| Date | November 2012 | Marks available | 1 | Reference code | 12N.3.HL.TZ0.7 |
| Level | Higher level | Paper | Paper 3 | Time zone | TZ0 |
| Command term | State | Question number | 7 | Adapted from | N/A |
Question
Soil contaminated with crude oil contains a very high amount of hydrocarbons, which may be an environmental hazard. In order to understand how bacteria could be helpful to remedy such a situation, scientists created laboratory samples of soil contaminated with crude oil and analysed the bacteria growing in it by measuring the respiratory activity and C23O gene ratio. The respiratory activity is an indication of the total amount of live bacteria in soil. The C23O gene ratio is an indication of the proportion of soil bacteria capable of hydrocarbon degradation compared to the total amount of bacteria.
State the respiratory activity when the C23O gene ratio first reached its highest level.
Describe the respiratory activity as the soil treatment progresses.
The data in the graph indicates that hydrocarbon degradation occurred during the first 30 days of the experiment. Explain the evidence for this conclusion.
Scientists are interested in inserting the C23O genes into bacteria to clean up oil spills in the sea. State the term used to qualify the bacteria that are able to survive in a saline habitat.
Markscheme
30 (arbitrary units) (accept answer in the range of 29 to 31 (arbitrary units))
rapid increase at the beginning/up to around day 8;
stable phase between days 7/8 to 15;
keeps increasing (not as much) after plateau / gradual increase after day 15;
increase in respiration means more bacteria are present;
increase in gene ratio means the numbers of bacteria with C23O gene are increasing;
more bacteria with C23O gene breaks down more hydrocarbons;
after day 30 proportion of bacteria with C23O gene decreases so no longer effective/required;
halophiles
Examiners report
Many candidates gave a correct answer in (a), but a large number stated 44 a.u. instead of 30 a.u., indicating a confusion between variables and axes.
For (b), there were good answers, but many candidates did not relate to the number of days or were too vague in their description.
There were mixed answers for (c). The significance of the relationship between C23O gene ratio and hydrocarbon degradation was missed by many weaker candidates who also had difficulty in other questions, including (d) and (e).
The significance of the relationship between C23O gene ratio and hydrocarbon degradation was missed by many weaker candidates who also had difficulty in other questions, including (d) and (e).
