Which of the following statements correctly describes the characteristics and primary function of second messengers in intracellular signaling pathways?
- A) They are transmembrane protein receptors that undergo conformational changes upon binding extracellular ligand molecules.
- B) They are large protein kinases that enter the nucleus to directly phosphorylate structural chromosomal proteins.
- C) They are hydrophobic steroid hormones that cross the plasma membrane to act directly as transcription factors.
- D) They are small, non-protein, water-soluble molecules or ions that diffuse rapidly through the cytosol to relay and amplify signals.
