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GATE 2020 BT – Question 15

Genetics, Cellular and Molecular Biology · Cell Biology: Eukaryotic cell structure, cell cycle and growth control, cell-cell communication, signalling and signal transduction · 1 mark · Multiple choice

Determine the correctness of Assertion [a] and Reason [r] regarding mammalian cells.

[a] Cells use $Ca^{2+}$, and not $Na^+$, for cell-to-cell signaling.

[r] In the cytosol, concentration of $Na^+$ is lower than that of $Ca^{2+}$.

  1. Both [a] and [r] are true and [r] is the correct reason for [a].
  2. Both [a] and [r] are true but [r] is not the correct reason for [a].
  3. Both [a] and [r] are false.
  4. [a] is true but [r] is false.

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Correct answer: (D) [a] is true but [r] is false.

Explanation

In the intended second-messenger context, calcium is maintained at a very low resting free cytosolic concentration, allowing transient increases to carry a signal. Sodium concentration in cytosol is far higher than free calcium, so the stated reason reverses the relationship. Thus the keyed answer is D: [a] true in that intended context, [r] false. The assertion is broader than biology warrants: sodium flux also participates in electrical signalling; it should not be read as denying that role.

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