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Cell Signaling and Communication

posted Oct 21, 2012, 1:34 PM by ana benitez
Why communicate? - to coordinate reactions and responses, maintain homeostasis 

Short distance - cells are close together 
  •  direct - cell junctions (plasmadesmota in plants) - cytoplasmic connections that allow molecules (signals ) to diffuse directly to each other   
  • indirect - paracrine ( a secreting cell sends local regulators to nearby cells) VS synaptic ( a neuron releasing neurotransmitters to target cells) 
Long distance - hormones, use blood to travel to target cells, will bind at receptors to start the signal transduction pathway 

Signal transduction pathway: 
 1. reception - signal molecules (ligand) attaches to receptor   (first messenger) 
  • membrane receptors - found in the plasma membrane, bind with polar(hydrophilic) ligands 
    • G-protein receptors - Reception with ligand triggers GDP (guanine disphosphate) to get replaced with GTP (guanine triphosphate)  --> G-Protein is activated --> G-protein attaches to a protein/enzyme --> transduction begins.. When ligand leaves the receptor --> GTP loses a phosphate forming GDP, G - protein becomes inactive [ G-protein animation}
    • Tyrosine Kinase Receptor - reception occurs --> ATP is used to phosphorylate Tyrosine Kinase  (a phosphate group is removed from ATP forming ADP) --> Tyrosine Kinase has phosphate which will be used to activate relay proteins or enzymes --> transduction starts [Tyrosine animation ]
    • Ion Channel Receptors - signal molecule attaches to receptor, specific ions are allowed through a channel protein, when ligand leaves, ions cannot pass through [ Ion channel receptor]
  • intracellular receptors 
    • receptor is found in the cytoplasm or by nucleus 
    • ligands are small hydrophobic molecules (can pass through the phospholipids of the cell membrane)
    • can often affect gene regulation ( trigger transcription --> mRNA --> translation --> specific protein) [animation]
2. transduction - cascade of events, domino effect of molecules, assembly line that leads to a response , amplify signal, at each step the signal is transduced into another form (conformational change) 
  • second messengers - small, nonprotein, water soluble molecules or ions. diffuse through out the cell 
    • cAMP - Adenylyl cyclase converts ATP into cAMP; 
      cAMP usually activates protein kinase A, which phosphorylates various other proteins

  • calcium ions (Ca2+) - second messengers & activate proteins 
  • cAMP and CA ions animation

3. response - reactions in cytoplasm or organelles; or regulation of nucleus activity (turn on/off genes)

overview of the three phases/steps: