Biological Systems MCAT Practice Exam

During repolarization, what happens to the K+ channels?

1. They remain inactive

2. They open slowly

3. They close rapidly

4. They open quickly

The correct answer is: They open slowly

During repolarization, the K+ channels play a crucial role in restoring the membrane potential after depolarization. When an action potential is generated, voltage-gated Na+ channels open first, leading to a rapid influx of Na+ ions and a positive shift in membrane potential. Following this rapid depolarization, the K+ channels respond to the change in voltage. These voltage-gated K+ channels do not open immediately; they have a slower activation phase compared to Na+ channels. During repolarization, they open slowly after the peak of the action potential has been reached. The delayed opening allows K+ ions to flow out of the cell, which helps to bring the membrane potential back down toward its resting state. This characteristic of K+ channels is essential for the proper timing of the action potential. During repolarization, while the Na+ channels are nearing inactivation and beginning to close, the K+ channels are being activated slowly, allowing K+ to exit efficiently and facilitate the restoration of the resting membrane potential. Thus, the key feature of K+ channels during this phase is their slow opening in response to the depolarization that has just occurred.