Ca2+ is a critical signaling molecule in muscle cells. its cytosolic concentration is very low, but high concentrations of ca2+ can be found in the sarcoplasmic reticulum where ca2+ is stored. which of the following statements is most correct to explain the fast release of ca2+ from the sarcoplasmic reticulum into the cytosol? o a ca2+ uniporter that takes advantage of the steep electrochemical gradient for ca2+ is necessary for this regulated transport step o an atp-driven ca2+ pump is necessary to transport ca2+ from the sarcoplasmic reticulum into the cytosol o a stimulus promotes the opening of a ca2+ channel present in the membrane of the sarcoplasmic reticulum. o ca2+ moves across the membrane by diffusion, which is made possible by the steep electrochemical gradient across the membrane of the sarcoplasmic reticulum and the reduced thickness of the membrane of this er-like organelle. o a ca2+ channel with a selectivity filter for ca2+ is necessary in the plasma membrane