So this is the calcium Atpas, the lumen side of the sarcoplas reticulum, the cytoplasm side of the sarcoplas reticulum. Now, in the e one stage in the e one state, notice that this structure basically is open to this side of the cytoplasm. And two, not one, but two calcium ions can move into this pocket of the transmembrane domain. So it's the transmembrane domain that is responsible for actually binding those calcium ions. And it's two calcium ions per one ATP that is actually pumped to the other side of that particular membrane. So in the e one state, we see that the ATP isn't actually bound and these calcium ions can move into the transmembrane domain, as shown here.