The sodium-potassium pump is primarily responsible for
A. maintaining isotonic water balance.
B. the excretion of salts.
C. the contraction of muscle fibers.
D. the resting potential.
E. the action potential.
Answer: D
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Phosphorylation cascades involving a series of protein kinases are useful for cellular signal transduction because they _____
A) are species specific B) always lead to the same cellular response C) amplify the original signal many times D) counter the harmful effects of phosphatases E) use a small and fixed number of molecules
The promoters of positively controlled operons require activator proteins, because
A) RNA polymerase easily recognizes the consensus sequence. B) they are required to inactivate the repressor proteins. C) the promoters have nucleotide sequences that bind RNA polymerase weakly, which are not close matches to the consensus sequence. D) they are needed to bind to the allosteric site of RNA polymerase.
Refer to the accompanying figure. Which cell type or types are primarily responsible for the release of histamine in
injured tissues and in allergic responses? a. A only b. B only c. C only d. D only e. A and cell type D
Hypothesis I: A flavin pigment absorbs blue light; this pigment then activates the proton pump.Hypothesis II: When CO2 concentration rises, a membrane receptor inactivates the proton pump.Stomata closure in darkened environment, at time zero, with air (0.03% CO2) or CO2-free airflow through leaf. (Note: A decrease in conductance indicates inactivation of the proton pump.) 
A. Supports both hypothesis I and II. B. Supports only hypothesis I. C. Supports only hypothesis II. D. Supports neither hypothesis.