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  4. In an experiment in which photosynthesis is performed during the day, you provide a plant with radioactive carbon dioxide (14CO2) as a metabolic tracer. The 14C is incorporated first into oxaloacetic acid. The plant is best characterised as a

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Q. In an experiment in which photosynthesis is performed during the day, you provide a plant with radioactive carbon dioxide $(^{14}CO_{2})$ as a metabolic tracer. The ${14}^C$ is incorporated first into oxaloacetic acid. The plant is best characterised as a

Photosynthesis in Higher Plants

Solution:

In $C_{4}$ plants, the first stable photosynthetic product is a $4$-carbon compound, i.e., oxaloacetic acid $(OAA)$, which is formed by initial fixation of $CO_{2}$ by the carboxylation of phosphoenol pyruvate in the mesophyll cells. So, when radioactive $^{14}CO_{2}$ is introduced into the reaction, it is first incorporated into the $OAA$.
$PEP + ^{14}CO_{2} + H_{2}O \xrightarrow{\text{PEP carboxylase}} $ $OAA+H_{3}PO_{4}$
In a $CAM$ plant, although the first product formed is $OAA$, but here the initial $CO_{2}$ fixation occurs at night.