And clearly, this is a much higher concentration than this. So the pump uses ATP to establish this gradient. And then when an action potential actually comes into that muscle cell, the protein channels not shown in this diagram open up. And because of the existence of this electrochemical gradient, the calcium ions can quickly and effectively move into the cytoplasm where they are needed to basically contract that muscle. So let's take a look at the following diagram. This is basically the generalization of how the ptype ATPases actually work.