Which statement about indirect signal transduction is accurate?

• A. It involves a second messenger in the cytoplasm mediating interaction between receptor and effector proteins
• B. It uses direct transcriptional changes without cytoplasmic messengers
• C. It does not involve receptor proteins
• D. It occurs exclusively at the cell surface

Answer: (A)

Indirect signal transduction uses cytoplasmic second messengers to relay the signal from the receptor to intracellular targets. When a ligand binds a receptor, the receptor typically activates a cascade—such as G proteins or kinases—that generates messengers like cAMP, IP3, DAG, or Ca2+. These second messengers diffuse or travel within the cytoplasm to activate effector proteins, ultimately producing the cellular response. This framework distinguishes indirect signaling from direct transcriptional changes that occur without such cytoplasmic messengers. It also requires a receptor to initiate the process and isn’t confined to events at the cell surface, since the critical actions happen inside the cell after the receptor is engaged. So the statement describing a second messenger in the cytoplasm mediating interaction between receptor and effector proteins best captures indirect signal transduction.