<p>Chapter 11: Cell Communication</p><p>I. External signals are converted into responses within the cell A. Signal Transduction Pathway – the process which a signal on a cell’s surface is converted into a specific response B. Local Signaling – cells usually use chemicals as signals 1. Direct contact a. Cell junctions: (gap jct – animal, plasmodesmata – plant) b. “Cell-cell recognition” - contact between surface membrane proteins c. Local signals are called local regulators. Ex: growth factors C. Long-Distance Signaling – cells use chemicals over long distances 1. Hormones. Enter blood stream to reach far-away target cells 2. Nerve Impulses (chemical and electrical signaling) D. Three Stages: Sutherland’s experiments 1. Reception - signal molecule (ligand) binds receptor protein on target cell a. Receptor protein changes conformation upon ligand binding b. Most receptor proteins are in the membrane, some are intracellular c. Intracellular receptors are either in the cytoplasm or the nucleus i. example: Nitrous Oxide, diffuses through membrane, acts as transcription factor ii. example: Hormones d. Three major membrane receptors: i. G-Protein-linked Receptors - Signal molecule binds to G-protein-linked receptor - Conformation change, causing binding with G-protein - GDP displacement in G-protein, GTP activates it - Activated G-protein activates membrane enzyme - Activated enzyme triggers signal transduction - G-protein self-hydrolyzes back to GDP, inactive ii. Tyrosine-Kinase Receptors - Can trigger multiple signal transduction pathways at once (key difference from G-protein-linked receptors) - Ligand binding on receptor causes dimerization between two receptors - Dimerization activates tyrosine-kinase region - Enzyme is a part of the receptor and therefore phosphorlyates itself: ATP  Tyrosine-P + ADP - Fully activated receptor recognized by relay proteins - Relay proteins serve in signal transduction iii. Ion-channel Receptors - Channels are ligand-gated. Gated is key word. - Binding of ligand to receptor causes channel to open - Flow of ions triggers ion concentration changes - Many relay proteins sensitive to ion concentration - Very important in nervous system 2. Transduction – Converts signal to a form that can bring about a specific cellular response a. Cascades of molecular interactions relay signals from receptors to target molecules in the cell b. Signal Transduction Pathways: interaction of proteins is a major theme in cellular regulation. <SIGNAL AMPLIFICATION – another key concept> c. Phosphorylation: i. Protein Kinase = attached phosphate ii. Protein Phosphatase = removes a phosphate iii. Many relay proteins are protein kinases iv. Usually phosphorylation activates, and dephosphorylation inactivates proteins d. Small Molecules and Ions as Secondary Messengers: i. Are non-protein, waterp-soluble molecules ii. Diffuse through the cell easily iii. Examples: cAMP and Calcium ions iv. Adenylyl cyclase: ATP  cAMP 3. Response – Triggering of the actual cellular response a. Transcription factors – alter transcription of certain enzymes b. Concentration changes that affect metabolism (feedback inhibition) 4. Scaffolding proteins increase efficiency by bringing relay proteins/enzymes in close proximity to each other so that cascades happen more quickly</p><p>EXAMPLE PATHWAY:</p><p>1. Signal molecule (ligand/primary messenger) binds to G-protein-linked receptor. 2. Activation of G-protein (GDP  GTP) leads to the activation of enzyme: Phospholipase C 3. Activated Phospholipase C cleaves PIP2 molcules into DAG and IP3 4. DAG relays to signal to one pathway, IP3 to another 5. IP3 (secondary messenger) binds to a ligand-gated receptor protein on the membrane of the ER 6. IP3 binding causes channel to open, releases Calcium ions into the cytoplasm 7. Increase in Calcium ion concentration in the cytoplasm causes a cellular response</p>