Final answer:
The claim that hydrophobic extracellular signaling molecules must bind to cell-surface receptors is false. These molecules can directly diffuse through the cell membrane and interact with internal receptors to modulate transcription and gene expression.
Step-by-step explanation:
The statement that extracellular signaling molecules that are hydrophobic must bind to cell-surface receptors to signal a target cell to change behavior is false. Hydrophobic signaling molecules, such as steroid hormones, are able to diffuse across the plasma membrane due to their ability to move through the hydrophobic lipid bilayer. Once inside the cell, they can bind to internal receptors which are often transcription factors. These internal receptors then enter the nucleus and initiate transcription, affecting gene expression.
In contrast, extracellular signaling molecules that are hydrophilic are unable to cross the plasma membrane and thus have to bind to cell-surface receptors. This binding initiates a signaling cascade through a variety of signal transduction mechanisms, eventually resulting in cellular responses. Therefore, cell-surface receptors are generally associated with hydrophilic, not hydrophobic, signaling molecules.