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, for 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) requires ATP; works without ATP
B) is small; is large
C) transports calcium slowly; transports calcium rapidly
D) works without ATP; requires ATP
E) is large; is small
Answer: D
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