Explain: Who is the primary acceptor of $CO_2$?

(N/A) The primary acceptor of $CO_2$ in $C_3$ plants is a $5$-carbon ketose sugar known as Ribulose $1,5$-bisphosphate $(RuBP)$.
Initially,scientists hypothesized that since the first stable product of the Calvin cycle is a $3$-carbon compound ($3$-phosphoglycerate),the primary acceptor must be a $2$-carbon compound.
However,after extensive research,it was discovered that the actual acceptor is the $5$-carbon molecule $RuBP$,which reacts with $CO_2$ to form two molecules of $3$-phosphoglycerate.