Ligand responsiveness — For a cell to respond to a particular signaling molecule, what must be true?

Introduction:
Specificity in signaling arises because only cells with the appropriate receptor can detect and initiate a response to a given ligand. This question tests the core requirement for ligand responsiveness.

Given Data / Assumptions:

• A defined extracellular ligand is present.
• Cells differ in receptor expression.
• Downstream signaling machinery is otherwise intact.

Concept / Approach:
The presence of the correct receptor (cell surface or intracellular) is the minimal requirement for detection. Binding triggers conformational changes and downstream transduction. Production site, tissue type, or proximity to the secreting cell are not prerequisites if the receptor and delivery route exist.

Step-by-Step Solution:

Verification / Alternative check:
Transfecting a receptor into a nonresponsive cell line confers ligand responsiveness, confirming the necessity and sufficiency of receptor expression with intact downstream pathways.

Why Other Options Are Wrong:

• Heart muscle location: irrelevant unless the ligand targets heart specific receptors.
• Site of production: autocrine signaling is not required; paracrine/endocrine signaling also occurs.
• Incapable of transduction: contradicts responsiveness.
• Adjacency to the gland: endocrine signals travel via blood to distant targets.

Common Pitfalls:
Assuming that any cell exposed to a hormone will respond. Only receptor positive cells can respond appropriately.

Final Answer:
It contains the receptor for the signaling molecule.