In bottom sediments of lakes and streams, organic matter is broken down by heterotrophic microorganisms and ammonia is released. Under aerobic conditions, specialized bacteria convert ammonia to nitrate (nitrification). This nitrate, together with nitrate from other sources, diffuses into the deeper sediments, where it may undergo anaerobic conversion to nitrogen gas (denitrification). These sediments typically contain oligochaete worms that live with their heads buried and their tails waving back and forth in the overlying water. Plastic columns were packed with freshly collected stream sediments and then  covered  with  layers  of  nitrate enriched  water.  A similar  set  of  columns  was packed with sediment that had been sterilized and then covered with either nitrate-enriched water or distilled water. Oligochaete  worms  were  collected  and  acclimated  to  20°(C)Following  acclimation,  worms  were  rinsed  in  distilled water and then added to three sediment columns. The columns were incubated in the dark at 20°C and monitored every three days for changes in the concentration of nitrate in the overlying water. Nitrate concentrations in each of the experimental treatments were plotted against time, as shown in the graph below

It can be concluded from the figure that

#Question id: 1243

#Unit 4. Cell Communication and Cell Signaling

One form of receptor desensitization can be mediated by negative feedback involving phosphorylation of the receptor itself by a kinase activated downstream of a second messenger. The phosphorylated receptor likely:

#Question id: 1242

#Unit 4. Cell Communication and Cell Signaling

Rhodopsin, a light-sensitive GPCR whose role in vision is dependent on its phosphorylation status, is influenced in part by the protein arrestin.  Which of the following is TRUE with regard to rhodopsin and vision?

#Question id: 1241

#Unit 4. Cell Communication and Cell Signaling

Which of the following is a common step in the opening/closing of ion channels by acetylcholine and rhodopsin binding to their receptors?

#Question id: 1239

#Unit 4. Cell Communication and Cell Signaling

GTPases serve in many signal transduction pathways and the presence of GTP or GDP dictates whether the pathway is on or off, respectively. Which of the following statements is TRUE regarding guanine nucleotide exchange factors (GEF) and the role in these signaling pathways?

#Question id: 1238

#Unit 4. Cell Communication and Cell Signaling

Which of the following can lead to loss-of-heterozygosity in a tumor-suppressor gene?

a. deletion of the normal copy                       

b. non dysjunction during mitosis

c. somatic mutation of the normal copy

d. mitotic recombination between a wild-type and mutant allele

#Question id: 1237

#Unit 4. Cell Communication and Cell Signaling

Arrange the following events in the proper order in which they occur during transduction of the Notch/Delta signaling pathway:

a. cleavage of Notch by TACE                                  

b. binding of Delta to Notch

c. translocation of Notch segment to the nucleus      

d. interaction of Notch with transcription factors     

e. cleavage of Notch by Presenilin 1