Section 4: Bioprocess Engineering and Process Biotechnology
Bioreactor types, scale-up
Subtopic: Fundamentals of Bioreactor types, scale-up
Question Statement:
A bioreactor is scaled up based on equal impeller tip speed. Consider the following parameters for
small and large bioreactors:
Parameters Small biorea ctor Large bioreactor
Impeller speed N1 N 2
Diameter of impeller D 1 D 2
Power consumption P 1 P 2
Assuming geometrical similarity and the bioreactors are operated in turbulent regime, what will be
P
2/P1?
(A) (D1/D2)2 (B) (D 2/D1)2 (C) (D 1/D2)5 (D) (D 2/D1)5
Options:
(A) (D1/D2)2
(B) (D 2/D1)2
(C) (D 1/D2)5
(D) (D 2/D1)5
Official Answer Key:
C
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2016 Q.49)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (2 Marks)
- **Topic**: Cell Signaling / Biochemistry (G-Protein Coupled Receptor Signaling)
**2. Detailed Step-by-Step Analysis**:
- Let us evaluate each statement concerning heterotrimeric and monomeric G proteins:
- **Statement A**: *GDP is bound to G protein in the resting stage.* This is **TRUE**. In the basal, inactive resting state, the $\alpha$ subunit of heterotrimeric G proteins is bound to GDP and firmly associated with the $\beta\gamma$ heterodimer.
- **Statement B**: *GTP bound $\alpha$ subunit cannot reassemble with $\beta\gamma$ dimer.* This is **TRUE**. When GPCR activation induces exchange of GDP for GTP on $G_\alpha$, a conformational change in the switch regions causes $G_\alpha\cdot\text{GTP}$ to dissociate from $G_{\beta\gamma}$. It cannot reassociate with $\beta\gamma$ until its intrinsic GTPase hydrolyzes GTP back to GDP.
- **Statement C**: *All G proteins are trimeric.* This is **INCORRECT / FALSE**. While receptor-coupled G proteins are heterotrimeric ($\alpha\beta\gamma$), there is an enormous superfamily of **small monomeric G proteins / small GTPases** (such as Ras, Rho, Rac, Cdc42, Rab, Ran, and Arf) consisting of a single polypeptide chain (~21 kDa). Thus, claiming that *all* G proteins are trimeric is fundamentally false.
- **Statement D**: *Activation of G protein may result in activation or inhibition of the target enzymes.* This is **TRUE** (e.g., $G_{\alpha s}$ activates adenylate cyclase, whereas $G_{\alpha i}$ inhibits adenylate cyclase).
- Therefore, the **INCORRECT statement** is **Statement C**.
- **Option-by-Option Evaluation**:
- **Option (A) [TRUE/NOT ANSWER]**: Accurately describes the resting GDP-bound state.
- **Option (B) [TRUE/NOT ANSWER]**: Accurately describes the structural dissociation of GTP-bound $G_\alpha$.
- **Option (C) [INCORRECT STATEMENT / CORRECT OPTION]**: Falsely states that all G proteins are trimeric, ignoring the large Ras superfamily of monomeric GTPases.
- **Option (D) [TRUE/NOT ANSWER]**: Accurately describes stimulatory ($G_s$) and inhibitory ($G_i$) pathways.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **C**
- **Scoring Criteria**: +2.0 Marks (Negative marking: -0.67 marks).
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