The lab you work in has discovered a previously extracellular signal molecule called QGF, a protein. You add QGF to different types of cells to determine its effect on these cells. When you add QGF to heart muscle cells, you observe an increase in cell contraction. When you add it to they undergo cell division. When you add it to nerve cells, they die. When you add it to glial cells, you do not see any effect on cell division or survival. Given these observations, which of the following statements is most likely to be true? 1. Because it acts on so many diverse cell types, QGF probably diffuses across the plasma membrane into the cytoplasm of these cells. 2. Glial cells do not have a receptor for QGF. Q QGF activates different intracellular signaling pathways in heart muscle cell, and nerve cells to produce the different responses observed. 3. Heart muscle cells, and nerve cells must all have the same receptor for QGF