Final answer:
Hormones are chemical signaling molecules secreted by endocrine cells that control specific physiological processes.
Step-by-step explanation:
Hormones are chemical signaling molecules secreted by endocrine cells that control specific physiological processes. In terms of their mechanism of action, hormones can either bind to intracellular hormone receptors and modulate gene expression or bind to cell surface receptors and stimulate signaling pathways. For example, steroid hormones such as the thyroid hormones enter the cell and enhance enzyme synthesis by binding to specific receptor proteins, forming a receptor protein-hormone complex that binds to DNA and initiates or enhances the synthesis of enzymes.