Final answer:
Extracellular signal molecules are chemicals like ligands that bind to cell surface receptors to initiate signal transduction, leading to cell responses. Various signaling types include paracrine, endocrine, autocrine, and gap junction signaling. Signaling pathways often involve second messengers and are amplified through enzymatic cascades.
Step-by-step explanation:
Extracellular signal molecules, often hydrophilic, communicate with cells by binding to the extracellular portion of receptor proteins. These receptors, which span the plasma membrane, lead to signal transduction, converting an extracellular message into an intracellular one. Errors in receptor protein structures can result in various diseases including hypertension, asthma, heart disease, and cancer. In addition, the initial extracellular signal can be greatly amplified through enzymatic cascades within the target cell.
There are four main types of signaling in multicellular organisms: paracrine signaling, endocrine signaling, autocrine signaling, and direct signaling through gap junctions. Ligand molecules, typically small and often volatile or soluble, are the specific chemical signals that bind to receptors. The binding of a ligand to its receptor initiates a signaling pathway or cascade that can involve second messengers and a series of phosphorylation events by kinases, ultimately causing a physiological response within the cell.