I am a cell biology tutor and today a student came to me with an exam question regarding an SGLT1-like symporter that I couldn't answer -- hope someone can help me out.
The question describes an Na+/glucose co-transporter in a reconstituted lipid vesicle. It then asks the student to describe the flux of Na+ and glucose under different experimental conditions (e.g., higher concentration of Na+ extracellularly compared to intracellularly and higher concentration of glucose intracellularly compared to extracellularly). One of these conditions is:
equal concentration of Na+ inside and outside of the cell and higher concentration of glucose intracellularly compared to extracellularly.
Since co-transport requires an electrochemical gradient of one species to power transport of another species against its concentration gradient, I would argue that nothing would happen. I know that these transporters can operate in the reverse direction but I suspect intracellular Na+ concentration would have to be higher than extracellular concentration to do this.
The professor's answer was that that "the transporter will run in the opposite direction and use the energy of the glucose gradient to transport Na+ out of the vesicles."
Sorry if I'm missing something really obvious here and embarrassing myself but is this possible? Can a concentration gradient established by a polar molecule power the active transport of an ion in the absence of any other energy input? I've done a few searches on google and read the relevant section in Molecular Biology of the Cell and still can't make heads or tails of this answer.
Any help would be appreciated!