Question

Nitrate assimilation in leaves occurs in two sequential steps catalysed by Nitrate reductase (NR) and Nitrite reductase (NiR) enzymes respectively. Which of the following statements is true about these two enzymes?

• A. The NR is chloroplastic while NiR is cytosolic
• B. Both NR and NiR are cytosolic
• C. Both NR and NiR are chloroplastic
• D. The NR is cytosolic and NiR is chloroplastic

Hint:

Remember the sequence as Nitrate $\rightarrow$ Nitrite $\rightarrow$ Ammonium.
For plant nitrogen assimilation, remember that nitrate reductase is cytosolic, while nitrite reductase is chloroplastic in green leaves.
A useful memory aid is: NR starts in the cytosol, NiR continues in the chloroplast.
In competitive examinations, questions often test the locations of enzymes along with the sequence of nitrate reduction, so memorise both together.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
Nitrate assimilation is the process by which plants convert absorbed nitrate into forms of nitrogen that can be incorporated into amino acids and other nitrogen-containing compounds.
The conversion occurs through two consecutive enzymatic reactions involving nitrate reductase (NR) and nitrite reductase (NiR).
The question specifically tests the cellular locations of these two enzymes in plant cells.

• Key Formula or Approach:
The important sequence to remember is:
\[ \mathrm{NO_3^- \xrightarrow{Nitrate\ reductase} NO_2^- \xrightarrow{Nitrite\ reductase} NH_4^+} \] Nitrate reductase is mainly located in the cytosol, whereas nitrite reductase is located inside plastids, particularly chloroplasts in green leaves.

•

Step 2: Detailed Explanation:

• Nitrate \( \left(\mathrm{NO_3^-}\right) \) absorbed by plant roots is transported to plant tissues where it undergoes reduction before being incorporated into organic nitrogen compounds.

• The first reaction is catalysed by nitrate reductase (NR).

• Nitrate reductase is a cytosolic enzyme. It converts nitrate into nitrite according to the reaction:
\[ \mathrm{NO_3^- \xrightarrow{NR} NO_2^-} \]
• The nitrite produced in the cytosol is transported into the chloroplasts of green plant cells.

• The second reaction is catalysed by nitrite reductase (NiR).

• Nitrite reductase is located in chloroplasts in photosynthetic tissues. It converts nitrite into ammonium:
\[ \mathrm{NO_2^- \xrightarrow{NiR} NH_4^+} \]
• The resulting ammonium is subsequently incorporated into amino acids through pathways such as the glutamine synthetase--glutamate synthase pathway.

• Therefore, the correct cellular arrangement is cytosolic nitrate reductase followed by chloroplastic nitrite reductase.

• Statement (A) is incorrect because nitrate reductase is not chloroplastic.

• Statement (B) is incorrect because nitrite reductase is not cytosolic.

• Statement (C) is incorrect because nitrate reductase is cytosolic rather than chloroplastic.

• Statement (D) correctly describes the locations of both enzymes.

•

Step 3: Final Answer:
The nitrate assimilation pathway involves nitrate reductase in the cytosol and nitrite reductase in the chloroplast.
Hence, the correct option is:
\[ \boxed{\text{(D) The NR is cytosolic and NiR is chloroplastic}} \]