RuBisCO Enzyme – Complete Concept through MCQs

अब हम तुम्हारा Rubisco enzyme topic जड़ से लेकर ऊपरी स्तर तक clear करेंगे 🔬

यह 15 MCQs इस तरह डिज़ाइन किए गए हैं कि इनकी practice से तुम्हारा concept, logic और memory तीनों strong हो जाएंगे  -

🌿 Topic: RuBisCO Enzyme – Complete Concept through MCQs

MCQs (1–15)

Q1. RuBisCO enzyme catalyzes which reaction in the Calvin cycle?

A. Reduction of 3-PGA

B. Carboxylation of RuBP

C. Regeneration of RuBP

D. Conversion of PGA to G3P

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Q2. The full form of RuBisCO is:

A. Ribose bisphosphate carboxyase

B. Ribulose-1,5-bisphosphate carboxylase/oxygenase

C. Ribulose-1,6-bisphosphate carboxylase

D. Ribose-5-phosphate oxidase

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Q3. RuBisCO is localized in which part of the chloroplast?

A. Thylakoid membrane

B. Inner membrane

C. Stroma

D. Intermembrane space

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Q4. RuBisCO is composed of:

A. 2 large + 2 small subunits

B. 4 large + 4 small subunits

C. 8 large + 8 small subunits

D. 16 identical subunits

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Q5. The large subunit of RuBisCO is encoded by:

A. Nuclear DNA

B. Chloroplast DNA (rbcL gene)

C. Mitochondrial DNA

D. Ribosomal RNA genes

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Q6. The small subunit of RuBisCO is encoded by:

A. Nuclear DNA (rbcS gene)

B. Chloroplast DNA

C. Mitochondrial DNA

D. Cytoplasmic RNA

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Q7. The metal ion essential for the catalytic activity of RuBisCO is:

A. Mg²⁺

B. Mn²⁺

C. Zn²⁺

D. Fe²⁺

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Q8. Which one of the following molecules acts as a substrate for RuBisCO during photosynthesis?

A. CO₂ and RuBP

B. CO₂ and PGA

C. O₂ and PGA

D. G3P and CO₂

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Q9. Under high O₂ concentration, RuBisCO acts as an:

A. Only carboxylase

B. Only oxygenase

C. Both carboxylase and oxygenase

D. None of these

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Q10. The oxygenase activity of RuBisCO leads to formation of:

A. Two molecules of 3-PGA

B. One molecule of 3-PGA and one molecule of 2-phosphoglycolate

C. Two molecules of 2-phosphoglycolate

D. One molecule of RuBP and CO₂

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Q11. Which of the following compounds acts as a natural inhibitor of RuBisCO activity?

A. 3-phosphoglycerate

B. 2-carboxyarabinitol-1-phosphate (CA1P)

C. Glyceraldehyde-3-phosphate

D. ATP

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Q12. The activation of RuBisCO enzyme requires which process?

A. ATP phosphorylation

B. Carbamylation of lysine residue in the active site

C. Glycosylation

D. Sulfation

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Q13. The activation of RuBisCO is assisted by which enzyme?

A. RuBisCO activase

B. Ferredoxin reductase

C. ATP synthase

D. Phosphoribulokinase

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Q14. In C₄ plants, RuBisCO is primarily found in which type of cells?

A. Mesophyll cells

B. Bundle sheath cells

C. Guard cells

D. Epidermal cells

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Q15. Which of the following statements about RuBisCO is NOT true?

A. It is the most abundant protein on Earth

B. It fixes atmospheric nitrogen

C. It can act both as a carboxylase and oxygenase

D. Its activity depends on stromal pH and Mg²⁺ concentration

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✅ Answers with Concept Capsules

No. Answer Concept Capsule

1 B RuBisCO catalyzes the carboxylation of RuBP (Ribulose-1,5-bisphosphate) → forming 2 molecules of 3-PGA.

2 B Full form – Ribulose-1,5-bisphosphate carboxylase/oxygenase. It has dual activity.

3 C RuBisCO is a stromal enzyme – operates in the Calvin cycle (which occurs in the chloroplast stroma).

4 C Functional enzyme = 8 large (L) + 8 small (S) subunits → L₈S₈ complex.

5 B The rbcL gene is present in the chloroplast genome and codes for the large subunit.

6 A The rbcS gene is nuclear-encoded → small subunit is synthesized in cytosol and imported into chloroplast.

7 A Mg²⁺ ion binds with carbamylated lysine → essential for catalytic activity.

8 A RuBisCO reacts with CO₂ and RuBP as substrates → forming 3-PGA.

9 C RuBisCO acts as both a carboxylase (with CO₂) and oxygenase (with O₂).

10 B Oxygenase activity forms one 3-PGA + one 2-phosphoglycolate → starts photorespiration.

11 B 2-carboxyarabinitol-1-phosphate (CA1P) binds active site and inhibits RuBisCO at night.

12 B Lysine residue in active site gets carbamylated → essential for CO₂ fixation.

13 A RuBisCO activase (an ATPase) removes inhibitors like CA1P and helps activation.

14 B In C₄ plants, RuBisCO is restricted to bundle sheath cells → prevents oxygenase activity.

15 B RuBisCO fixes carbon dioxide, not nitrogen.

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🧠 Summary Snapshot:

Location: Chloroplast stroma

Structure: L₈S₈ holoenzyme

Genes: rbcL (chloroplast), rbcS (nuclear)

Function: CO₂ fixation → 3-PGA formation

Dual role: Carboxylase + Oxygenase

Cofactor: Mg²⁺

Regulation: Carbamylation + RuBisCO activase

Inhibitor: CA1P

C₄ localization: Bundle sheath cells