Final answer:
Non-steroid hormones bind to cell membrane receptors, triggering a second messenger system, differing from steroid hormones that can cross membrane barriers and influence gene transcription directly.
Step-by-step explanation:
Unlike steroid hormones, non-steroid hormones, composed of amino acids or polypeptides, cannot diffuse across the plasma membrane due to their lipid insolubility. Instead, they bind to hormone receptors on the cell membrane. The process involves a non-steroid hormone approaching and binding to a receptor on the cell's surface, triggering an enzyme within the cell membrane. This activates a molecule known as a second messenger, which in turn initiates various intracellular activities. An example of this chain reaction in action involves the hormone epinephrine, which, when it binds to its receptor, activates a G-protein that then influences the formation of cAMP, another second messenger, ultimately resulting in a cellular response such as converting glycogen to glucose.
Steroid hormones, on the other hand, such as estrogen, are able to cross the plasma membrane and bind with intracellular receptors in the cytoplasm or nucleus, directly influencing gene transcription and protein synthesis. Lipid-soluble hormones that are not steroids, like vitamin D and thyroxin, also associate with receptors inside the nucleus and play a role in gene transcription.