Suppose that 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?
a. The membrane potential of the cell will become less negative (depolarization).
b. K+ ions will flow out of the cell through the activated K+ channels.
c. K+ ions will flow out of the cell but Na+ ions will flow in at a greater rate, resulting in a net inward current.
d. Insufficient information is given to answer this question.
Question 2
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?
a. ECl is the same as the resting potential.
b. ECl is more positive than the resting potential.
c. ECl is more negative than the resting potential.
d. ECl is the same as the potassium equilibrium potential.
Question 3
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.
a. The hypoosmolarity of the KCl solution stimulated the animals’ nervous systems, leading them to produce light and thus become visible.
b. 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.
c. 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.
d. 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.