Ligand-Receptor Binding in Cell Signaling

Steps:

• Signal molecules, or ligands, travel to target cells and interact with their surface.
• Detection occurs when the ligand fits specifically into a receptor protein on the cell membrane.
• This binding triggers intracellular responses, like enzyme activation or ion channel opening.
• Specificity ensures only target cells respond, based on complementary molecular shapes.

Why A is correct:

• Receptors act as locks, and signal molecules as keys with complementary shapes, enabling specific recognition per the lock-and-key model in biochemistry.

Why the others are wrong:

• B: Diffusion explains molecule transport, not detection by the target cell.
• C: Endocytosis internalizes some signals but is not the primary detection mechanism.
• D: Binding is specific to matching receptor types, not any receptor.

Final answer: A