Problem 1. Inspired by the natural design of cell membranes, you have invented a

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Problem 1. Inspired by the natural design of cell membranes, you have invented a strange new semi-permeable membrane consisting of a water-impermeable base layer interspersed with hydrophilic transmembrane glycoproteins which form three types of channels. Channel type 'A' is approximately 3 angstroms in diameter and lined with negative Channel type 'B' is approximately 3 angstroms in diameter also but lined with positive Channel type 'C" is approximately 4 angstroms in diameter and electrically neutral charges charges This membrane separates two ionic solutions containing lithium (Li*) and potassium (K+) ions, as well as some impermeable anions (A), having the following concentrations: ION Li SOL#1 30 mM 170 mM 200 mM SOL#2 120 mM 80 mM 200 mM Assume solution 1 as your "reference" solution A. Speculate on the specificity of the three channel types. Which channels might you expect to be Lif and K channels and why? B. Assuming that this membrane is permeable to only Lit, what electric potential would you measure across the cell membrane (assume solution #1 is the reference or ground)? C. What would the potential be if the membrane was permeable to only K? D. What would the membrane potential be if the membrane permeability to Li+ was 10% that of the permeability to K? E. You discover that the Li+ permeability is daylight-sensitive, increasing linearly from its rest value of 10% that of K+ at 6AM daylight to a peak of 5-times that of K+ at high noon. You therefore place your membrane in the kitchen window to use as a "sundial". This morning when you woke up, you noticed that the sun shining in through the window created a -10 mV potential

Explanation / Answer

Answers

A). Channel type A is specific for negatively charged ions to pass through it. Channel type B is specific for positively charged molecules and Channel type C is nonspecific. Channel type B might be K+ or Li+ ion channels because of its positive charged.

B). By assuming that this membrane is only permeable to Li+ ,the movement of fluid occur from Solution 2 to Solution 1 because of concentration gradient. Electrical potential generated through membrane will be positive and in the direction of solution 2 to solution 1.

C). If membrane is permeable only to K+ then electrical potential will be positive and in direction of solution 1 to solution 2 because of concentration gradient of K+.

D). According to Goldman-Hodgkin-Katz equation,

VM= RT/Fln (pkLi+/PKK+)

By putting data into this equation one can calculate the membrane potential (Vm)

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