Q1. Source: CSIR NET NOV 20; Session II
A student added DMEM culture medium which was pink in color to growing liver cells. Three days later the medium color was yellow. This indicated-
a. Change in cell morphology
b. Change in pH of the medium
c. Depletion of nutrients in the medium
d. Lack of antibiotics in the culture
Statement Analysis:
DMEM (Dulbecco’s Modified Eagle Medium) commonly contains phenol red, a pH indicator used to monitor the condition of the culture medium.
Phenol red changes color depending on the pH:
- Pink/red= neutral or slightly basic pH
- Yellow= acidic pH
As liver cells grow in culture, they continuously metabolize nutrients like glucose and release acidic metabolic byproducts such as lactic acid. Over time, this lowers the pH of the medium, causing the color to shift from pink to yellow.
a. Change in cell morphology (incorrect)
Changes in cell shape or morphology do not directly alter the color of the culture medium.
b. Change in pH of the medium (correct)
Correct answer. The yellow color specifically indicates acidification of the medium due to cellular metabolism.
c. Depletion of nutrients in the medium (incorrect)
Although nutrients may gradually deplete over time, the color change itself is linked to pH alteration, not nutrient depletion directly.
d. Lack of antibiotics in the culture (incorrect)
Absence of antibiotics may increase the risk of contamination, but it does not inherently cause the medium to turn yellow.
Correct Answer: b. Change in pH of the medium
Exam Insights:
Whenever you see culture media changing color in cell biology questions, immediately think of pH indicators like phenol red and cellular metabolism.
Q2. Source: GATE 2019
Which of the following techniques can be used to study conformational changes in myoglobin?
P. Mass spectroscopy
Q. Fluorescence spectroscopy
R. Circular Dichroism Spectroscopy
S. Light microscopy
a. P only
b. P and S only
c. Q and R only
d. S only
Statement Analysis:
Conformational changes refer to alterations in the three-dimensional structure of proteins. Since myoglobin is a protein, techniques that detect changes in protein structure are most appropriate here.
P. Mass spectroscopy (incorrect)
Mass spectrometry is mainly used to determine molecular mass, peptide composition, or protein identification. It is not typically the primary technique used to directly monitor conformational changes in proteins in such questions.
Q. Fluorescence spectroscopy (correct)
Correct. Fluorescence spectroscopy is widely used to study protein folding and conformational changes because amino acids like tryptophan exhibit fluorescence that changes depending on their environment within the protein structure.
R. Circular Dichroism Spectroscopy (correct)
Correct. Circular Dichroism (CD) spectroscopy is a powerful technique used to analyze secondary structures such as alpha-helices and beta-sheets, making it ideal for studying conformational changes in proteins like myoglobin.
S. Light microscopy (incorrect)
Proteins are far too small to be visualized using conventional light microscopy due to its limited resolution.
Correct Answer: C. Q and R only
Exam insights:
For protein conformational studies, techniques like fluorescence spectroscopy and circular dichroism are highly important because they provide insights into protein folding and structural organization.