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Q.
First $ C{{O}_{2}} $ receptor in $ {{C}_{4}} $ plants is
Bihar CECEBihar CECE 2007
Solution:
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\xrightarrow{PEP\text{ }carboxylase} $ oxaloacetic acid $ +{{H}_{3}}P{{O}_{4}} $ Oxaloacetic acid is reduced to malic acid, Oxaloacetic-acid $ +NADPH\xrightarrow{dehydrcgenas} $ 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}}\xrightarrow[Carboxylase]{RuBP} $ 2-carboxy, 3-keto, 1, 5-biphosphoribotol 2 carboxy 3-keto 1-5 biphosphoribotol $ +{{H}_{2}}O\xrightarrow{{}}2\,PGA $ .