Question

First $C{O}_2$ receptor in $C_4$ plants is

• A. PGA
• B. PEP
• C. RuBP
• D. OAA

Answer 1

Answer: (B)

In $C_4$ plants initial $C{O}_2$ fixation occurs in mesophyll cells of leaf. The first $C{O}_2$ acceptor is a 3-C compound Phosphoenol Pyruvate (PEP). It combines with $C{O}_2$ in the presence of PEP carboxylase to form 4-C compound oxaloacetic acid. $PEP+C{O}_2+H_{2}O\stackrel{PEPcarboxylase}{\to }$ oxaloacetic acid $+H_{3}P{O}_4$ Oxaloacetic acid is reduced to malic acid, Oxaloacetic-acid $+NADPH\stackrel{dehydrcgenas}{\to }$ malic acid $+NAD{P}^{+}$ In $C_3$ plants the first $C{O}_2$ acceptor is ribulose 1-5 biphosphate. It combines with $C{O}_2$ to produce a transient intermediate compound called 2-carboxy 3-keto 1, 5 biphosphoribotol. It immediately splits to form two molecules of 3 phosphoglyceric acid or PGA. RuBP $+C{O}_2\underset{Carboxylase}{\overset{RuBP}{\to }}$ 2-carboxy, 3-keto, 1, 5-biphosphoribotol 2 carboxy 3-keto 1-5 biphosphoribotol $+H_{2}O\to 2PGA$ .