Advanced Placement (AP) Biology Practice Exam

What is typically called the signal molecule that binds to a receptor?

1. Hormone

2. Ligand

3. Enzyme

4. Transmitter

The correct answer is: Ligand

The term that is typically used to describe a signal molecule that binds to a receptor is "ligand." In cellular signaling, ligands can include a wide variety of molecules such as hormones, neurotransmitters, or even drugs that interact with specific receptors on the surface of cells. When a ligand binds to its receptor, it triggers a response within the cell, leading to a cascade of biochemical events that ultimately result in a specific physiological effect. This definition highlights the importance of the ligand's role in initiating signaling pathways. It is essential for understanding cellular communication and the mechanisms through which cells respond to their environment. Ligands can be as small as ions or gases or as complex as peptides and proteins, underscoring the diverse nature of the molecules involved in signaling processes.