What characterizes receptors on target cells?

• A. Low specificity
• B. Permanent binding
• C. Proteins with specific binding
• D. Always located in cytosol

Answer: (C)

Receptors on target cells are primarily characterized by their nature as proteins that possess specific binding sites for particular ligands, such as hormones, neurotransmitters, or other signaling molecules. This specificity allows receptors to selectively interact with certain substances, enabling cells to respond appropriately to various signals in their environment. The unique shape and chemical properties of these receptors ensure that they only bind to specific molecules, leading to precise biological responses.

In contrast, low specificity would imply that receptors could bind a wide range of molecules without discrimination, which is not true for most receptor systems. Permanent binding suggests that once a molecule is bound to the receptor, it remains there indefinitely, which is not generally the case; most receptor-ligand interactions are transient. While some receptors are located in the cytosol (intra-cellular receptors), not all receptors are found there, as many are located on the cell membrane, highlighting that this is not a universal characteristic. Therefore, the defining feature of receptors on target cells is their status as proteins with specific binding sites tailored to interact with specific ligands.