Testosterone does not affect all cells of the body because __________.

Receptors for signal molecules __________.

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?

Steroid hormones can enter a cell by simple diffusion. Therefore, steroids __________.

G-protein-coupled receptors __________, whereas receptor tyrosine kinases __________.

Which of the following is activated when the binding of single molecules causes it to form a dimer?

Domoic acid is a neurotoxin produced by some species of alga. Domoic acid binds to the kainate receptor on neurons in parts of the brain. The kainate receptor facilitates the movement of calcium into the cell. Excess domoic acid–induced stimulation of the neural kainate receptors causes neural damage and short-term memory loss. Based on this, which of the following is likely true?

A G protein is active when __________.