Final answer:
Internal or intracellular receptors require hydrophobic ligands that can cross the plasma membrane, forming a complex that interacts directly with DNA in the nucleus to regulate gene expression.
Step-by-step explanation:
Type of Cell Signal Receptors and Ligand Binding
The type of cell signal receptor that requires a ligand-receptor complex to be free in the cytosol or located in the nucleus is commonly referred to as internal receptors or intracellular receptors. These receptors are distinct because they bind to hydrophobic ligands, such as steroids, which can cross the plasma membrane and enter the cell due to their nonpolar nature. Internal receptors and their ligands form complexes that interact directly with cellular DNA in the nucleus to regulate gene expression and initiate the process of transcription and translation.
It's important to distinguish between internal and cell-surface receptors. Cell-surface receptors, on the other hand, attach to hydrophilic ligands, which cannot easily cross the plasma membrane, resulting in signal transduction through a series of intracellular events rather than direct DNA interaction.
Overall, it is the hydrophobic, small molecular ligands that can diffuse through the cell membrane's lipid bilayer and bind to internal receptors. The resulting ligand-receptor complex moves to the nucleus, where it has the capability to affect transcriptional activity and thereby impact protein synthesis.