In а resting heаrt muscle cell the cоncentrаtiоn оf Ca2+ ions is low in the cytosol, high in the extracellular space, and high in the sarcoplasmic reticulum (a specialized version of the endoplasmic reticulum). To coordinate muscle contraction, the proteins labeled 1 and 2 are activated and Ca2+ enters the cytosol. To coordinate muscle relaxation, the protein labeled 3 moves Ca2+ back into the extracellular space and the protein labeled 4 moves Ca2+ back into the sarcoplasmic reticulum; this restores the resting state. Protein 1 is a [protein1] and this is an example of [example1]. Protein 2 is a [protein2] and this is an example of [example2]. Protein 3 is a [protein3] and this is an example of [example3]. Protein 4 is a [protein4] and this is an example of [example4]. Ca2+ ions require proteins to move across the membrane because [reasons].