Question

During dark reaction, the three carbon atoms of 3-PGA are derived from

Answer 1

Formation of 3-PGA

3-PGA is formed by the fixation of carbon dioxide by a five carbon compound, RuBP, catalysed by RuBP carboxylase during dark reaction. One RuBP (5C) and one $C{O}_2$ molecule (1C), after rearrangements, forms 2 molecules of 3-PGA (3C). Hence, 3-PGA is derived from both RuBP and $C{O}_2$.

$\text{RuBP}+C{O}_2\stackrel{\text{RuBP carboxylase}}{⟶}\begin{array}{c}2\times 3-\text{Phospho-}\\ \text{glyceric acid)}\end{array}$

Final answer
$\text{(C) RuBP}+C{O}_2$