Chap 16

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Chapter 16

  • Cells in multicellular organisms communicate through a large variety of extracellular chemical signals.
  • In animals, hormones are carried in the blood to distant target cells but most other extracellular signal molecules act over a short range. Neighboring cells often communicate through direct cell-to-cell contact.
  • Extracellular signal molecules stimulate a target cell when they bind to and activate receptor proteins. Each receptor protein recognizes a particular signal molecule.
  • Small hydrophobic extracellular signals, such as steroid hormones and nitric oxide, can diffuse directly across the plasma membrane; they activate intracellular proteins, which are usually either transcription regulators or enzymes.
  • Most extracellular signal molecules cannot pass through the plasma membrane; they bind to cell-surface receptor proteins that convert (transduce) the extracellular signal into two different intracellular signals.
  • There are three main classes of cell-surface receptors: (1) ion-channel -coupled receptors, (2) G-protein-coupled-receptors (GPCRs), and (3) enzyme-coupled receptors.
  • GPCRs and enzyme-coupled receptors respond to extracellular signals by activating one or more activating one or more intracellular pathways that alter the behavior of the cell.
  • Turning off signaling pathways is as important as turning them on. Each activated component in a signaling pathway must be subsequently inactivated or removed for the pathway to function again.

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