Figure 1. Calcium pumps transport Ca2+ from the cytosol to the lumen of the sarcoplasmic/endoplasmic reticulum (SER). Calcium pumps establish a large concentration gradient of calcium between the lumen and the cytosol. When calcium channels open, Ca2+ floods out of the SER which allows muscle contration. Calcium pumps transport Ca2+ back into the lumen to facilitate relaxation.
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