Final answer:
There is much greater diversity in ligands that bind to cell-surface receptors compared to those that can pass through the membrane unaided because water-soluble ligands are polar and cannot cross the hydrophobic plasma membrane.
These ligands, including small molecules, peptides, and proteins, bind to cell-surface receptors which span the plasma membrane and convert external signals into internal responses.
Step-by-step explanation:
The diversity of signal molecules, or ligands, that bind to the surface of a cell is much greater compared to those that travel through the membrane unaided due to the nature of the cell membrane and the characteristics of ligands.
Water-soluble ligands are polar and generally cannot pass through the hydrophobic plasma membrane without assistance; they are often too large as well. These ligands include a diverse group such as small molecules, peptides, and proteins.
They bind to the extracellular domain of cell-surface receptors, which are special membrane-anchored proteins that span the plasma membrane and are responsible for signal transduction, a process that converts an extracellular signal into an intracellular one.
On the other hand, nonpolar molecules, like steroids, can diffuse across the cell membrane and bind to internal receptors within the cell. Polar ligands need cell-surface receptors because their polarity prevents them from passing through the lipid bilayer of the plasma membrane.
Cell-surface receptors allow for a highly specific interaction with ligands, resulting in a cascade of intracellular events leading to a cellular response. This specificity contributes to the diversity of extracellular signals that a cell can discern.