Final answer:
The correct term for a hormone that initiates cell-signaling pathways is c) First messenger. This hormone binds to cell surface receptors and sets off a signaling cascade within the cell, leading to various cellular responses.
Step-by-step explanation:
Among the options provided, the term for a hormone that initiates cell-signaling pathways is c) First messenger. This hormone is released in response to a stimulus and travels through the bloodstream to reach the target cells. Upon binding to hormone receptors on the cell surface, it triggers a series of events internally, without crossing the cell membrane. This is because hormones like amino acid-derived and polypeptide hormones are not lipid-soluble and bind to extracellular receptors, initiating a signaling cascade inside the cell. Such a hormone is known as a first messenger, which activates subsequent cellular changes through second messengers such as cAMP, enzymes, and other intracellular proteins, ultimately resulting in a cellular response.
In signal transduction, the events following the binding of the hormone are likened to a cascading river flow, with the events categorized as upstream (before activation) and downstream (after activation). The hormone, functioning as a first messenger, is fundamental in this process because it serves as the initial trigger for the signaling pathway.