Question

Given below are two statements
Statement I: Plastids are site of purine synthesis.
Statement II: Cytosol is the site of pyrimidine synthesis.
In light of the above statements, choose the most appropriate answer from the options given below

• A. Both Statement I and Statement II are true
• B. Both Statement I and Statement II are false
• C. Statement I is true but Statement II is false
• D. Statement I is false but Statement II is true

Hint:

In plant cells, the plastid acts as the "metabolic hub."
Almost all nitrogen-containing compounds, including amino acids and nucleotides (both purines and pyrimidines), are synthesized primarily in the plastids.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question concerns the intracellular localization of primary metabolic pathways for nucleotide biosynthesis in higher plants.

Step 2: Detailed Explanation:

• Purine Biosynthesis (Statement I): In plants, the entire de novo biosynthetic pathway for purines (from PRPP to IMP and finally to AMP and GMP) is localized within the plastids (chloroplasts and non-green plastids).
This is supported by the fact that many enzymes of this pathway contain N-terminal transit peptides for plastid targeting.
Therefore, Statement I is correct.

• Pyrimidine Biosynthesis (Statement II): Unlike animals, where part of the pyrimidine pathway is mitochondrial and part is cytosolic, in plants, almost the entire de novo pyrimidine pathway is localized in the plastids.
Key enzymes like carbamoyl phosphate synthetase (CPSase), aspartate transcarbamoylase (ATC), and dihydroorotase (DHO) are found in the plastid stroma.
The only exception is dihydroorotate dehydrogenase (DHODH), which is often associated with the mitochondrial membrane, but the subsequent steps leading to UMP also occur in the plastid.
Labeling Statement II as "Cytosol is the site" is incorrect for the primary biosynthetic pathway in plants.
Final Answer:
Statement I is true because purines are synthesized in plastids.
Statement II is false because de novo pyrimidine synthesis in plants is primarily a plastidic process, not cytosolic.
The correct option is (C).