Final answer:
Endocrine signaling involves the longest time interval between the release of a signaling molecule and receptor activation due to hormone travel through the bloodstream.
Step-by-step explanation:
Among the four categories of chemical signaling - paracrine signaling, endocrine signaling, autocrine signaling, and direct signaling across gap junctions - the one that involves the longest time interval between the release of a signaling molecule and activation of a receptor is endocrine signaling. This is because endocrine signals are carried long distances through the bloodstream by hormones to distant cells. Due to relying on the circulatory system for transport, the onset of the cellular response can be slow and may vary from seconds to hours, depending on the hormone.
The signaling system that involves the longest time interval between release of a signaling molecule and activation of a receptor is endocrine signaling. Endocrine signaling uses the circulatory system to transport ligands, such as hormones, to distant target cells. This distance and reliance on the bloodstream for transport results in a longer time period between release of the signaling molecule and activation of the receptor.