E) All of the answers are correct.
Ion channels are proteins located in the membrane that allow ions to pass through the channel pore. Ion channels can be classified (by what opens and closes the channels) into few groups:
• Voltage-gated channels-open and close in response to membrane potential
Examples: Voltage-gated proton channels, Voltage-gated sodium channels, Voltage-gated potassium channels
• Ligand-gated (neurotransmitter) or ionotropic receptors (ligand binds to extracellular domain of the receptor
Examples: "nicotinic" Acetylcholine receptor, ionotropic glutamate-gated receptors
• Lipid-gated- lipid molecules bind to the transmembrane domain of the channel
Example: Inward-rectifier potassium channels
• Light-gated channels-like rhodopsin channel that is directly opened by photons
• Mechanosensitive (e.g. stretch, pressure) ion channels.