Refer to the figures below. Researchers investigating the uptake of dipeptides by bacterial cells constructed model cells by embedding the membrane protein responsible for peptide transport in a lipid bilayer.They loaded the interior of the model cell with potassium ions and pyranine, a fluorescent molecule that undergoes a decrease in its fluorescence intensity at low pH.The researchers used the small molecule valinomycin to interact with the membrane lipids of the model cell to release potassium ions into the surrounding environment (no membrane protein is needed for this release).Upon doing this, the researchers observed an almost immediate change in fluorescence of the pyranine still inside the model cells, but only under certain conditions, as detailed in the graph.What do the results shown in the graph suggest about the membrane protein?
A) The membrane protein is a uniporter that transports dipeptides against their concentration gradients.
B) The membrane protein is a symporter that transports dipeptides only at the same time it transports hydrogen ions.
C) The membrane protein is a symporter that transports dipeptides only at the same time it transports potassium ions.
D) The membrane protein is an antiporter that transports dipeptides in exchange for hydrogen ions.
E) The membrane protein is an antiporter that transports dipeptides in exchange for potassium ions.

Question

Refer to the figures below.  Researchers investigating the uptake of dipeptides by bacterial cells constructed model cells by embedding the membrane protein responsible for peptide transport in a lipid bilayer.They loaded the interior of the model cell with potassium ions and pyranine, a fluorescent molecule that undergoes a decrease in its fluorescence intensity at low pH.The researchers used the small molecule valinomycin to interact with the membrane lipids of the model cell to release potassium ions into the surrounding environment (no membrane protein is needed for this release).Upon doing this, the researchers observed an almost immediate change in fluorescence of the pyranine still inside the model cells, but only under certain conditions, as detailed in the graph.What do the results shown in the graph suggest about the membrane protein?
A) The membrane protein is a uniporter that transports dipeptides against their concentration gradients. 
B) The membrane protein is a symporter that transports dipeptides only at the same time it transports hydrogen ions. 
C) The membrane protein is a symporter that transports dipeptides only at the same time it transports potassium ions. 
D) The membrane protein is an antiporter that transports dipeptides in exchange for hydrogen ions. 
E) The membrane protein is an antiporter that transports dipeptides in exchange for potassium ions.

Quizplus · Accepted Answer

The graph shows that the fluorescence intensity of pyranine inside the model cell decreases only when both dipeptides and hydrogen ions are present in the external environment, suggesting that the membrane protein is a symporter, meaning it transports two molecules simultaneously in the same direction. The fact that hydrogen ions also need to be present suggests that they are being transported alongside the dipeptides by the membrane protein. Therefore, option B is the best choice.

Refer to the Figures Below