Final answer:
A hormone, neurotransmitter, or paracrine messenger must first bind to a specific receptor on the target cell, leading to a sequence of intracellular events that mediate the cell's response.
Step-by-step explanation:
A hormone, neurotransmitter, or paracrine messenger must first bind to a specific receptor on the target cell. This receptor can be located on the surface of the cell or within the cell, and it is this specific binding that allows the hormone to exert its effect only on cells that have the necessary receptors. Hormone receptors are crucial for mediating the effects of hormones, as they can initiate signal transduction pathways leading to various cellular responses. Hormones that are not lipid-soluble typically bind to cell surface receptors, inducing signaling pathways without entering the cell. These hormones are known as first messengers. The sequence of events includes the hormone binding to its receptor, activating G proteins and effector proteins, and possibly leading to the generation of second messengers within the cell that amplify the signal and lead to the desired cellular response.