Q.5 Determine the correctness or otherwise of the following Ascention (a) and the
Reason (r) regarding mammalian cells.
Assertion (a): Cells use Ca2+, and not Na+, for cell–to–cell signalling
Reason[r): In the cytosol, concentration of Na+ is lower than that of Ca2+ ?
(A) Both (a) and (r) are true and (r) is the correct reason for [a].
(B) Both (a) and rare true but [c] is not the correct reason for (a).
(C) Both (a) and care false
(D) [a] is true but is false.

Cells in mammalian systems primarily rely on Ca²⁺ ions for cell-to-cell signaling due to their tightly regulated cytosolic levels, unlike Na⁺ which maintains higher steady-state concentrations. This assertion-reason question tests understanding of ion gradients and signaling physiology relevant for IIT JAM Biotechnology and Life Sciences exams. The correct option emerges from evaluating both parts independently.

Assertion Analysis

The assertion states that mammalian cells use Ca²⁺, not Na⁺, for cell-to-cell signaling. This holds true because Ca²⁺ acts as a universal second messenger, enabling rapid signal transduction via channels like IP₃ receptors and voltage-gated Ca²⁺ channels, triggering responses such as muscle contraction and neurotransmitter release. Na⁺ primarily drives membrane potential and action potentials but lacks the dynamic cytosolic range (~100 nM resting to 1 µM peaks) needed for signaling specificity, as its levels stay elevated (~10-15 mM) without sharp transients.​

Reason Evaluation

The reason claims cytosolic Na⁺ concentration is lower than Ca²⁺. This is false, as resting cytosolic [Ca²⁺] is ~100 nM, vastly lower than [Na⁺] at 10-15 mM, maintained by Na⁺/K⁺-ATPase pumps against a high extracellular gradient (~140 mM). Even during activity, Na⁺ does not drop below Ca²⁺ peaks, making the reason incorrect.​

Option Breakdown

• (A) Both true, (r) correct reason: Incorrect. Assertion true, reason false.

• (B) Both true, (r) not reason: Incorrect. Reason false.

• (C) Both false: Incorrect. Assertion true.

• (D) (a) true, (r) false: Correct. Matches analysis—Ca²⁺ signals effectively due to low baseline enabling detectable rises, not because Na⁺ is “lower”.​

Key Signaling Insights

• Ca²⁺ Dynamics: Low cytosolic [Ca²⁺] (~10⁻⁷ M) vs. extracellular (~10⁻³ M) allows 10,000-fold spikes for signaling without toxicity.​

• Na⁺ Role: Supports excitability but buffered by NCX exchangers linking to Ca²⁺ indirectly.​

• Exam Tip: Focus on gradients—Ca²⁺’s rarity enables signaling; Na⁺’s abundance suits electrogenic roles.​