Which statement describes a common theme in signal transduction pathways?

• A. Receptors are always active without ligands
• B. Binding causes receptor to change shape and be activated
• C. Ligands are always internalized immediately
• D. No change occurs to the responding molecule

Answer: (B)

When a signal binds to a receptor, it usually causes the receptor to change shape and become activated, starting the signaling cascade inside the cell. This conformational shift acts as the switch that converts an extracellular cue into intracellular actions, such as activation of enzymes, opening/closing of channels, or changes in gene expression.

Receptors are not typically active without ligands, so the idea that they’re always on is incorrect because many receptors need the ligand to switch from an inactive to an active state. Ligands are not always internalized immediately; binding at the cell surface often triggers signaling right away, and internalization is not universal. And finally, signaling does involve changes to the responding molecules, such as phosphorylation or production of second messengers, so the notion that no change occurs is also inaccurate.