Uniporter

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A uniporter is an integral membrane protein that is involved in facilitated diffusion. They can be either a channel or a carrier protein.

Uniporter carrier proteins work by binding to one molecule of solute at a time and transporting it with the solute gradient. Uniporter channels open in response to a stimulus and allow the free flow of specific molecules. Uniporters may not utilize energy other than the solute gradient. Thus they may only transport molecules with the solute gradient, and not against it.

There are several ways in which the opening of uniporter channels may be regulated:

Voltage - Regulated by the difference in voltage across the membrane
Stress - Regulated by physical pressure on the transporter (as in the cochlea of the ear)
Ligand - Regulated by the binding of a ligand to either the intracellular or extracellular side of the cell

Uniporters are involved in many biological processes, including impulse transmission in neurons. Voltage-gated sodium channels are involved in the propagation of a nerve impulse across the neuron. During transmission of the signal from one neuron to the next, calcium is transported into the presynaptic neuron by voltage-gated calcium channels. Calcium released from the presynaptic neuron binds to a ligand-gated calcium channel in the postsynaptic neuron to stimulate an impulse in that neuron. Potassium leak channels, also regulated by voltage, then help to restore the resting membrane potential after impulse transmission.

In the ear, sound waves cause the stress-regulated channels in the ear to open, sending an impulse to the vestibulocochlear nerve (VIII).