chapter 7

• Signals and Signaling


  Affect cell function
• Receptors


  Bind signals to initiate a cellular response
• Response to a signal

  Spreads through the cell
• Cells


  Change in response to signals in several ways
• Adjacent cells in a multicellular organism


  Can communicate directly
• Signals targeting cells are classified by


  • Signal source
  • Signal delivery mode
• Environmental information


  Is provided as a signal, or stimulus
• Types of stimuli


  • Physical stimulus (e.g. light, temperature, fluid flow)
  • Chemical stimulus (e.g. taste or smell)
• Signals can come from


  • Cells very far away, but still within the organism
  • Cells outside the organism
  • Cells close to the receiving cell
• Both local and long-distance signaling are possible
• Types of local signals


  • Autocrine signals (affect the cells that made them)
  • Juxtracrine signals (affect only adjacent cells)
  • Paracrine signals (affect nearby cells)
• Types of long distance signals


  • Hormones (travel to distant cells, e.g. via the circulatory system)
• Receptor


  Proteins that have very specific binding sites for chemical signal molecules, or ligands
• Ligand binding


  Causes receptor protein to change shape
• Receptor-ligand binding


  Is reversible
• Ligand is not altered with receptor/ligand binding
• Intracellular receptors


  Interact with physical signals (e.g. light) or chemical signals that cross the cell membrane
• Dissociation constant (KD)


  Measure of the affinity of the receptor for its ligand
• Lower KD


  Greater affinity
• Receptors with very low KD values bind ligand at very low ligand concentrations
• Agonists


  Have the same effect as the ligand
• Antagonists


  Bind the receptor and prevent ligand binding
• Membrane receptors


  Bind large or polar ligands (e.g. insulin)
• Membrane receptor types


  • Gated ion channels, protein kinase, GPCRs
• Intracellular receptors


  Bind small or nonpolar ligands that can diffuse across the cell membrane (e.g. estrogen)
• Signal transduction pathway


  The sequence of events and chemical reactions that lead to a cell's response to a signal
• Signal transduction pathways convert signals received by cells into cellular responses
• What do signal transduction pathways require?

  • A signal
  • A receptor
  • Response(s)
• Possible cellular responses


  • Activation
  • Inactivation
• Cellular responses may involve


  • Enzymes that catalyze biochemical reactions
  • Transcription factors that turn gene expression on/off
• Cellular responses are always regulated
• Signal transduction pathway overview


  1. Signal molecule binds receptor

  2. Activated receptor activates signal-transduction pathway

  3. Activated signal-transduction molecule

  4. Inactive signal-transduction molecule

  5. Short-term changes: enzyme activation, cell movement

  6. Long-term changes: altered DNA transcription
• Cross-talk


  Results from interrelated signal transduction pathways
• Types of cell membrane receptors

  • Ion channel receptors
  • Protein kinase receptors
  • G protein-linked (coupled) receptors (GPCRs)
• Ion channel receptors


  Allow ions to enter or leave a cell
• Ion channel receptor


  • Acetylcholine receptor on muscle cells
• Protein kinase receptors


  Catalyze phosphorylation of themselves and/or other proteins
• Protein kinase receptors


  • Growth factors like EGF and VEGF
• Human insulin receptor


  Subunits phosphorylate one another, then insulin response substrates respond, resulting in glucose transporter insertion into cell membrane
• G protein-linked (coupled) receptors (GPCRs)


  Have many functions including photoreception, olfaction, and regulation of mood/behavior