- A student added DMEM culture medium which was pink in colour to growing liver cells. Three days later the medium colour was yellow. This indicated
(1) change in cell morphology
(2) change in pH of the medium
(3) depletion of nutrients in the medium
(4) lack of antibiotics in the cultureThe correct answer is (2) change in pH of the medium.
How DMEM color indicates pH
DMEM commonly contains phenol red, a pH indicator dye.
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At around physiological pH (~7.2–7.4), the medium appears red to pink.
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As cells metabolize glucose and produce CO₂ and organic acids, the medium becomes more acidic, and phenol red turns orange to yellow.
So, when pink DMEM becomes yellow after three days, it signals a drop in pH caused by metabolic activity and waste accumulation.
Option-wise explanation
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Change in cell morphology
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Morphological changes do not directly alter the color of the medium. Color is controlled by pH, not cell shape.
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Change in pH of the medium – correct
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The pink-to-yellow shift reflects acidification of the medium due to accumulation of acidic metabolites, indicating that the culture is overgrown or the medium is exhausted and needs changing.
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Depletion of nutrients in the medium
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Nutrient depletion often occurs along with pH change but does not itself cause the color shift; the indicator responds specifically to pH, not nutrient concentration.
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Lack of antibiotics in the culture
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Absence of antibiotics can lead to contamination, which may also acidify the medium, but the color change is still a pH effect, and antibiotics themselves do not directly control color.
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Thus, the yellow color after three days most directly indicates a change in pH of the medium.
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