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Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Receptor Proteins Cell Surface Receptors - Three-dimensional shape that fits shape of specific signal molecule.  Binding induces a change in receptor protein’s shape, ultimately producing a response in the cell.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Characterization Methods of receptors: 1. Monoclonal Antibodies 2.Gene Analysis (group receptor proteins into families)

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Cell Signaling Four Basic Mechanisms: 1. Direct Contact 2. Paracrine Signaling 3. Endocrine Signaling 4. Synaptic Signaling Autocrine Signaling - Secreting signals that bind to specific receptors on own plasma membrane.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Intercellular Receptors Gene Regulators  Some receptors act as regulators of gene transcription. - Each has a binding site for DNA.  When signal molecule binds to another site on the receptor, the inhibitor is released, exposing DNA binding site.  Receptor binds to specific nucleotide sequence on DNA, activating a particular gene.

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Cell Surface Receptors Cell Surface Receptors convert extracellular signal to intracellular signal.  Chemically Gated Ion Channels - Multipass transmembrane protein that opens when a neurotransmitter binds to it.  Enzymatic Receptors - Binding of signal molecule activates enzyme - usually protein kinases.

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Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Cell Surface Receptors  G-Protein-Linked Receptors - Guanosine triphosphate binding system. - Initiate diffusible signal in cytoplasm.  Largest family of cell surface receptors.  May have evolved from sensory receptors of single-celled ancestors.

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Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Initiating the Intracellular Signal Many receptors carry signal’s message into the target cell using intracellular mediators.  cAMP - Used by all animal cells.  Calcium (Ca++) - Widely used.

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Amplifying the Intracellular Signal Most signaling molecules exist in such low concentrations that diffusion across cytoplasm would take great deal of time without amplifying the signal.  Chain of protein messengers amplify the signal. - Examples of these proteins can be cAMP, protein kinases

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Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Expression of Cell Identity During development, each animal cell type acquires a unique set of cell surface molecules.  Glycolipids  MHC Proteins - (Major Histocompatibility Complex)

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Intercellular Adhesion Cell clusters form long-lasting or permanent connections with each other (cell junctions).  The nature of the connections determines what the tissue is like.

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Tight Junctions Tight junctions connect plasma membranes of adjacent cells in a sheet.  Prevent small molecules from leaking between cells and through the sheet.  Effectively segregate proteins on opposite sides of the sheet.  Fuse like a zipper  Ex. Prevent digestive enzymes from seeping into blood

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Anchoring Junctions Anchoring junctions mechanically attach cytoskeleton of one cell to cytoskeleton of other cells, or to the extracellular matrix.  Desmosomes - Connect cytoskeletons of adjacent cells; prevent cells subjected to mechanical stress from pulling apart; they are buttonlike thickenings of plasma membrane  Hemidesmosomes - Anchor epithelial cells to basement membrane.

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Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Anchoring Junctions  Cadherins - Connect actin frameworks of cells in cadherin-mediated junctions.  Adherens - Connect actin filaments of one cell with those of neighboring cells or with extracellular matrix. - Integrin proteins

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Communicating Junctions Communication junctions pass chemical signals directly from one cell to adjacent cells.  Establish direct physical connections. Animals  Gap Junctions - Connexons. Plants  Plasmodesmata

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