| Date | May 2011 | Marks available | 1 | Reference code | 11M.3.HL.TZ1.13 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Calculate | Question number | 13 | Adapted from | N/A |
Question
Helicobacter pylori infection is a cause of stomach ulcers. It affects SLC26A9, which is a membrane protein present in the epithelial lining of the stomach. SLC26A9 takes part in the reversible transport of chloride and hydrogen carbonate ions into and out of the epithelial cells in order to raise the pH at the membrane to neutral levels. Entry of chloride ions into epithelial cells and removal of hydrogen carbonate ions both cause extracellular pH to increase.
To assess the function of SLC26A9, this process was reversed by artificially raising the external pH. The rate of change of extracellular pH was measured with normal epithelial cells and with modified cells with extra SLC26A9. The tests were also performed in the presence of DIDS, an inhibitor of SLC26A9.
Calculate the difference in the rate of decrease of pH between the control cells and the modified cells without DIDS.
.................. pH min–1
State the effect of DIDS on the rate of decrease of the extracellular pH.
Scientists hypothesized that Helicobacter pylori alters the ability to maintain neutral pH at the epithelial cell surface by inhibition of SLC26A9. Evaluate this hypothesis.
In further experiments, scientists observed that the levels of mRNA of SLC26A9 increased in epithelial cells when infected by Helicobacter pylori. Suggest a possible explanation for this increase.
Predict, with a reason, the effect of DIDS on stomach pH if given to an experimental subject.
Markscheme
0.13 (pH min−1) (Allow values between 0.125 and 0.135)
(DIDS) reduces the rate of decrease of (extracellular) pH;
rate of decrease reduced less than control cells / some SCL26A9 are not inhibited;
(hypothesis supported as) SCL26A9 in excess means more transport of ions;
(hypothesis supported as) when inhibited there is less transport of ions (needed to maintain neutral pH);
host cells increase transcription/ protein synthesis to make more carriers
pH will fall;
SLC26A9 transports less chloride/hydrogen carbonate ions;
Examiners report
Most were able to correctly do the calculation required.
Most were able to correctly state that DIDS reduces the rate of decrease of pH.
Candidates struggled to evaluate the given hypothesis and most did not relate the support for the hypothesis to the movement of ions.
Some candidates were able to get the mark for indicating that a possible explanation for the observation was that host cells increased transcription to make more carriers.
Some candidates correctly predicted that pH will fall but few related this to transport of ions by SLC26A9.
