How do the principles of protein-ligand interactions provide a foundation for understanding signal transduction pathways in cells?

• A. They demonstrate how ligands are permanently bound to receptors, preventing any further signaling.
• B. They show how proteins are converted into ligands during signal transduction.
• C. They explain how ligands are converted into products during signal transduction.
• D. They illustrate how ligands can bind to receptors, initiating a cascade of cellular responses.

Answer: (D)