Final answer:
Ligand-gated channels open when a signaling molecule binds to the extracellular region, causing a conformational change that allows ions to pass through the channel and change the cell's charge.
Step-by-step explanation:
A ligand-gated channel opens because a signaling molecule, a ligand, binds to the extracellular region of the channel. This type of channel is also known as an ionotropic receptor because when the ligand, known as a neurotransmitter in the nervous system, binds to the protein, ions cross the membrane changing its charge. The conformational change occurs when the ligand binds to the extracellular region of the channel, allowing ions to pass through the channel and affecting the cell's charge.