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) CO2, would have 3 carbons (of PGA)? The studies very unexpectedly showed that the accepter molecule was a 5-carbon ketose sugar – it was 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 C3 acid, the primary acceptor would be a 2-carbon compound; they spent many years trying to identify a 2-carbon compound before they discovered the 5-carbon RuBP.