Channel Protein

A channel protein is an integral membrane protein that spans a cell membrane and contains a hydrophilic pore through which specific ions or molecules can pass. Channel proteins facilitate passive transport (facilitated diffusion) - they allow substances like ions or water to flow down their concentration gradients across the membrane.

• Channel proteins are typically <u>selective</u> for particular substances: e.g., an ion channel might allow Na+ but not K+ through, and aquaporins are channel proteins specific for water.
• Channels can be <u>gated</u> or open: a gated channel protein opens only in response to a stimulus (such as a chemical ligand or a voltage change), whereas some channels (like certain leak channels) remain open all the time.
• Channels move solutes much faster than carrier proteins - millions of ions per second. Unlike carrier transporters, channel-mediated diffusion does not exhibit saturation at physiological concentrations in the same way, due to not binding each individual molecule.

• A classic comparison question: *channels vs carriers*. Correct answer: <u>channel proteins</u> provide a pore for substances to diffuse rapidly across membranes (passive, no energy required), whereas carrier proteins bind substances and undergo shape changes - making carriers slower and sometimes able to perform active transport.
• Scenario recognition: if a question describes ions flowing into a neuron when a membrane protein opens in response to a signal, that membrane protein is a gated ion <u>channel</u>. For instance, Na+ rushing into a neuron during an impulse indicates voltage-gated Na+ channels opening.