An experiment was conducted to investigate the effect of extracellular sodium ion (Na+\text{Na}^+Na+) concentration on the rate of alanine (an amino acid) uptake by epithelial cells lining the ileum.
The results are shown in the table below:
| Extracellular Na+\text{Na}^+Na+ concentration / mmol dm−3\text{mmol dm}^{-3}mmol dm−3 | Rate of alanine uptake / arbitrary units\text{arbitrary units}arbitrary units |
|---|---|
| 0 | 2.0 |
| 20 | 8.0 |
| 40 | 17.0 |
| 80 | 32.0 |
| 120 | 44.0 |
| 140 | 44.0 |
Calculate the percentage increase in the rate of alanine uptake when the extracellular Na+\text{Na}^+Na+ concentration increases from 20 mmol dm−320\text{ mmol dm}^{-3}20 mmol dm−3 to 80 mmol dm−380\text{ mmol dm}^{-3}80 mmol dm−3. Show your working.
Explain how the movement of Na+\text{Na}^+Na+ into the cell allows the absorption of alanine, and why the rate of alanine uptake plateaus above 120 mmol dm−3120\text{ mmol dm}^{-3}120 mmol dm−3.
The active transport of Na+\text{Na}^+Na+ out of the cell is coupled to the action of a membrane-bound ATP hydrolase. Explain the function of this ATP hydrolase in maintaining alanine absorption.