Which type of protein allows ions to pass through the cell membrane?

Ion channels are specialized types of proteins that facilitate the movement of ions across the cell membrane. These channels provide a passageway that is selective for specific ions, allowing them to flow in and out of the cell according to their concentration gradients. This movement is crucial for various cellular functions, including the generation of electrical signals in nerve and muscle cells.

Ion channels typically open or close in response to certain stimuli, such as changes in voltage across the membrane or the binding of signaling molecules. This dynamic regulation helps maintain ion balance, which is essential for processes like nerve impulse transmission and muscle contraction.

In contrast, carrier proteins and transport proteins serve different roles. Carrier proteins bind to specific substances and undergo a conformational change to shuttle those substances across the membrane, but they do not form open channels through which ions can pass freely. Receptor proteins are involved in signal transduction and do not facilitate the direct transport of ions across the membrane. Therefore, ion channels are the specific proteins designed for the passage of ions, making them the correct answer.