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?

• A. Nontarget cells possess enzymes that immediately degrade the molecules as they enter the cell.
• B. Nontarget cells lack the inactive enzymes that the signal molecules activate.
• C. The signal molecules diffuse from the cell before an effective concentration can be achieved.
• D. In nontarget cells, these signal molecules cross the membranes of the endoplasmic reticulum and are captured by vesicles.
• E. Nontarget cells lack the intracellular receptors that, when activated by the signal molecule, can interact with genes in the cell's nucleus.

Answer: (E)

Verified step by step guidance

1. Understand that testosterone and estrogen are lipid-soluble hormones, which means they can easily pass through the lipid bilayer of the plasma membrane by simple diffusion.
2. Recognize that these hormones can enter all cells due to their ability to diffuse across cell membranes, but not all cells will respond to them.
3. Identify that the key factor determining a cell's response to these hormones is the presence of specific intracellular receptors. Only target cells have these receptors.
4. Learn that when these hormones bind to their specific intracellular receptors, the receptor-hormone complex can then interact with the cell's DNA, leading to changes in gene expression.
5. Conclude that nontarget cells do not respond to these hormones because they lack the specific intracellular receptors necessary for the hormone to exert its effect on gene expression.