# Molecular Signaling --- **Molecular signaling** refers to [[cell signaling]] between cells that are distant from one another, either as part of a larger organism, or different single cellular organisms to one another. [[Cytokines]] are the [[signaling molecule|signaling molecules]] of the immune system. ## Signaling Between Unicellular Organisms [[receptor proteins|Signal transduction]] pathways are remarkably similar in unicellular organisms as they are in multicellular organisms, though the "conversations" may be quite different. Unicellular organisms can use a method called **quorum sensing** to sense when their population has reached a certain threshold. ## How Molecular Signaling Works ### Step 1: The Signaling Molecules The **[[signaling molecule|signaling molecules]]** such as hormones are released into the system, and when they find their receptor, they bind with it. Something that is attaches to a receptor is called a "ligand", and *receptors can identify their ligands with remarkable specificity* ### Step 2: The Receptor is Activated Once bound with the [[signaling molecule|ligand]], the **[[receptor proteins]] is activated**, which causes a conformational change, and kicks off the response that changes the behavior of the cell. #### Step 2a: Lipid Soluble Ligands If the signaling molecule is a [[hormone|steroid hormone]], that is lipid soluble, and can therefore pass right through the [[plasma membrane]]. These generally have a more acute effect ### Step 3: Signal Transduction Once it's received the signal, signal transduction passes that signal on through the cell and ends in some sort of reaction, whether that be to divide, or to express a certain gene, or what have you. ___