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zen_arcade zen_arcade
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12 years ago
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!
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wrote...
Staff Member
12 years ago
The Na+/K+ ATPase pump on the basolateral membrane of the proximal tubule cell uses ATP to move 3 sodium outward into the blood, while bringing in 2 potassium. This creates a downhill sodium gradient inside the proximal tubule cell in comparison to both the blood and the tubule. The SGLT proteins use the energy from this downhill sodium gradient created by the ATPase pump to transport glucose across the apical membrane against an uphill glucose gradient.

I think that's how the professor derived this statement:

"the transporter will run in the opposite direction and use the energy of the glucose gradient to transport Na+ out of the vesicles."
- Master of Science in Biology
- Bachelor of Science
zen_arcade Author
wrote...
12 years ago
Not sure I understand your answer.

The question does not ask about the cells of the proximal tubule or even about SGLT specifically. It describes a reconstituted lipid vesicle that has been engineered with only Na+/glucose symporters in the membrane, and only Na+ and glucose in the system.

Under those conditions, it asks, what would happen if Na+ concentrations inside and outside of the cell were equal but glucose concentration was higher intracellularly than extracellularly. Would Na+ or glucose move in or out?

Since the symporter requires an electrochemical gradient established by unequal concentrations of Na+ on either side of the membrane, I argued that nothing would happen. Sure there's a gradient of glucose but a glucose gradient cannot transport ions that are equilibrated across the membrane, right? How would that work?
wrote...
Staff Member
12 years ago
The question does not ask about the cells of the proximal tubule or even about SGLT specifically. It describes a reconstituted lipid vesicle that has been engineered with only Na+/glucose symporters in the membrane, and only Na+ and glucose in the system.

My apologies. I referenced that because that's were these symporters are typically found and studied.

As in:



In the human gut, glucose is taken up by the sodium-glucose transport protein (SGLT)1 symporter along with Na+. The concentration gradient of Na+ over the cell membrane provides the driving force for uptake of glucose by this symporter. 



Since this symporter doesn't require energy (it's facilitated diffusion), you're absolutely right.

The professor may have mistaken this with an antiporter.
- Master of Science in Biology
- Bachelor of Science
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