An in vitro study was conducted on rat ileal tissue to investigate how the concentration of extracellular sodium ions (Na+\text{Na}^+Na+) affects the rate of absorption of galactose (a monosaccharide carbohydrate) by enterocytes lining the mucosal surface.
The experimental findings are 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 | 4.5 |
| 30 | 11.5 |
| 60 | 22.5 |
| 100 | 31.5 |
| 120 | 31.5 |
Calculate the percentage increase in the rate of galactose uptake when the extracellular Na+\text{Na}^+Na+ concentration increases from 15 mmol dm−315\text{ mmol dm}^{-3}15 mmol dm−3 to 60 mmol dm−360\text{ mmol dm}^{-3}60 mmol dm−3. Show your working.
Explain how the transport of sodium ions (Na+\text{Na}^+Na+) into the enterocyte facilitates the absorption of galactose, and suggest why the rate of uptake becomes constant at concentrations above 100 mmol dm−3100\text{ mmol dm}^{-3}100 mmol dm−3.
The active transport of sodium ions out of the enterocyte is mediated by the membrane-bound sodium-potassium pump (Na+/K+\text{Na}^+/\text{K}^+Na+/K+-ATPase). Explain how the action of this pump maintains the continuous absorption of galactose.
