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In bicarbonate-buffered culture media, what primarily sets and stabilizes the initial pH of the medium under standard incubator conditions?

Difficulty: Easy

Presence of CO₂ alone in the incubator atmosphere
Presence of the bicarbonate buffer system in the medium
Addition of strong bases only (for example, NaOH) after inoculation
No specific control; pH drifts to neutrality
Use of nitrogen gas to displace oxygen

Correct Answer: Presence of the bicarbonate buffer system in the medium

Explanation:

Introduction / Context:
Effective pH control is essential for enzyme function, membrane transport, and product quality in mammalian cell culture. Most common media rely on the CO₂/HCO₃⁻ buffer system for physiological pH.

Given Data / Assumptions:

Media contain sodium bicarbonate at defined concentrations.
Incubators provide controlled CO₂ (about 5%).
We refer to the initial pH set before major metabolic shifts occur.

Concept / Approach:
According to the Henderson–Hasselbalch relationship, the ratio of bicarbonate to dissolved CO₂ determines pH. The medium’s bicarbonate content is the primary buffer; CO₂ sets the gas equilibrium component. Together they stabilize pH near 7.2–7.4 without continuous base addition.

Step-by-Step Solution:

Identify the buffer present: bicarbonate system in the medium.Recognize CO₂ as the gas-phase partner of the buffer, not the buffer itself.Select the option naming the bicarbonate buffer as the primary pH control.

Verification / Alternative check:
Media datasheets specify bicarbonate concentration and recommended CO₂ level to achieve the target pH, confirming buffer primacy.

Why Other Options Are Wrong:

CO₂ alone: needs bicarbonate to form a functional buffer.Strong base addition: used for adjustment, not continuous control.No control or nitrogen flushing: unrelated to establishing physiological pH.

Common Pitfalls:
Using mismatched CO₂ level for a given bicarbonate concentration, causing drift toward acidity or alkalinity.

In bicarbonate-buffered culture media, what primarily sets and stabilizes the initial pH of the medium under standard incubator conditions?

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