Zoo Psych Neuro 523 Answers to Study Qs for Exam 1 probset 3 2016 1 Calculate the number of coulombs on a univalent ion from the Faraday Constant and Avogadro s number 1 Faraday 96500 coulombs mole Avogadro s 6 02 X 1023 molecules or ions mole so coulombs ion 96 500 6 02 X 1023 1 60 X 10 19 coulombs ion 2 A 10 megohm resistor has a current of 1 mA flowing through it What is the voltage across the resistor What is the conductance of this resistor Note I do expect you to know that mega means 106 milli means 10 3 etc V I R 107 X 10 3 volts 104 volts Keep your fingers away The conductance of a 10 megohm resistor is 1 10 000 000 Siemens 0 1 microS 3 What is the total resistance of a 1 megohm and a 10 megohm resistor a in series b in parallel In series R 11 megohms or 1 1 X 107 ohms In parallel 1 R 1 106 1 107 11 107 so R 0 9 X 106 ohms 4 What is the potassium equilibrium potential of a cell permeable only to K ions with 10 mM KCl outside and 200 mM KCl inside EK 58 log 10 200 MILLIVOLTS at 20 C 75 5 mV 5 How many univalent ions per square cm of membrane would be separated across the membrane capacitance if the membrane potential is 120 mV Q V C C 1 microfarad sq cm so Q 10 6 X 0 12 coulombs sq cm 10 6 X 0 12 1 6 X 10 19 0 75 X 1012 ions sq cm 6 What factors limit the maximum value of a nerve signal The maximum value of a nerve signal is limited by the equilibrium potential of the ion that flows when the signal is initiated For example a nerve signal that is due to the opening of sodium channels can never take the membrane potential more positive than E Na 7 A cell has a membrane potential of 40 mV If the potassium conductance is 10 times higher than the sodium conductance and the internal concentration of potassium is 50 mM what is the external concentration of potassium Ignore any effects of chloride Assume ENa 55 mV This is another example of the use of the EXTREMELY IMPORTANT EQUATION EK gNa gK 1 mV or 40 0 1 X 55 EK 1 1 mV 44 5 5 EK so EK 49 5 mV Ek 58 log K out Kin so 49 5 58 log Kout 50 K out 7 mM Vm gNa gK ENa 8 An artificial membrane permeable only to potassium ions has a conductance of 4 0 mS It separates two KCl solutions 0 1 M KCl and 1 0 M KCl If the membrane potential is brought to 18 mV what is the magnitude of the resultant current flow Ignore osmotic effects This question has 2 answers depending on which side of the membrane has the higher concentration of K ions I ll just work through one assuming that the membrane starts at Ek rather than Ek EK 58 mV IK gk Vm EK 4 X 10 3 18 58 X 10 3 amps 160 X 10 6 amps 0 16 mA 9 Why are transmembrane domains of proteins frequently alpha helices Transmembrane domains typically consist of stretches of the amino acid sequence of the membrane protein in which there are 20 25 amino acids all of which have hydrophobic side chains While the side chains of this region are hydrophobic the backbone of the polypeptide chain the NH CH CO n part onto which the side chains are covalently bonded contains hydrophilic groups the NH and the CO groups In order to keep these groups satisfied in a hydrophobic environment they hydrogen bond to each other One very common way for them to do this is in an alpha helix In this structure the backbone is wound into a helix that brings the CO of one amino acid close to the NH of the fourth amino acid away toward the C terminal end of the chain and nicely aligned to make a hydrogen bond between these 2 groups The hydrogen bonds are nearly parallel to the axis of the helix So the final structure has all its hydophilic groups on the inside interacting with each other instead of with water and the hydrophobic sidechains sticking out making the whole structure nice and greasy and happily at home in a membrane 10 Define briefly Hydrophobic amino acid An amino acid with a side chain that is not ionized or does not hydrogen bond with water Phosphatidylethanolamine A phospholipid formed from glycerol esterified to 2 fatty acids and phosphate bonded to ethanolamine It is located on the internal monolayer of the cell membrane Epitope An antigenic determinant Part of an antigen e g a protein that is recognized by an antibody Tony Stretton Department of Zoology