Q.12 Determine the correctness or otherwise of the following Assertion [a] and the
Reason [r].
Assertion [a]: Nitric oxide is involved in transient paracrine and autocrine signaling.
Reason [r]: Nitric oxide is highly reactive, with a lifetime of few seconds, yet can
diffuse freely across membranes
(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) Only [a] is true but [r] is false

Both Assertion [a] and Reason [r] are true, and [r] correctly explains [a]. The correct answer is (A).

Nitric oxide (NO) functions as a key signaling molecule in biological systems, particularly through transient paracrine and autocrine pathways due to its unique chemical properties. This makes it highly relevant for CSIR NET Life Sciences exam preparation in topics like cell signaling and physiology.​

Assertion [a] Analysis

Assertion [a] states that nitric oxide is involved in transient paracrine and autocrine signaling. Nitric oxide acts as both an autocrine (affecting the same cell) and paracrine (affecting nearby cells) signal, modulating processes like vasodilation, neurotransmission, and myocardial function. Its signaling is transient because of rapid degradation, limiting effects to short durations and distances.​

Reason [r] Analysis

Reason [r] claims nitric oxide is highly reactive with a lifetime of a few seconds, yet diffuses freely across membranes. NO has a biological half-life ranging from milliseconds to a few seconds (often <6 seconds) due to reactions with oxygen, hemoglobin, and superoxide, confirming its high reactivity. Despite this, NO freely diffuses across lipid membranes because of its small size and lipophilicity, similar to oxygen.​

Option Evaluation

• (A) Both true, [r] explains [a]: Correct. NO’s short lifetime ensures transient signaling, while free diffusion enables paracrine/autocrine action without receptors or vesicles—directly linking [r] to [a].​

• (B) Both true, but [r] not explanation: Incorrect. The properties in [r] precisely cause the transient nature in [a].

• (C) Both false: Incorrect. Scientific evidence confirms both statements.​

• (D) Only [a] true, [r] false: Incorrect. [r] accurately describes NO’s reactivity, lifetime, and diffusion.​

Nitric oxide transient paracrine autocrine signaling is a cornerstone concept in cell biology for CSIR NET Life Sciences aspirants. This assertion reason question tests understanding of NO as a gaseous signaling molecule that diffuses freely yet acts transiently due to its reactivity.​

Why NO Excels in Paracrine/Autocrine Roles

NO, produced by nitric oxide synthase enzymes, bypasses traditional receptor-mediated pathways. It diffuses across membranes to nearby cells (paracrine) or back to the producing cell (autocrine), regulating vascular tone, neuronal plasticity, and immune responses. Its effects remain transient—lasting seconds—preventing prolonged activation.​

Chemical Basis: Reactivity Meets Diffusion

Highly reactive NO reacts rapidly with hemoglobin, superoxide, and oxygen, yielding a half-life of 0.1–6 seconds in tissues. Yet, its uncharged, lipophilic nature allows unrestricted membrane crossing, enabling local signaling without transporters. This balance defines nitric oxide transient paracrine autocrine signaling.​

CSIR NET Exam Insights

In assertion reason formats, link properties (short lifetime, free diffusion) directly to functions (transient signaling). Option (A) fits perfectly, as seen in vascular homeostasis and myocardial modulation. Practice similar questions on gasotransmitters like NO, CO, and H2S.​