| Date | May 2009 | Marks available | 2 | Reference code | 09M.2.HL.TZ1.1 |
| Level | Higher level | Paper | Paper 2 | Time zone | Time zone 1 |
| Command term | Evaluate | Question number | 1 | Adapted from | N/A |
Question
Genetic engineering allows genes for resistance to pest organisms to be inserted into various crop plants. Bacteria such as Bacillus thuringiensis (Bt) produce proteins that are highly toxic to specific pests.
Stem borers are insects that cause damage to maize crops. In Kenya, a study was carried out to see which types of Bt genes and their protein products would be most efficient against three species of stem borer. The stem borers were allowed to feed on nine types of maize (A–I), modified with Bt genes. The graph below shows the leaf areas damaged by the stem borers after feeding on maize leaves for five days.
Before the use of genetically modified maize as a food source, risk assessment must be carried out. A 90-day study was carried out in which adult male and female rats were fed either:
• seeds from a Bt maize variety
• seeds from the original non-Bt maize variety
• commercially prepared rat food.
All the diets had similar nutritional qualities.
Studies have shown that Bt proteins are released by plant roots and remain in the soil. One study looked at the biomass of microorganisms in soil surrounding the roots of:
• Bt maize
• non-Bt maize
• non-Bt maize with an insecticide (I).
The graph below shows the biomass of microorganisms at two different times in the growth cycle of the plants (Flower and Harvest). Error bars represent standard error of the mean.
Bt proteins act as toxins to insects, primarily by destroying epithelial cells in the insect’s digestive system. Below is the three-dimensional structure of one such protein.
Calculate the percentage difference in leaf area damaged by Sesamia calamistis between the control and maize type H. Show your working.
Discuss which species of stem borer was most successfully controlled by the genetic engineering of the maize plants.
Calculate the change in mean mass of male and of female rats fed on Bt maize from day 14 to 42.
Evaluate the use of Bt maize as a food source on the growth of the rats.
Comment on the use of Bt maize as a food source compared to the other diets tested.
Compare the biomass of microbes in the soils surrounding the roots of Bt maize and non-Bt maize.
The researchers’ original hypothesis stated that microorganisms would be negatively affected by the Bt protein released by the plant roots. Discuss whether the data supports the hypothesis.
State the type of structure shown in the region marked A in the diagram above.
Outline how this structure is held together.
Region A inserts into the membrane. Deduce, with a reason, the nature of the amino acids that would be expected to be found in this region.
Markscheme
50 12 38 (mm ); Accept 12 50 = 38
(38 50) 100 ( )76(%); (ECF)
Sesamia (was most successfully controlled);
in control plants Sesamia caused most damage;
all types of Bt/genetically modified maize/A–I show (significant) decrease in damage by Sesamia;
mark for correct numerical comparison;
Sesamia caused no damage to type E/ in one instance;
Busseola not controlled/affected by Bt/genetically modified maize/caused largest amount of damage in types A–I/increased damage in some varieties;
Eldana controlled by some types of maize / B/C/D but not others / Eldana caused least damage in control and not much difference in many maize types;
males: (440 – 325 =)115g ; (Accept answers in range 105–125 g)
females: (268 – 215 =)53g ; (Accept answers in range 51–57 g)
Units required, no workings required.
(promotes) highest rate of growth at start of study / tapering off later in the study;
Bt maize appears to cause less growth/mass gain than rat food / vice versa;
more pronounced difference in females;
no difference in growth/mass gain between Bt and non-Bt maize;
(Bt) maize may not be as good as the (commercially prepared) rat food;
Bt maize appears to be as good a food source as non-Bt maize;
Bt maize an acceptable/safe food source;
Answers require a judgement about Bt maize as a food source rather than a description.
(for both groups) overall biomasses were higher during flowering than harvest / vice versa
the microbial biomass for the Bt crop was (slightly) lower than for the non-Bt crops at flower time;
the microbial biomass for the Bt crop was (slightly) higher than for the non-Bt crops at harvest time;
data does not support the hypothesis as there is little difference between biomass found in the soil (surrounding) roots (of the Bt and non-Bt) at either time;
data does not support the hypothesis as there is a slightly positive effect at harvest;
data supports hypothesis as there is a slightly negative effect at flowering;
helix / alpha helix
hydrogen bonds;
between the turns of the helix (rather than between R-groups);
bonds between carboxyl and NH groups/C-O---H-N;
non-polar amino acids/R-groups;
(inner part of phospholipid) bilayer is hydrophobic/non-polar;
Examiners report
In comparison to similar questions in previous years, candidates were relatively successful in answering this question. Where candidates did not answer correctly, it was due to a lack of ability to calculate percent difference rather than a problem with interpreting the data.
Most candidates scored at least one mark. A common error was to interpret the results without comparison to the control.
Most candidates calculated the mean masses correctly and included the correct units.
Most candidates scored at least one mark. A common error was to focus on the difference between male and female rats rather than the food source and to not make reference to growth.
Most candidates gained the mark, but some simply repeated their answer to (d). The command term "comment‟ requires candidates to give a judgment. Commonly, candidates mistakenly described the data in response to this command term.
Most candidates gained both the marks by recognizing the difference between harvest and flowering. Like answer (f), word choice affected performance with candidates referring to the biomass of flowers for example rather than biomass of soil microbes.
Many candidates scored both marks. A common error was to answer without reference to the hypothesis.
Many candidates identified the alpha helix, though a surprising number referred to the double helix.
Most candidates identified hydrogen bonds as stabilizing the structure but very few could identify the parts of the molecule that were connected by H-bonds.
Only a minority of candidates recognized the importance of the hydrophobic nature of membrane proteins.
