Chapter 9: Cell CommunicationI. Introduction & Pupillary Reflex A. Definitions1. Communication – producing, detecting, and/or respondingto signals2. Signal – any “agent” that affects a cella. Ex. a molecule (ligand), a force, lighti. *Ligands bind to receptors noncovalently3. Receptor : a cellular structure, usually a protein, thatresponds to a signalB. Unicellular organisms communicate with neighbors or the environment (Fig. 9.1a)II. When a cell receives a signal…(Fig. 9.3) A. Receptor ActivationB. Signal Transduction (conversion to a differentform)C. Cell Response to the signal1. Ex. enzyme activation, cytoskeletal change, ortranscriptionD. Termination of ResponseIII. Types of cell signaling (Fig. 9.4) A. Endocrine 1. Signals travel long distances through the bloodstream2. Signals are called hormonesB. Paracrine signaling1. Signaling cell lack receptors; target cell is localC. Autocrine signaling1. Signaling cell receives its own signal2. Ex. Looking in a mirrorD. Juxtacrine signaling1. Contact-dependent2. Cell that produces the signal must directly contact receptor cell to communicate (signal never enters extracellular space)3. Ex. Development of a zygoteIV. Ligand-Receptor Activity 9.1:9.39.4A. Cell surface receptors interact with molecules that can’t diffuse across membrane (Fig. 9.6)B. Signals that do diffuse across the membrane bind to intracellular receptors (fig. 9.6b)C. **Clicker question: Ligand//type of receptor (intra vs. extracellular) 1. Tryptophan: extracellular2. ATP: extracellular3. Estrogen: intracellular (all steroids diffuse through membranes) 4. Insulin: extracellular5. Adrenaline: extracellularD. ATP can be released as a signaling molecule1. 3 types of ATP receptors a. P1, P2X, P2YV. 3 classes of cell surface receptors (GPCR’s, Enzyme-Linked, & Ligand-Gated Ion Channels) 9.6A. G-Protein Coupled Receptors (G.P.C.R.) (Fig. 9.7b, 9.8) 1. Associate with other proteins when activated2. 5 steps of G.P.C.R. Activation 1) Signal binds to G.P.C.R.2) G.P.C.R. changes shape & binds to G protein complex3) G protein releases GDP from its -subunitα 4) GTP binds to the -subunit, taking the place of GDPα5) G protein complex dissociates (breaks apart) & activated α-subunit propagates response3. When GTP binds to a protein, the protein changes shapeB. Enzyme-Linked Receptors (Fig. 9.7c) 1. When activated, catalyze reactiona. Usually phosphorylate cytosolic proteinb. Fig. 9.7c: receptor proteins dimerize to form 2-part structure, then phosphorylate a target proteinC. Ligand-Gated Ion Channels (Fig. 9.7d) VI. Amplifying Signals (Secondary Messaging) 9.79.8:A. Initial signal can be amplifiedB. Signal/ligand (1st messenger) binds to receptorC. Small molecules/ions (2nd messengers) are produced, which convey information in cell1. 2nd messengers allow signal amplification & a speedy response D. Ex. Adrenaline binds to a G-protein-coupled receptor in heart muscle (fig. 9.9)1. Second messenger = cAMP (cyclic AMP) 2. Kinase : enzyme that phosphorylates another moleculea. Phosphorylaton = activation3. Signal amplification can occur at many levels ( Fig. 9.10) a. At each step, the signal amplitude is increased exponentially (i.e. 248…)4. Pathway : Adrenalinenoradrenergic receptor in membrane -α subunit of G-protein activatedG (s) activatesα adenylyl cyclaseactivated adenylyl cyclase converts ATP into cAMPcAMP activates protein kinase A (PKA)PKA activates target proteins (like CREB)E. cAMP is a common 2nd messenger1. Nucleotide2. Synthesized from ATP9.99.10:VII. Turning Off Signals A. Example:1. Enzymes convert cAMP to AMP (stops signal transduction pathway)2. Other enzymes (Phosphatases) remove phosphate groups from proteins3. Pumps sequester ionsVIII. Cell’s Proteome Determines Cell Response A. Proteome: set of proteins produced by a cell1. All cells have same genome, but produce different proteinsB. Receptor protein required for responseC. Receptor type (and response) can have different:1. Signal binding affinities2. Signal transduction pathways (STPs)3. STPs that cause different resultsFig.
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