An investigation was carried out to analyze the relationship between the extracellular sodium ion (Na+\text{Na}^+Na+) concentration and the rate at which galactose (a monosaccharide) is absorbed by the microvilli-bearing epithelial cells of the mammalian ileum.
The experimental data collected is presented in the table below:
| Extracellular Na+\text{Na}^+Na+ concentration / mmol dm−3\text{mmol dm}^{-3}mmol dm−3 | Rate of galactose uptake / arbitrary units\text{arbitrary units}arbitrary units |
|---|---|
| 0 | 1.5 |
| 15 | 6.0 |
| 30 | 15.0 |
| 90 | 39.0 |
| 120 | 45.0 |
| 150 | 45.0 |
Calculate the percentage increase in the rate of galactose uptake when the extracellular Na+\text{Na}^+Na+ concentration is increased from 15 mmol dm−315\text{ mmol dm}^{-3}15 mmol dm−3 to 90 mmol dm−390\text{ mmol dm}^{-3}90 mmol dm−3. Show your working.
Explain how the movement of Na+\text{Na}^+Na+ into the epithelial cells facilitates the absorption of galactose, and suggest why the rate of galactose uptake plateaus when the extracellular concentration of Na+\text{Na}^+Na+ exceeds 120 mmol dm−3120\text{ mmol dm}^{-3}120 mmol dm−3.
The active extrusion of Na+\text{Na}^+Na+ across the basolateral membrane of these cells is catalyzed by a membrane-bound ATP hydrolase. Explain the role of this enzyme in maintaining continuous galactose absorption.
