A protein, is a protein that changes conformation as each ion or molecule crosses the membrane it is embedded in. This is in contrast to a channel, which lets molecules through without changing conformation for each molecule or ion. Channels can change conformation to open, close or inactivate, but they do not change conformation with each molecule that crosses.

The correct answer is: carrier.

Explanation:

Carrier proteins are a type of integral proteins (meaning they're embedded in the membrane) that facilitate the transport of ions, charged particles, and polar molecules.

Carrier proteins undergo conformational changes when ions or polar molecules bind to them, which makes them rearrange in a way these particles and molecules can cross through the membrane.

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