Advanced Placement (AP) Biology Practice Exam

What function does a ligand-gated ion channel serve?

1. It alters the structure of signaling proteins

2. It transfers phosphate groups from ATP to proteins

3. It opens or closes in response to signaling chemicals

4. It influences gene expression within the cell

The correct answer is: It opens or closes in response to signaling chemicals

A ligand-gated ion channel serves the critical role of allowing ions to flow across a cell membrane in response to the binding of specific signaling molecules, or ligands. When a ligand, such as a neurotransmitter, binds to the channel, it causes a conformational change in the channel's structure, resulting in the opening or closing of the pore through which ions can pass. This process is essential for various physiological functions, including the transmission of nerve impulses and muscle contraction, as the flow of ions can lead to changes in the membrane potential of the cell. Other options describe different cellular processes: altering the structure of signaling proteins pertains to the role of allosteric regulators or kinases, transferring phosphate groups from ATP to proteins relates to kinase activity and phosphorylation events, and influencing gene expression is typically associated with transcription factors or signaling pathways that affect nuclear activity. These processes are distinct from the direct ion movement facilitated by ligand-gated ion channels.