lec 17 note - LECTURE 17 01 October 2010. (P. J....

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-1- LECTURE 17 01 October 2010. (P. J. Hollenbeck) BIOL231 ENERGY OF SOLUTE GRADIENTS ACROSS MEMBRANES Reading: same as lectures 15/16; Problems: 46-50; Exam II’05, #2; II’06, #5; ECB Q12-2 <The big questions: When you open a channel in the membrane, which way will solutes flow? Can their movement be coupled to something else to do useful work for the cell?> I. Charged & uncharged solutes <Remember WHY we can have disequilibria across the membrane in the first place - relative 22 impermeability! Although H O and some dissolved gases (NO, CO, O ) move freely across the membrane, none of the solutes that we’re interested in can do so. They require a protein channel or transporter in order to cross the membrane.> A. Concentration gradients (1) For uncharged species (e.g., glucose), the difference in concentration across the membrane ALONE determines what will happen when a “channel” is opened. (2) If it has an avenue across the membrane, an uncharged solute will simply flow in out down its concentration gradient until [solute] = [solute] . B. Charge gradients (1) For a charged species ( e.g. , Na , K ) we must consider 2 factors: the concentration ++ gradient and the charge gradient. Together we refer to these as an ELECTROCHEMICAL GRADIENT. We saw this intuitively for the disequilibria of Na and K across the axonal membrane in the previous lecture . We were able to predict that Na would flow into the cytoplasm if we opened a channel, but it was not + obvious which way K would flow. It turned out to be important: the two “arms” of + the action potential were driven by Na ions, and then K ions, flowing down their M electrochemical gradients and driving the V toward their respective equilibrium potentials. (2) So we often can’t always tell which way an ion will flow when we open a channel. We need a way to quantitatively integrate data about charge with data about concentration so that we can determine exactly what is going to occur.
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This note was uploaded on 09/18/2011 for the course BIOLOGY 231 taught by Professor Petethollenbeck during the Fall '10 term at Purdue University-West Lafayette.

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lec 17 note - LECTURE 17 01 October 2010. (P. J....

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