Question

The incubators used to culture mammalian cells are maintained at:

• A. 30% $CO_2$
• B. 2% $CO_2$
• C. 5--10% $CO_2$
• D. 15--20% $CO_2$

Hint:

Phenol red is commonly added to mammalian cell culture media as a pH indicator: it turns yellow under acidic conditions (excess lactic acid/metabolites) and purple/pink under alkaline conditions (loss of $CO_2$).

Answer 1

The Correct Option is C

Step 1: Concept
Mammalian cell culture media typically rely on a sodium bicarbonate-carbonic acid buffer system to maintain physiological pH (around 7.2 to 7.4), imitating blood serum chemistry.

Step 2: Carbon Dioxide Role
The bicarbonate buffer in the liquid medium exists in chemical equilibrium with gaseous carbon dioxide ($CO_2$) in the incubator's atmosphere:
$$CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$$ Maintaining a stable concentration of gaseous $CO_2$ prevents the loss of dissolved carbon dioxide from the medium, which would otherwise cause the pH to become dangerously alkaline.

Step 3: Standard Operating Values
The standard atmosphere in mammalian cell culture incubators is controlled at $37^\circ\text{C}$ with a constant, humidified supply of $5% - 10%$ $CO_2$ (most commonly $5%$).

Step 4: Conclusion
This directly corresponds to option (C). Final Answer: (C)