Testosterone and estrogen are lipid-soluble signal molecules that cross the plasma membrane by simple diffusion. If these molecules can enter all cells, why do only specific cells respond to their presence?
Nontarget cells possess enzymes that immediately degrade the molecules as they enter the cell.
Nontarget cells lack the inactive enzymes that the signal molecules activate.
The signal molecules diffuse from the cell before an effective concentration can be achieved.
In nontarget cells, these signal molecules cross the membranes of the endoplasmic reticulum and are captured by vesicles.
Nontarget cells lack the intracellular receptors that, when activated by the signal molecule, can interact with genes in the cell's nucleus.