Final answer:
Target cells have either internal receptors that interact with hydrophobic ligands inside the cell or cell-surface receptors that translate extracellular signals into intracellular responses without the ligand entering the cell.
Step-by-step explanation:
Target cells typically have types of receptors that are specialized for binding specific ligands. There are two principal kinds of receptors: internal receptors and cell-surface receptors. Internal receptors, also known as intracellular or cytoplasmic receptors, are located within the cytoplasm and interact with hydrophobic ligand molecules that can cross the cell membrane. They play a key role in regulating gene expression through direct action on DNA or through other intracellular signaling pathways.
Cell-surface receptors, on the other hand, are integral proteins embedded in the plasma membrane of the cell. They operate through signal transduction, converting an extracellular signal into an intracellular one without the ligand having to enter the cell. These receptors include ion channel-linked receptors, which open ion channels in response to a ligand, allowing specific ions to pass through the membrane, changing the cell's internal conditions. Cell-surface receptors act as markers because they are specific to individual cell types and play crucial roles in processes like immune responses and cell communication.