What mechanism binds to receptors on the target cell membrane, triggering second messengers to affect the cell's activities?

Nonsteroid hormones bind to receptors on the target cell membrane and initiate a cascade of events that involve second messengers to exert their effects on the cell's activities. Unlike steroid hormones, which are lipid-soluble and can pass through the cell membrane to directly alter gene expression, nonsteroid hormones are usually water-soluble and cannot penetrate the lipid bilayer.

When a nonsteroid hormone, such as a peptide or protein hormone, binds to its specific receptor on the cell surface, this interaction activates the receptor and triggers a change in the receptor's structure. The activated receptor then communicates with intracellular signaling pathways, often involving second messengers like cyclic AMP (cAMP), calcium ions, or inositol triphosphate (IP3). These second messengers amplify the signal within the cell, leading to a variety of biological responses such as metabolic changes, gene expression alterations, or the activation of other signaling pathways.

This mechanism highlights the importance of nonsteroid hormones in cellular communication and regulation through receptor-mediated actions and signaling pathways.