BENG 140B Lecture 3: Chapter 4

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University of California - San Diego
Gabe Silva

April 11, 2017 Chapter 4: Movement of Water Across Cell Membranes = Transmembrane Traffic Diffusion: solute moves down its concentration gradient 1) Simple diffusion: Small (e.g. oxygen, carbon dioxide) Lipid soluble (e.g. steroids) 2) Facilitated diffusion: Requires transporter (e.g. glucose) Active transport: solute moves against its concentration gradient 1) Primary active transport: ATP directly consumed 2) Secondary active transport: energy of ion gradient (usually Na ) used to move second solute (e.g. nutrient absorption in gut) Large scale movement of molecules Exocytosis: movement from inside the cell out of the cell Endocytosis: movement from outside the cell into the cell – Net flux accounts for solute movements in both directions. Magnitude is given by gradient of concentration, higher for first arrow. Direction is given by difference Plasma membrane embedded with proteins (a)to (b) is due to attraction between side chains Diffusion of only ions, not proteins or large molecules. Ion channels are selective so they specifically enable diffusion by diameter and charge distribution in internal walls of complex. A thin shell of positive and negative charge provides the electrical gradient that drives ion movement across the membranes of excitable cells. Ions are charged and their movement depends on electrical force that results from membrane potential The opening and closing of ion channels results from conformational changes in integral proteins. Discovering these factors that cause these changes is key to understanding excitable cells. Voltage gated channels- close and open depending on membrane potential-voltage at the membrane (important for generation of action potential in neurons). Ligand gated ion channels-open and close after binding with ligands (modulators) Mechanically gated ion channel- mechanical stress on plasma membrane, change of conformation of protein Model of a carrier-meditated transport Diffusion flux rate limited by concentration gradient. Mediate transport by the number of available carriers In both simple and facilitated diffusion, solutes move in the direction predicted by the concentration gradient. In active transport, a protein associated with energy is needed. Solutes move opposite to the direct
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