Answer:
B) Decrease extracellular pH
Explanation:
A Glucose- H+ symporter does simultaneously transport H+ (protons) and glucose from extracellular to intracellular (cytoplasmatic) medium through the same channel (protein complex). By doing so, it acts as a co-transporter (in the same direction).
To do so it uses the electrochemical gradient of the protons to move glucose against its concentration gradient (since it would be more concentrated in the inside in comparison with the extracellular medium).
But before taking advantage of protons's electrochemical gradient, this gradient has to be developed. Thus, there is an active (with energy investment) transport of H+ from the inside to the outside of the cell thank to the activity of a H+ ATPase that pumps H+ to the outside.
Then, when protons accumulate in the outside and want to flow into the inside due to the developed proton motive force, the Glucose- H+ symporter acts as a channel that allows protons to flow to the inside accompanied with glucose.
If the extracellular glucose concentration is decreased (as stated in option A), that won't lead to an increase in glucose transport as it will reduce the amount of glucose available for actually being transported (reducing the potential gradient of glucose).
If the cytoplasmatic pH is decreased (as stated in option C ) then there will be much more protons inside the cell and then the proton gradient will be less effective to force protons to flow from the outside into the inside (the electrochemical gradient will be less developed and effective).
But if the extracellular pH is decreased (as stated in option B ) then there will be much more protons outside and then the proton gradient will much better developed and effective in forcing protons to flow to the inside. Then they will pass through the symporter and by doing so they will drag more glucose into the inside.
If one adds an inhibitor that blocks the regeneration of ATP (as stated in option D) , the cell will run out of ATP and the H+ pump simply will stop pumping protons to the outside for it consumes ATP as a source of energy to pump protons against its gradient. Thus, this will prevent proton's electrochemical gradient from being developed, leading to a reduced glucose transport to the inside.
If one adds a substance that makes the membrane more permeable to protons, then they will flow through it more easily and the symporter will be less effective as a proton transporter, leading to a reduced glucose transport to the inside.
