Question

The first carbon dioxide acceptor in ${C_4}$ plants
a. Phosphoenolpyruvate
b. Oxaloacetic acid
c. Phosphoglyceric acid
d. Ribulose 1, 5 diphosphate

Answer 1

Hint: The ${C_4}$ cycle is a dark reaction of photosynthesis that is the cycle mostly occurs in the absence of light. It is a cyclic reaction mostly adapted by the tropical and dry region plants such as sugarcane, maize, sorghum etc. The ${C_4}$ cycle in the plants is seen in the mesophyll and bundle sheath cells.

Complete answer:
> Option A: Phosphophenol pyruvate-
It is a high energy compound, it is also considered as the second source of ATP. The phosphophenol pyruvate is mostly seen in the chloroplast of the plastid cells, during the ${C_4}$ cyclic reaction the carbon dioxide which is produced is first accepted by the phosphophenol pyruvate by the enzyme phosphophenol pyruvate carboxylase.

> Option B: Oxaloacetic acid-
The energy formed after accepting the carbon dioxide by carboxylase enzyme is used to form the oxaloacetic acid which is a four carbon compound and passed in the next part of the reaction in the bundle sheath.

> Option C: Phosphoglyceric acid-
The phosphoglyceric acid is the first product of the ${C_3}$ cycle unlike the ${C_4}$ cycle. The carbon dioxide is released after the formation of phosphoglyceric acid which is then accepted by the phosphophenol pyruvate.

> option D: Ribulose 1,5 diphosphate-
Ribulose 1,5 diphosphate carboxylase is an enzyme seen in the mesophyll cells of CAM plants it helps in the formation of carbon compounds in the CAM cycles.

Hence, the correct answer is option (A).

Note: In ${C_4}$ cycle the chloroplast is dimorphic. The leaf anatomy of ${C_4}$ plants differ from the ${C_3}$ plants, the leaf surface is highly exposed to higher levels of oxygen which makes it atmospheric where the sheath makes it difficult to diffuse into the cells.