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Date	May 2022	Marks available	7	Reference code	22M.2.SL.TZ2.7
Level	Standard level	Paper	Paper 2	Time zone	Time zone 2
Command term	Explain	Question number	7	Adapted from	N/A

Question

The growing human population has an increasing demand for energy derived from crop plants. At the same time, increasing droughts that are part of climate change make it difficult to grow crops in some parts of the world.

Outline energy flow through a community in a natural ecosystem.

[5]

Explain how natural selection can cause traits such as drought resistance to develop in wild plants.

[7]

Suggest possible benefits and risks of using genetic modification to develop varieties of crop plant with traits such as drought resistance.

[3]

Markscheme

communities are made up of populations of different species;
plants receive energy from the sun/light;
convert it to chemical energy through photosynthesis;
chemical energy is stored in organic/C-compounds;
the energy is passed to other organisms through feeding / reference to food chain;
respiration (of plants and animals) converts the chemical energy (of C-compounds) to other useful forms of energy;
eventually the chemical energy is lost as heat energy;
energy is non-recyclable/lost from a community/ecosystem;
energy losses between trophic levels limit food chains/mass of top trophic levels/only about 10 % of energy is transferred;

(natural selection occurs if) there is variation in degree of drought resistance among members of a population/same species;
variation is caused by mutations (when changes occur in the DNA/nucleic bases/chromosomes);
variation during meiosis occurs (with separation of chromosomes);
variation occurs during sexual reproduction (as different alleles combine);
some variations make some plants more drought-resistant;
example of variations: deeper roots/more storage tissue for water/thicker cuticles/less opening of stomata/other verifiable variations;
these variations let some survive and reproduce better/have more offspring
OR
(these variations) confer selective advantage;
these variations/characteristics are passed onto offspring which survive better;
natural selection increases the frequency of these characteristics;
eventually leads to changes/evolution in the species / more drought-resistant plants;

Benefits:

increase crop growth/food productivity;
with limited water/ less water is used;
increase amount of land available for food production in dry areas;
Risks:
these plants may out-compete other species in the community/may cause extinction of some species/affect the food chains in the community;
the modified gene/recombinant DNA may pass to other organisms;
more grain requires more nutrients from the soil so its quality may diminish/monoculture issues;
GMO may have health effects in consumers / OWTTE;

Must include at least one benefit and one risk for [3 max].

Examiners report

This was well attempted with an average score of 3. The question was about energy, but weaker students also tended to include biomass. Fortunately, very few students failed to grasp the idea that the energy is lost from the ecosystem and not recycled.

The inclusion of ‘drought resistance’ in the stem of the question should have been a suggestion to include it, but many answers were far to general with a vague attempt to explain Natural Selection in general without explaining how the initial variation in the population came about. There were quite a few ‘Lamarckian’ answers with claims of individuals adapting to cope with the change. The average for this question was only 2.8, putting it as one of the most poorly answered.

Again, drought resistance was in the stem, but not always used. There were many general ‘rants’ about GMO crops without setting out the facts.

Syllabus sections

Core » Topic 5: Evolution and biodiversity » 5.2 Natural selection

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