Anumber of systems for pumping ions across membranes are powered by atp. such atp-powered pumps are often called atpases although they don't often hydrolyze atp unless they are simultaneously transporting ions. because small increases in calcium ions in the cytosol can trigger a number of different intracellular reactions, cells keep the cytosolic calcium concentration quite low under normal conditions, using atp-powered calcium pumps. for example, muscle cells transport calcium from the cytosol into the membranous system called the sarcoplasmic reticulum (sr). if a resting muscle cell's cytosol has a free calcium ion concentration of 10-7 while the concentration in the sr is 10-2, then how is the atpase acting?