Which one of the following metabolic processes in mammalian cells does NOT occur in the
mitochondria?
Options:
(A) Citric acid cycle
(B) Oxidative phosphorylation
(C) Fatty acid β-oxidation
(D) Glycolysis
Official Answer Key:
D
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2018 Q.20)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Biochemistry / Cell Biology (Metabolic Pathways & Subcellular Compartmentalization)
**2. Detailed Step-by-Step Analysis**:
- Let us review the subcellular compartmentalization of the four metabolic processes in mammalian cells:
1. **Tricarboxylic acid (TCA) cycle**: Occurs inside the **mitochondrial matrix** (with succinate dehydrogenase embedded in the inner mitochondrial membrane).
2. **$eta$-oxidation of fatty acids**: Occurs inside the **mitochondrial matrix** (after carnitine shuttle transport) as well as peroxisomes.
3. **Oxidative phosphorylation**: Occurs across the **inner mitochondrial membrane** via Complexes I–IV and $F_o F_1$-ATP synthase.
4. **Glycolysis (Embden-Meyerhof-Parnas pathway)**: Occurs exclusively in the **cytosol** (cytoplasm) of mammalian cells. All ten glycolytic enzymes (from hexokinase to pyruvate kinase) are soluble cytosolic proteins.
- Therefore, glycolysis is the process that does **NOT** occur in the mitochondria.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: TCA cycle is mitochondrial.
- **Option (B) [INCORRECT]**: $eta$-oxidation of fatty acids takes place in mitochondria.
- **Option (C) [INCORRECT]**: Oxidative phosphorylation is a hallmark mitochondrial process.
- **Option (D) [CORRECT]**: Glycolysis takes place entirely in the cytosol.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **D**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).
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