Epinephrine is ______ and binds to cell-surface receptors to activate ____ _____ and produce the second messenger ______.

Epinephrine is known to be a hydrophilic molecule, which means it is water-soluble and cannot easily pass through the lipid bilayer of cell membranes. Instead, it binds to cell-surface receptors, specifically adrenergic receptors, which are G protein-coupled receptors.

Upon binding to these receptors, epinephrine activates adenylate cyclase, an enzyme that catalyzes the conversion of ATP to cyclic AMP (cAMP), which serves as a second messenger within the cell. cAMP plays a crucial role in signal transduction pathways, leading to various physiological effects, such as increased heart rate and energy mobilization.

This understanding of epinephrine's nature as a hydrophilic molecule and its interaction with adenylate cyclase to produce cAMP is foundational in biochemistry and cellular biology, highlighting how extracellular signals can influence cellular responses through secondary messengers.