GATE 2020 BT Q.13: solution and answer

GATE 2020 · BTQ.13 · 1 mark · MCQ

Which one of the following media components is used to maintain pH in mammalian cell culture?

(A)
$CaCl_{2}$
(B)
$MgSO_{4}$
(C)
$NaCl$
(D)
$NaHCO_{3}$

See the question in the paper

Answer and solution

ANSWER

The answer is D, $NaHCO_{3}$ , because sodium bicarbonate forms the bicarbonate and carbon dioxide buffer that holds mammalian culture medium near pH 7.4.

Official answer: DSource: Official answer key, GATE 2020 BT (IIT Delhi)

Mammalian cells grow best near pH 7.2 to 7.4, so the culture medium must contain a buffer that resists pH change as the cells release acidic metabolites such as lactic acid and $CO_{2}$ .
Option D, $NaHCO_{3}$ , is the buffer component. The bicarbonate in the medium is in equilibrium with dissolved $CO_{2}$ from the incubator atmosphere, usually 5 percent: $CO_{2} + H_{2} O ⇌ H_{2} CO_{3} ⇌ H^{+} + HCO_{3}^{-}$ Added acid or base shifts this equilibrium and the pH changes only slightly, so D is correct.
Option A, $CaCl_{2}$ , supplies calcium ions, needed for cell adhesion and signalling. It is not a buffer, so A is wrong.
Option B, $MgSO_{4}$ , supplies magnesium ions, which act as enzyme cofactors. It does not buffer pH, so B is wrong.
Option C, $NaCl$ , is the main contributor to osmolarity and sodium balance. It is a neutral salt with no buffering action, so C is wrong.

KEY IDEA

Mammalian culture media are buffered by sodium bicarbonate in equilibrium with 5 percent $CO_{2}$ in the incubator. Salts such as NaCl, CaCl2 and MgSO4 supply osmolarity and ions, not buffering.

WATCH OUT FOR

Choosing NaCl because it is the most abundant salt in the medium. It sets osmolarity but has no buffering action.

From the GATE 2020 BT paper. Every question of this paper · Download the full paper · All 5 Animal cell culture questions, with notes

GATE 2020 BT Q.13: solution and answer

Answer and solution

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