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Pg. 651-671.docx

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John Coleman

Pg. 651-671 Membrane Transport of Small Molecules and the Electrical Properties of Membranes Principles of Membrane Transport  Describe the permeability properties of the membrane Protein-Free Lipid Bilayers are Highly Impermeable to Ions  Molecules using diffuse from high concentration to low concentration  The rate of diffusion in the membrane depends on the size of the molecule and how soluble it is  Small nonpolar molecules can diffuse easily into the membrane Ex. O 2nd CO 2  Large molecules however can’t diffuse easily into the membrane Ex. Water and urea  Small ions such as Na+ and K+ cannot pass through  Large uncharged polar molecules such a glucose and sucrose can sometimes pass There are Two Main Classes of Membrane Transport Proteins: Transporters and Channels  Membrane transport proteins: transfers important solutes across the membrane  Transporters (carriers): bind to specific solute and changes conformation to let it pass through the membrane  Channels: interact weakly with solutes Active Transport is mediated by Transporters Coupled to an Energy Source  Passive Transport/Facilitated diffusion: transport molecules from a high concentration to a low concentration  A solute carrying a net charge can diffuse into the membrane by an electrochemical gradient  Active Transport: when diffusion happens from low concentration to high concentration and involves ATP Transporters and Active Membrane Transport  Each transporter has a specific binding site for the solute  This changes conformation when the solute binds, allowing the solute to pass through the membrane Active Transport can be Driven by Ion Gradient  Uniporters: transfer one molecule against its concentration gradient  Symporters (coupled co-transporters): transfer one type of molecule against its concentration gradient and one type of molecule down its electron gradient  Ex. Na+ symporter  Ex. Lactase permease: transports lactose across the plasma membrane Transporters in the Plasma Membrane regulate Cytosolic pH  Most protein operate at a specific pH  Ex. Lysosomal enzymes regulate at a pH of 5; cytosolic enzymes pH of 7.2  Most cells use Na+ antiporters to transfer Na+ into the cytosol  The Na+ gradient allows for the pump of H+ ions into the l  Na+ antiporter and the Cl HCO wo3k together to pump H+ into or out of the cytosol to maintain the pH at the same level An Asymmetric Distribution o
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