Final answer:
Receptor proteins must be present in the target cell that is receiving the signal.
Step-by-step explanation:
True. Receptor proteins must be present in the target cell that is receiving the signal. Cell-surface receptors, also known as transmembrane receptors, are integral proteins that bind to external ligand molecules. These receptors span the plasma membrane and perform signal transduction, converting an extracellular signal into an intracellular signal. Ligands that interact with cell-surface receptors do not have to enter the cell that they affect. Internal receptors, on the other hand, are found in the cytoplasm of target cells and respond to hydrophobic ligands that can cross the plasma membrane.
While there are also internal receptors for hydrophobic ligands, cell-surface receptors typically deal with hydrophilic molecules. The cell-surface receptor-ligand binding does not require the ligand to enter the cell, as they can activate a response simply by binding at the cell's surface.