Signal molecules are chemical substances that facilitate communication between cells in living organisms. They can be hormones, neurotransmitters, or other types of signaling compounds that transmit information, triggering specific responses in target cells. This process is essential for regulating various physiological functions, such as growth, metabolism, and immune responses. These molecules bind to specific receptors on the surface of target cells, initiating a cascade of biochemical events. For example, in the case of insulin, a hormone produced by the pancreas, it helps regulate blood sugar levels by signaling cells to absorb glucose. This intricate signaling system is vital for maintaining homeostasis in the body.