Subtopic: Fundamentals of Downstream processing (chromatography, filtration)
Question Statement:
Which of the following are true with regard to anaerobic respiration in bacteria?
P. The final electron acceptor is an inorganic substance other than molecular oxygen
Q. The number of ATP molecules produced per glucose molecule is more than that produced
in aerobic respiration
R. The number of ATP molecules produced per glucose molecule is less than that produced
in aerobic respiration
S. Only substrate level phosphorylation is used to generate ATP
Options:
(A) P and S only
(B) Q and S only
(C) P and R only
(D) P, Q and S only
Official Answer Key:
C
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2018 Q.45)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (2 Marks)
- **Topic**: Microbiology & Microbial Metabolism (Anaerobic Respiration vs. Aerobic Respiration)
**2. Detailed Step-by-Step Analysis**:
- Let us evaluate each statement regarding bacterial anaerobic respiration:
- **Statement P**: *The final electron acceptor is an inorganic substance other than molecular oxygen.*
- **TRUE**: By definition, anaerobic respiration couples an electron transport chain to terminal electron acceptors such as nitrate ($ ext{NO}_3^-$), sulfate ($ ext{SO}_4^{2-}$), ferric iron ($ ext{Fe}^{3+}$), or carbonate ($ ext{CO}_3^{2-}$), rather than $O_2$.
- **Statement Q & R**: *Number of ATP molecules produced per glucose molecule.*
- Because alternative terminal electron acceptors (like nitrate, $E^\circ' = +0.42 ext{ V}$, or sulfate, $E^\circ' = -0.22 ext{ V}$) have significantly less positive reduction potentials than oxygen ($E^\circ' = +0.82 ext{ V}$), the total free energy released ($\Delta G^\circ' = -nF\Delta E^\circ'$) is substantially lower. Consequently, fewer protons are pumped per electron pair, generating **fewer ATP molecules per glucose than aerobic respiration**.
- Hence, **Q is FALSE** and **R is TRUE**.
- **Statement S**: *Only substrate level phosphorylation is used to generate ATP.*
- **FALSE**: Anaerobic respiration operates an electron transport chain coupled to proton translocation across the membrane, synthesizing ATP primarily via **oxidative phosphorylation** (chemiosmosis). Relying *only* on substrate-level phosphorylation is the definition of **fermentation**, not anaerobic respiration.
- Therefore, statements **P and R only** are true.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: S confuses anaerobic respiration with fermentation.
- **Option (B) [INCORRECT]**: Q and S are both false.
- **Option (C) [CORRECT]**: Statements P (inorganic terminal electron acceptor other than $O_2$) and R (ATP yield is less than aerobic respiration) are biologically accurate.
- **Option (D) [INCORRECT]**: Includes false statements Q and S.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **C**
- **Scoring Criteria**: +2.0 Marks (Negative marking: -0.67 marks).
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