ABSTRACT
Cells communicate with one another in multicellular organisms using extracellular signaling molecules or hormones. The hormone is secreted by the signaling cell and then binds to a receptor on the target cell, initiating a response in that cell. In endocrine signaling the hormone acts at a distant site in the body from where it was produced, in paracrine signaling the hormone acts on nearby cells, and in autocrine signaling the hormone acts on the same cell from which it was secreted. The principal way by which cells communicate with each other is by means of extracellular signaling molecules or hormones. Hydrophilic and some lipophilic hormones bind to cell surface receptors. These are integral membrane proteins situated in the plasma membrane that bind the signaling molecule with high affinity. Ion channel-linked receptors are involved in the rapid synaptic signaling between electrically excitable cells.
