Which one of the following is used as a pH indicator in animal cell culture medium?
- (A)
Acridine orange
- (B)
Phenol red
- (C)
Bromophenol blue
- (D)
Coomassie blue
GATE 2019 BT Q.2: solution and answer
Answer and solution
ANSWER
The answer is (B) Phenol red.
Official answer: BSource: Official answer key, GATE 2019 BT (IIT Madras)
Animal cell culture media are buffered near pH 7.2 to 7.4. Cell metabolism produces acids (lactic acid, ), so the pH of the medium must be easy to monitor.
Phenol red is a weak-acid dye added to culture media. It is red at physiological pH, yellow when the medium turns acidic and purple-pink when it turns alkaline, so the colour of the medium shows its pH at a glance. Option B is correct.
Acridine orange is a fluorescent nucleic acid stain, used for DNA/RNA staining and for viability and apoptosis assays. It is not a pH indicator for the medium. Option A is wrong.
Bromophenol blue is mainly a tracking dye in gel electrophoresis loading buffers. As a pH indicator it changes colour only at about pH 3.0 to 4.6, far from the physiological range, so it is not used in culture media. Option C is wrong.
Coomassie blue is a protein stain for gels and protein assays, not a pH indicator. Option D is wrong.
KEY IDEA
Phenol red is the pH indicator built into most animal cell culture media. It changes colour (red at about pH 7.4, yellow when acidic, purple-pink when alkaline) in the range where cells grow, so you can judge the medium's pH by eye.
WATCH OUT FOR
Picking bromophenol blue because it is also a pH-sensitive dye. Its colour change is at about pH 3.0 to 4.6, nowhere near the neutral range of culture media. Acridine orange and Coomassie blue are stains, not indicators.
From the GATE 2019 BT paper. Every question of this paper · Download the full paper · All 5 Animal cell culture questions, with notes