Advanced Placement (AP) Biology Practice Exam

What happens when a signal molecule binds to a receptor protein?

1. It degrades

2. It changes shape

3. It replicates

4. It exits the cell

The correct answer is: It changes shape

When a signal molecule binds to a receptor protein, the receptor undergoes a conformational change, or change in shape. This alteration is crucial because it activates the receptor, enabling it to initiate a series of intracellular processes. The binding of the signal molecule can trigger various effects, such as the activation of enzymes, opening of ion channels, or the initiation of signaling cascades that impact gene expression or cellular responses. This process is fundamental in cellular communication and signaling pathways, allowing cells to respond appropriately to external signals and maintain homeostasis. The other options do not accurately describe the receptor's response to signal molecule binding. For instance, degradation and replication are not characteristic responses of receptor proteins upon ligand binding, and exiting the cell is not applicable since receptors are typically situated on the cell surface or within the cell.