Calcium Ions (Ca²⁺) Are Present in the Endoplasmic Reticulum at Concentrations

Multiple Choice

Calcium ions (Ca²⁺) are present in the endoplasmic reticulum at concentrations 1,000 times higher than in the cytosol (the gel-like part of the cell outside organelles) . One protein on the endoplasmic reticulum membrane is devoted to allowing Ca²⁺ to move from the endoplasmic reticulum to the cytosol, and another protein moves Ca²⁺ from the cytosol to the endoplasmic reticulum. These processes are not trivial-Ca²⁺ released into the cytosol allows, for example, muscle contraction and communication between nerve cells. In this pair of Ca²⁺ transport proteins, you would predict that the protein moving Ca²⁺ from the endoplasmic reticulum to the cytosol ________ and the protein that moves Ca²⁺ from cytosol to the endoplasmic reticulum ________.

A) transports calcium slowly; transports calcium rapidly
B) works without ATP; requires ATP
C) requires ATP; works without ATP
D) is large; is small
E) is small; is large