Question

In CAM plants, the acceptor of $C{O_2}$ is
A. Pyruvic acid.
B. Oxaloacetate.
C. Phosphoenol pyruvate.
D. Nucleic acid.

Answer 1

The pathway for carbon fixation which has been evolved in some plants as adaptation to arid conditions is known as CAM photosynthesis. This kind of adaptation allows a plant to photosynthesize during the day, but only gas exchange at night. A plant which uses full CAM, the stomata present in the leaves remain shut during day which reduces evapotranspiration, but they open at night and collect carbon dioxide which allows it to diffuse into the mesophyll cells.

Complete Explanation
Option A - Pyruvic acid: Pyruvic acid is a $2$-oxo monocarboxylic acid which is the $2$-keto derivative of propionic acid which is $2$-oxo monocarboxylic acid, is known as the pyruvic acid. It is one of the important metabolites which is obtained during glycolysis. One of the important roles that pyruvate plays is as a fundamental metabolite and a cofactor. It is derived from propionic acid. There is no presence of pyruvic acid in CAM plants.
Hence option A is not correct.
Option B - Oxaloacetate: Oxaloacetate is involved in many processes such as gluconeogenesis, the urea cycle, the glyoxylate cycle, amino acid synthesis, and fatty acid synthesis. The process of phosphorylation in CAM plants can dramatically enhance the PEP carboxylase enzyme's capability to catalyze formation of oxaloacetate. This can be then further transformed into malate by $NA{D^ + }$ malate dehydrogenase. Oxaloacetate is not the acceptor of $C{O_2}$.
Hence option B is not correct.
Option C - Phosphoenol pyruvate: PEP or Phosphoenol Pyruvate is the first acceptor of $C{O_2}$ in CAM plants like the $C4$ plants. In Crassulacean Acid Metabolism (CAM) plants, the spatial separation of initial PEP carboxylase fixation occurs at night and final Rubisco fixation of $C{O_2}$ occurs at night. The fixation occurs in the same cell. Examples include plants like opuntia, pineapple and vanilla.
Hence option C is the correct answer.
Option D - Nucleic acid: The main information-carrying molecules of the cell are the nucleic acids. The main function of nucleic acid is directing the process of protein synthesis and also determining the inherited characteristics of every living thing. Deoxyribonucleic acid or DNA and ribonucleic acid or RNA are the two main nucleic acids. Nucleic acids are not the acceptor of $C{O_2}$ in CAM plants.
Hence option C not correct.

Hence, Option C. Phosphoenol pyruvate is the correct answer.

Note:
The carbon dioxide collected in CAM plants are stored as four-carbon acid malic acid in vacuoles at night. Then in daytime, the malate is transported to chloroplasts where it is then converted back into carbon dioxide, which is then used during the process of photosynthesis. The carbon dioxide which is pre-collected is then concentrated by the help of enzyme RuBisCO, which further increases photosynthetic efficiency.