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OperonBiology
Prediction gridDifficulty 3/5 · ChallengingOlympiadArgumentationQuestions & methods

Blocking the sodium–glucose symporter

Topic: Membrane transport

Intestinal epithelial cells take up glucose through a Na⁺–glucose symporter (SGLT1), which carries Na⁺ and glucose into the cell together. The Na⁺/K⁺ pump uses ATP to keep Na⁺ low inside the cell. Researchers bathe isolated cells in medium with 140 mM Na⁺ and 0.2 mM of a radioactive glucose analogue that enters only through the symporter and is not metabolised; at this concentration the symporter is far from saturated. They measure the uptake rate over 30 seconds immediately after a treatment starts, and again over 30 seconds after 30 minutes of continuous treatment. Earlier work showed that once the pump stops, Na⁺ leaking into these cells runs the Na⁺ gradient down within about 10–20 minutes.

Experimental setup

Question
What powers glucose uptake through the Na⁺–glucose symporter?
Independent variable
Treatment applied to the cells
Dependent variable
Rate of uptake of the radioactive glucose analogue
Kept constant
Temperature (37 °C) · Cell type and number · 30-second measuring window
Groups
  • Control: Normal medium, no treatment
  • Treatments: The five changes listed in the grid, compared with the control

Compared with untreated cells, predict the rate of glucose uptake immediately after each treatment starts and after 30 minutes of it.

  1. Ouabain (blocks the Na⁺/K⁺ pump)

    First 30 s of treatment
    After 30 min of treatment

  2. Metabolic poisons that drain the cell's ATP within a minute

    First 30 s of treatment
    After 30 min of treatment

  3. Na⁺ in the medium replaced by choline⁺ (Na⁺-free)

    First 30 s of treatment
    After 30 min of treatment

  4. Phlorizin (binds and blocks the symporter itself)

    First 30 s of treatment
    After 30 min of treatment

  5. Glucose analogue raised from 0.2 to 0.4 mM

    First 30 s of treatment
    After 30 min of treatment

Answer every part to submit.