Subtopic: Fundamentals of Cell structure and signaling
Question Statement:
Determine the correctness or otherwise of the following Assertion [a] and the
Reason [r] regarding mammalian cells.
Assertion [a]: Cells use Ca'*, and not Na*, for cell-to-cell signaling
Reason [r]: In the cytosol, concentration of Na* is lower than that of Ca?+
Options:
(A) Both [a] and [r] are true and [r] is the correct reason for [a].
(B) Both [a] and [r] are true but [r] is not the correct reason for [a].
(C) Both [a] and [r] are false.
(D) [a] is true but [r] is false.
Official Answer Key:
D
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2020 Q.15)**
**1. Governing Concept**:
- **Topic**: Cell Biology & Second Messenger Signal Transduction
- Evaluating Assertion and Reason:
- **Assertion [a]**: *"Cells use $\text{Ca}^{2+}$, and not $\text{Na}^+$, for cell-to-cell / intracellular signaling."* $\rightarrow$ **TRUE**. $\text{Ca}^{2+}$ is a ubiquitous intracellular second messenger whose cytosolic concentration changes rapidly by 10- to 100-fold upon receptor activation.
- **Reason [r]**: *"In the cytosol, concentration of $\text{Na}^+$ is lower than that of $\text{Ca}^{2+}$."* $\rightarrow$ **FALSE**.
- Resting cytosolic $[\text{Ca}^{2+}]$ is extremely low ($\approx 100\text{ nM} = 10^{-7}\text{ M}$).
- Resting cytosolic $[\text{Na}^+]$ is relatively high ($\approx 10-15\text{ mM} = 10^{-2}\text{ M}$).
- Thus, cytosolic $[\text{Na}^+]$ is $\approx 100,000$-fold *higher* than $[\text{Ca}^{2+}]$, directly contradicting [r].
**2. Option Analysis**:
- **Option (A) is INCORRECT**: Reason [r] is completely false.
- **Option (B) is INCORRECT**: [r] is false.
- **Option (C) is INCORRECT**: Assertion [a] is true.
- **Option (D) [[a] is true but [r] is false] is CORRECT**: Resting cytosolic calcium is nanomolar, orders of magnitude lower than cytosolic sodium.
**3. Final Answer**:
- **Correct Choice**: **D**
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