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Lecture #5 Membrane Transport & Cell Communication Membrane Transport i. diffusion ii. facilitated diffusion iii. active transport Diffusion O2 EXTRACELLULAR INTRACELLULAR CO2 -molecules pass through membrane -movement is spontaneous -molecules travel down the concentration gradient -goal: equlibrium -speed is limited by rate of diffusion Osmosis • movement of water down a concentration gradient • difference in concentration of solute across a membrane • intracellular [solute] vs. extracellular [solute] • osmotic, hyperosmotic, hyposmotic Facilitated Diffusion molecule is transported down its concentration gradient faster and more efficient than diffusion uses transport proteins 2 kinds: carrier proteins channel proteins GLUT – glucose transporter proteins GLUT1 – red blood cells, adipose cells, muscle cells GLUT4 – liver cells, adipose cells, muscle cells glucose EXTRACELLULAR insulin INTRACELLULAR after glucose uptake Active Transport • pumping a molecule against its concentration gradient • requires energy (ATP) Ca2+-ATPase – skeletal muscle Ca2+ CYTOPLASM Ca2+ Ca2+ ATP Ca2+ Ca2+ P Ca2+ ER LUMEN Ca2+ Ca2+ Ca2+ 2+ Ca Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ 2+ 2+ Ca Ca Ca2+ Ca2+ Ca2+ Ca2+ 2+ Ca Ca2+ Ca2+ Na+-K+-ATPase – nerve stimulation and membrane potential EXTRACELLULAR K2+ 2+ K K2+ K2+ ATP ADP Na2+ Na2+ Na2+ stimulation release of energy resting stimulated Na2+ Na2+ Na2+ INTRACELLULAR K2+ 2+ K Na2+ Na2+ Na2+ Active Transport and Co-transport blood gut epithelial cell glucose glucose glucose Na2+ Na2+ Na2+ Na2+ intestinal lumen ATP glucose Na2+ ADP K2+ 2+ K tight junctions Other forms of transport • Exocytosis • Endocytosis • phagocytosis • pinocytosis • receptor-mediated endocytosis Cell Signalling • Signals: • local regulators – e.g. growth factors • long-distance regulators – e.g. hormones • 3 stages: • Reception • Transduction • Response Non-hormonal cell-to-cell communication Reception – Plasma Membrane Receptors • 3 major types: • G-protein linked receptors • receptor tyrosine kinases • ion channel receptors G-protein linked receptors ligands: e.g. some hormones (epinephrine) neurotransmitters Epinephrine – adrenergic receptor • epinephrine – produced by the adrenal gland • enters the bloodstream during short-term stress response • liver - a1 receptors • blood vessels - b2 receptors Liver a1 receptor blood vessels - heart, lung, cerebral cortex adenylate cyclase b2 receptor GTP a1 receptor adenylate cyclase GTP adenylate cyclase GTP b2 receptor cAMP ATP adenylate cyclase GTP cAMP ATP PKA glycogen breakdown – release glucose PKA inhibits MLCK – vasodilation blood vessels – skin and gut a2 receptor adenylate cyclase GDP a2 receptor GDP adenylate cyclase X ATP vasoconstriction cAMP Receptor Tyrosine Kinases • kinase: an enzyme that phosphorylates another protein – catalyzes the transfer of phosphate groups • RTKs – catalyze the transfer of phosphate to side chain of tyrosine amino acid in a protein • ligands – e.g. growth factors – EGF, FGF, PDGF, etc. Transduction • protein phosphorylation – kinases • second messengers: • cAMP • Ca2+ Transduction • phosphorylation - kinases – growth factors » stimulate cell division • cAMP – adenylate cyclase » activates PKA » various effects – e.g. glycogen breakdown or vasodilation • Ca2+ - PLC produces IP3 and DAG » activates PKC » various effects – e.g. muscle contraction Response • Cytoplasmic responses: – – – – opening or closing of a gated ion-channel alteration in metabolism regulation of enzyme activity change in cytoskeletal organization • Nuclear responses – activation or repression of gene expression » activation or inhibition of cell cycle » induction of differentiation » cell committment
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