Final answer:
Internal receptors bind hydrophobic ligands that can cross the plasma membrane and directly interact with DNA, whereas cell-surface receptors bind hydrophilic ligands that signal through membrane-associated mechanisms, influencing cellular processes without entering the cell.
Step-by-step explanation:
The differences in the signal or ligand composition between an externally received signal and internal one pertain to the solubility and receptor interaction of the ligands. Internal receptors are located in the cell cytoplasm and bind hydrophobic ligands that can cross the plasma membrane.
These receptor-ligand complexes then move to the nucleus to interact directly with DNA.
On the other hand, hydrophilic ligands cannot pass through the plasma membrane, hence they bind to cell-surface receptors, which transmit the signal inside the cell through various mechanisms like ion channel-linked receptors, G-protein-linked receptors, and enzyme-linked receptors.
For example, steroid hormones, which are small and hydrophobic, can easily penetrate the cell membrane and activate internal receptors, directly influencing gene expression.
Contrastingly, insulin, a water-soluble peptide hormone, must bind to cell-surface enzyme-linked receptors to exert its effects.