Final answer:
Ion channels are protein conduits in the plasma membrane that allow specific ions to pass through. They facilitate electrical transmission in nerve cells and muscle contraction in muscle cells. Ion channels can be always open or gated, and they respond to different types of stimuli.
Step-by-step explanation:
Ion channels are protein conduits in the plasma membrane that are permeable to specific ions. They allow the movement of ions such as sodium, potassium, calcium, and chloride through the membrane, facilitating electrical transmission in nerve cells or muscle contraction in muscle cells. Ion channels are specific and can be always open or "gated", meaning they can be opened or closed in response to different types of stimuli.