### **Official Solution & Step-by-Step Derivation (GATE BT 2016 Q.51)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (2 Marks)
- **Topic**: Animal Cell Culture & Bioprocess Engineering (Microcarrier Technology for Anchorage-Dependent Cells)
**2. Detailed Step-by-Step Analysis**:
- Let us evaluate each statement regarding microcarriers (e.g., Cytodex dextran beads):
- **Statement P**: *Higher cell densities can be achieved using microcarriers.*
- **TRUE**. Suspended spherical microcarriers in stirred bioreactors dramatically increase volumetric productivity, achieving cell densities orders of magnitude higher (
gt; 10^7\text{ cells/mL}$) than traditional 2D static T-flasks.
- **Statement Q**: *Microcarriers increase the surface area for cell growth.*
- **TRUE**. Thousands of small spherical beads provide massive surface-area-to-volume ratios (
gt; 10,000\text{ cm}^2/\text{L}$).
- **Statement R**: *Microcarriers are used for both anchorage- and nonanchorage-dependent cells.*
- **FALSE**. Nonanchorage-dependent cells (e.g., hybridomas, suspension CHO, Jurkat) grow freely suspended in liquid medium without needing any solid attachment surface; microcarriers are engineered specifically and exclusively for **anchorage-dependent cells**.
- **Statement S**: *Absence of surface charge on microcarriers enhances attachment of cells.*
- **FALSE**. Cell attachment requires a **positive surface charge** (e.g., $N,N$-diethylaminoethyl groups) or extracellular matrix proteins (collagen, gelatin) to mediate electrostatic adhesion with negatively charged cell surface glycoproteins. Uncharged microcarriers fail to attach cells.
- Therefore: **P-T, Q-T, R-F, and S-F**.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: Q is true, R is false.
- **Option (B) [CORRECT]**: P is True, Q is True, R is False, and S is False.
- **Option (C) [INCORRECT]**: P and Q are true.
- **Option (D) [INCORRECT]**: P is true.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **B**
- **Scoring Criteria**: +2.0 Marks (Negative marking: -0.67 marks).