Subtopic: Fundamentals of Biomolecules, enzyme kinetics
Question Statement:
Which one of the following is used as a pH indicator in animal cell culture medium?
Options:
(A) Acridine orange
(B) Phenol red
(C) Bromophenol blue
(D) Coomassie blue
Official Answer Key:
B
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2019 Q.12)**
**1. Governing Concept**:
- **Topic**: Animal Cell Culture & Chemical pH Indicators
- In mammalian cell culture media (e.g., DMEM, RPMI 1640), **Phenol red** (phenolsulfonphthalein) is universally added as a visible, non-toxic pH indicator:
- At normal physiological pH ($7.2 - 7.4$): Medium is **red / bright pink**.
- Acidic conditions ($\text{pH} < 6.8$, due to lactic acid accumulation from overgrowth): Medium turns **yellow**.
- Alkaline conditions ($\text{pH} > 7.8$, due to $\text{CO}_2$ degassing): Medium turns **purple / magenta**.
**2. Option Analysis**:
- **Option (A) [Acridine orange] is INCORRECT**: Fluorescent dye for nucleic acid staining (intercalates in DNA/RNA).
- **Option (B) [Phenol red] is CORRECT**: The universal visual pH indicator in animal cell culture media.
- **Option (C) [Bromophenol blue] is INCORRECT**: Tracking dye in agarose and SDS-PAGE gel electrophoresis.
- **Option (D) [Coomassie blue] is INCORRECT**: Total protein staining dye in SDS-PAGE.
**3. Final Answer**:
- **Correct Choice**: **B**
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