Handout Week 8

Handout Week 8 - Jonathan Goldstone jongoldstone@ucsd.edu...

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Jonathan Goldstone 5/22/09 jongoldstone@ucsd.edu BIBC 100 - Week 8  Membrane Transport Driving Forces for Membrane Transport o Concentration gradient Permeable ions pass from high concentration to low Osmosis is a specific example of the diffusion of water o Membrane Potential Voltage difference across a membrane due to a slight excess of ions (positive or negative) on one side of the membrane o Electrochemical gradient Combined influence of the concentration gradient and the membrane potential (difference in concentration and electrical charge) Simple Diffusion o Hydrophobic molecules can pass through easily O 2 , CO 2 , N 2 , benzene o Small uncharged polar molecules don’t pass through as well Water, urea, glycerol o Large uncharged polar molecules can barely pass through Glucose, sucrose o Ions cannot pass through cell membrane by themselves! Na + , K + , Ca 2+ Facilitated Diffusion – “Passive Transport”     o Diffusion of an impermeable molecule across the membrane via a protein channel down its concentration or electrochemical gradient o Spontaneous passage of uncharged and charged molecules o Protein examples – transporters, permeases, ion channels o MB proteins decrease activation energy for transport of a polar solute Fig 11-27 shows catalysis of membrane transport by protein channel o Aquaporins     – facilitates transport of water across membrane Helical protein with tetramer structure, each monomer has a pore Pore is 2-3 Å wide at narrowest region, just enough for water Water molecules pass through single file, but not as a chain Specificity filter so only water can pass through! Includes Arg residue to repel cations Other residues repel other things, so only water is allowed Asp interacts favourably with water to encourage passage
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Jonathan Goldstone 5/22/09 jongoldstone@ucsd.edu o Glucose Transporters     Fig 11-29 – glucose transporter w/ 12 transmembrane domains There are polar residues inside the membrane!! Transmembrane helixes are amphipathic , 4 helixes come together to make a channel with polar residues inside and hydrophobic residues outside, allows passage of glucose through pore! Very simple model for transport Two modes, T1 and T2, via conformational change T1 allows glucose to enter protein channel T2 allows glucose to enter cell o Diabetes Between meals most of the glucose transporters are stored in membrane vesicles inside the cell
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This note was uploaded on 01/07/2010 for the course BIBC BIBC 100 taught by Professor Buehler during the Spring '09 term at UCSD.

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Handout Week 8 - Jonathan Goldstone jongoldstone@ucsd.edu...

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