Which of the following signal molecules is not used for extracellular signaling?

(a) Autocrine

(b) Endocrine

(c) Cyclic AMP

(d) None of the above

The correct answer is (c) Cyclic AMP.

Signal molecules used for extracellular signaling are released by a cell and travel outside of it to communicate with other cells. Cyclic AMP is a classic example of an intracellular signal molecule, often called a second messenger.

Explanation

• Extracellular Signaling (First Messengers): This involves molecules that function outside of cells to transmit signals between them. Autocrine and endocrine signaling are two types of extracellular signaling that use molecules like hormones and growth factors as "first messengers."
  • Autocrine: A cell releases a signal that acts on itself. The molecule still has to exit the cell to bind to a surface receptor.
  • Endocrine: A cell releases a hormone that travels through the bloodstream to act on distant cells.
• Intracellular Signaling (Second Messengers): After an extracellular signal molecule (the first messenger) binds to a receptor on the cell's surface, the signal is relayed inside the cell. This is often done by small, non-protein molecules called second messengers. Cyclic AMP (cAMP) is synthesized inside the cell and activates other proteins to continue the signal cascade, leading to a cellular response. It does not leave the cell to signal other cells.