So channels are these transmembrane proteins that use the same electrochemical gradient that is established by these pumps to actually move ions and molecules across the membrane down that electric chemical gradient. And what that means is membrane channels, unlike membrane pumps, do not actually use any energy. So we see that membrane pumps use energy to establish the electric chemical gradients, but then the membrane channels use those same electrochemical gradients to then move these ions and molecules spontaneously across that cell membrane without actually using any energy. And these membrane channels basically respond to chemical or sometimes physical changes in the environment around that particular channel. And that's exactly what stimulates the opening or the closing of that channel as we'll discuss in just a moment. So we're going to focus on three different types of membrane channels.