Question

First stable product of $C_3$ pathway is:

• A. Oxalo-acetic acid
• B. 3 phosphoglyceric acid
• C. RuBP
• D. Phosphoenol pyruvate

Answer 1

Answer: (B)

In $C_3$ cycle of photosynthesis RuBP (Ribulose $1-5$-biphosphate) accept $C{O}_2$ and converted into a transient intermediate compound called $2$-carboxy $3$keto $1-5$-biphospho ribotol which immediately splits into $2$ molecules of $3$ phosphoglyceric acid (first stable product).
$RuBP+C{O}_2\stackrel{\mathrm{RuBp}\text{carboxylase}}{\to }2$ carboxy, $3-$ keto $1,5$ -biphosphoribotol In $C_4$ pants ( $C_4$-cycle) the $C{O}_2$ is accepted by phosphoenol pyruvate in mesophyll cells chloroplast. Oxaloacetic acid (oxaloacetate is formed by the reaction (first product).
$PEP+C{O}_2+H_{2}O\underset{\text{Carboxylase}}{\overset{\mathrm{PEP}}{\to }}$ Oxaloacetic acid $+H_{3}P{O}_4$