10-8-08_lecture_transportA

10-8-08_lecture_transportA -...

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My father once told me that respect for the truth comes close to being the basis for all morality. "Something cannot emerge from nothing," he said. This is profound thinking if you understand how unstable "the truth" can be. -from "Conversations with "Muad'Dib" by the Princess Irulan (Dune) In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move. Douglas Adams
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Announcements Quiz 4 due Wednesday 10/15 at noon, not Monday. Bioengineering hits it big! A Harvard alum donated $125 million for an Institute for Biologically Inspired Engineering
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Examples of osmosis Cycle of secretion and osmotic reabsorption in digestive tract Animal cells usually isotonic (a) Plant cells usually hypotonic. They are swollen and have an internal osmotic pressure called turgor . Turgor provides support for plants (b) If placed in a hypertonic solution (e.g. seawater), the cell will shrink away from the cell wall, or wilt, aka plasmolysis .
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Despite polar nature, water flows through membranes much faster than other polar solutes Water, as expected has a very low oil/water partition coefficient Its permeability is far above what would be expected based on this
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Like water for channels: Aquaporins Aquaporins are a family of water permeable channels discovered by Peter Agre’s group in the early 1990’s (Nobel Prize 2003) Left, one subunit, with multiple transmembrane portions Tetramers each with a specific, water permeable channel Very high flux rate, up to 1 billion waters/second King et al. 2004
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The free energy of solute movement Spontaneous movement from high to low concentration G= 1.4 log 10 [C in /C out ] for movement of a solute into a cell. So if C out is greater than C in , G is negative, because log 10 [C in /C out ] will be negative.
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What if the solute is an ion? Now must consider the concentration gradient and the voltage gradient, or the electric potential produced by an unequal balance of charges Together this is the electrochemical gradient. In millivolts, E ion =-60z log 10 [C in /C out ] This is the Nernst equation.
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Ion channels Integral membrane proteins that are specific for allowing Na+, K+, Ca++, Cl- through Most ion channels are gated, meaning that their permeability (open vs. closed state) is regulated Voltage-gated is when the membrane potential
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10-8-08_lecture_transportA -...

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