Receiving and decoding messages — who performs the task? When a signaling molecule reaches its appropriate target cell, which specialized proteins receive it and initiate cellular responses?

Introduction:
Cell communication requires the detection of extracellular cues by dedicated proteins that can translate binding into intracellular action. These detector proteins are receptors, which then engage transducers such as G proteins, kinases, or ion channels to produce a response.

Given Data / Assumptions:

• First messengers bind specific receptors based on shape and chemistry.
• Receptors can be GPCRs, receptor tyrosine kinases, ligand-gated channels, or intracellular nuclear receptors.
• Downstream components amplify and diversify the signal.

Concept / Approach:

Separate the “receiver” from the “transducer.” Receptors are the primary receiving molecules; G proteins and kinases are part of subsequent transduction. Thus, the proper completion of the statement assigns the reception step to receptors on/in the target cell.

Step-by-Step Solution:

Verification / Alternative check:

Pharmacology defines drug “receptors” precisely as the macromolecules with which drugs/ligands interact to initiate a biological response.

Why Other Options Are Wrong:

Option A/B misplace G proteins as primary receivers; they are downstream transducers.

Option D/E substitute unrelated components and break the logical flow of message reception.

Common Pitfalls:

Equating any downstream protein with the receptor, or assuming all receptors are at the surface; some are intracellular.

Final Answer:

Signaling molecule; target cell; receptors