when do the calcium channels on the sarcoplasmic reticulum normally open
Sarcoplasmic reticulum SERCA consists of 13 subunits (labelled M1-M10, N, P and A). Calcium ions bind to the M1-M10 subunits (which are located within the membrane), whereas ATP binds to the N,P and A subunits (which are located outside the SR). When 2 calcium ions, along with a molecule of ATP, bind to the cytosolic side of the pump (i.e the region of the pump outside the SR), the pump opens. This occurs because ATP (which contains three phosphate groups) releases a single phosphate group (becoming adenosine diphosphate). The released phosphate group then binds to the pump, causing the pump to change shape. This shape change causes the cytosolic side of the pump to open, allowing the two Ca2+ to enter. The cytosolic side of the pump then closes and the sarcoplasmic reticulum side opens, releasing the Ca2+ into the SR.[6]