Quiz 03 Results Question 1 Multiple Choice Average Score 0 40506 points Imagine a light sensitive photoreceptor cell in the eye of a particular invertebrate The cell membrane contains cGMP gated K channels In the dark this cell has low levels of cGMP in its cytoplasm such that the cGMP gated K channels are in the closed state When light strikes the cell a sequence of events is triggered that results in the activation of guanylyl cyclase Which one of the following is most likely to happen when the cell which had been in the dark is suddenly illuminated Correct Percent Answered The membrane potential of the cell will become less negative depolarization 6 962 K ions will flow out of the cell through the activated K channels 81 013 K ions will flow out of the cell but Na ions will flow in at a greater rate resulting in a net inward current 9 494 Insufficient information is given to answer this question 2 532 Unanswered 0 Question 2 Multiple Choice Average Score 0 41139 points Imagine that you treat a typical neuron with a drug that blocks Cl channels and you observe that the cell depolarizes slightly Which one of the following statements about the chloride equilibrium potential ECl is likely to be true Correct Percent Answered ECl is the same as the resting potential 2 532 ECl is more positive than the resting potential 14 557 ECl is more negative than the resting potential 82 278 ECl is the same as the potassium equilibrium potential 0 Unanswered 0 633 Question 3 Multiple Choice Average Score 0 4019 points Years ago when I was a graduate student at UCLA sorry I helped lead a field trip for an invertebrate physiology class I was TAing This was a 3 day camping trip to the intertidal rocks and mudflat at Puerto Penasco Mexico Field trips were fun back then Our professor who worked on bioluminescent animals took us out at low tide on a moonless night to search for bioluminescent species We were armed with squirt guns filled with 0 5 M KCl Why would squirting 0 5 M KCl on the rocks help us find our quarry In case you were wondering the osmolarity of seawater is about 1 000 mOsM Correct Percent Answered The hypoosmolarity of the KCl solution stimulated the animals nervous systems leading them to produce light and thus become visible 1 1 266 The composition of the squirted solution didn t matter because it was the mechanical impact of the stream that stimulated the animals to luminesce Squirting seawater would probably have worked just as well 1 266 Squirting KCl onto an animal lowered Na concentration in its immediate environment which shifted ENa the sodium equilibrium potential in a positive direction leading to depolarization and stimulation of the nervous system 17 089 When a luminescent animal was squirted the local increase in K concentration shifted EK the potassium equilibrium potential in a positive direction and given their typically high K permeability at rest this depolarized and excited neurons 80 38 Unanswered 0 Question 4 Essay Average Score 0 points Take another look at the slide entitled NaCl and H2O Transport in the Lung Colon from the Membrane Dynamics lecture Imagine that the cell shown is in the epithelium lining the intestine and the space above the apical membrane is the lumen of the intestine All other mechanisms are the same Now consider these two facts a The alpha subunit of the G protein trimer has an intrinsic GTPase activity which is important in terminating the action of G protein coupled systems b The CFTR is activated by cAMP Here s the question If you introduced a toxin into the intestinal cell that blocks the intrinsic GTPase activity of a stimulatory G protein Gs what would be the most significant effect on intestinal function Briefly explain the mechanism underlying your answer including the most important steps in the signal transduction pathway Given Answers If a toxin that blocks the intrinsic GTPase activity of a stimulatory G protein was introduced into the intestinal cell the G protein pathway that converts ATP to cAMP would not be able to inactivate and cAMP would be continually produced cAMP activates the CFTR so this chloride ion channel would be indefinitely open for Cl to diffuse into the intestinal lumen This subsequently higher ion concentration in the lumen would lead to more water entering into the lumen through osmosis via aquaporins This could lead to dehydration and diarrhea The diarrhea resulting from cholera is produced by a similar increase in cAMP concentration If the GTPase activity of the G protein was blocked the pathway would no longer have its terminating action Therefore the entire pathway would be overactivated Overactivation of the G protein coupled system would lead to excess amounts of cAMP and as stated in point b cAMP activates the CFTR Excess cAMP would likely cause excess CFTR activation which would then allow excess chloride to exist the cell When chloride exits the cell sodium will follow due to the electrical potential difference When these ions exit the cell in excess they create an osmotic gradient which then drives water across the apical membrane Overactivation of CFTRs will cause excess water to enter the lumen of the intestine This would have effects remarkably similar to those of cholera diarrhea and extreme water loss leading to dehydration and possibly death If there was a toxin in the intestinal cell that blocked the GTPase activity of a stimulatory G protein ion channels in the affected area would not close G protein coupled systems could not terminate because the GTPase would not remove the GTP from the alpha subunit leaving the system in the activated state where the GCPR is separated from the alpha subunit The activated GPCR converts ATP to cAMP which as stated above activates the CFTR This would lead to an oversecretion of the Cl ions and H20 and Na 2 would follow the Cl This would result in dehydration and excessive loss of electrolytes through the mucus of the intestine A GTP molecule activates cAMP which then activates the CFTR channel through phosphorylation If a toxin inhibits the GTPase activity of the G protein GTP will never get hydrolyzed and cAMP will remain active This will result in Cl being constantly pumped into the lumen of the intestine followed by H2O through aquaporin channels and Na that leaks through intercellular tight junctions The activity of the Na K 2Cl symporter will be increased due to the concentration gradient made by increased CFTR activity The final result of that is