<unk> s are depicted by the red circles. A gradient is represented by the different concentration of<unk> s on either side of the membrane. The open conformation of the<unk>  channel allows for the translocation of<unk> s across the cell membrane, while the closed conformation does not.<unk> s are a class of transmembrane proteins that form<unk>  channels that are activated by changes in the electrical membrane potential near the channel. The membrane potential alters the conformation of the channel proteins, regulating their opening and closing. Cell membranes are generally impermeable to<unk> s, thus they must diffuse through the membrane through transmembrane protein channels. They have a crucial role in excitable cells such as neuronal and muscle tissues, allowing a rapid and co-ordinated depolarization in response to triggering<unk>  change. Found along the axon and at the synapse,<unk> s directionally propagate electrical signals.<unk> s are usually<unk> -specific, and channels specific to sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl−)<unk> s have been identified. The opening and closing of the channels are triggered by changing<unk>  concentration, and hence charge gradient, between the sides of the cell membrane.
Voltage-gated ion channel
Cyclic nucleotide–gated ion channel
G protein-gated ion channel
Mechanosensitive channels
Neurotransmitter transporter
Glutamate transporter
Carboxylate transporter
Sodium-chloride symporter