# DBT JRF 2020 — Question 94

Exam: DBT JRF 2020 · Section: B · Topic: Cell Biology · Difficulty: Medium

## Question

Which of the following organelle is NOT a site for Reactive Oxygen Species (ROS) generation?

## Options

- **A.** mitochondria
- **B.** nucleus
- **C.** peroxisomes
- **D.** endoplasmic reticulum

## Answer

Option B

## Explanation

Reactive Oxygen Species (ROS) generation sites in eukaryotic cells: Mitochondria — the major source: electron leakage at Complex I (NADH dehydrogenase) and Complex III (ubiquinol-cytochrome c reductase) transfers single electrons to $\text{O}_2$ , forming superoxide ( $\text{O}_2^{\bullet-}$ ). Peroxisomes — $\beta$ -oxidation of very long chain fatty acids and amino acid catabolism produces $\text{H}_2\text{O}_2$ ; catalase converts it to $\text{H}_2\text{O} + \text{O}_2$ . Endoplasmic Reticulum — protein disulfide bond formation during oxidative folding by Ero1 (ER oxidoreductin 1) and PDI generates $\text{H}_2\text{O}_2$ . NADPH oxidase (NOX) at plasma membrane — in phagocytes, produces superoxide for pathogen killing. The NUCLEUS contains antioxidant enzymes (thioredoxin, PRDX) and DNA repair mechanisms but is NOT a significant ROS-generating site.

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