘dark reaction’. How many carbon atoms would a molecule have which after accepting (fixing) CO , would have carbons (of PGA)?
The studies very unexpectedly showed that the acceptor molecule was a -carbon ketose sugar – ribulose bisphosphate (RuBP). Did any of you think of this possibility? Do not worry; the scientists also took a long time and conducted many experiments to reach this conclusion. They also believed that since the first product was a C acid, the primary acceptor would be a -carbon compound; they spent many years trying to identify a -carbon compound before they discovered the -carbon RuBP.
. . The Calvin Cycle Calvin and his co-workers then worked out the whole pathway and showed that the pathway operated in a cyclic manner; the RuBP was regenerated. Let us now see how the Calvin pathway operates and where the sugar is synthesised.
Let us at the outset understand very clearly that the Calvin pathway occurs in all photosynthetic plants ; it does not matter whether they have C or C (or any other) pathways (Figure . ). For ease of understanding, the Calvin cycle can be described under three stages: carboxylation, reduction and regeneration. .
Carboxylation – Carboxylation is the fixation of CO into a stable organic intermediate. Carboxylation