Final answer:
First messengers are often hydrophilic hormones that bind to cell-surface receptors, initiating a signaling cascade, not hydrophobic as the statement suggests. Hydrophobic signaling molecules usually interact with intracellular receptors to directly affect gene expression.
Step-by-step explanation:
The statement that first messengers that activate membrane-bound receptors are hydrophobic is false. In biological systems, first messengers are often hydrophilic hormones that cannot diffuse through the lipid bilayer of the cell membrane. These hormones, also known as ligands, bind to cell-surface receptors, which are located on the exterior of the cell membrane. For example, amino acid-derived hormones, except for thyroid hormones, are not lipid-soluble and must activate these membrane-bound receptors to initiate a signaling cascade within the cell.
Once a hydrophilic first messenger binds to a receptor on the cell surface, it typically leads to the activation of a second messenger like cyclic AMP (cAMP) within the cell. This then triggers a series of events that result in a cellular response. On the other hand, hydrophobic signaling molecules can diffuse across the plasma membrane and interact with intracellular receptors in the cytoplasm. These receptors often function as transcription factors and can directly regulate gene expression.