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Date November 2018 Marks available 7 Reference code 18N.2.HL.TZ0.8
Level Higher level Paper Paper 2 Time zone TZ0 / no time zone
Command term Explain Question number 8 Adapted from N/A

Question

Extensive areas of the rainforest in Cambodia are being cleared for large-scale rubber plantations. Distinguish between the sustainability of natural ecosystems such as rainforests and the sustainability of areas used for agriculture.

[3]
a.

Describe the roles of the shoot apex in the growth of plants.

[5]
b.

Research suggests that many living plant species are polyploid. Explain how polyploidy occurs and, using a named example, how polyploidy can lead to speciation.

[7]
c.

Markscheme

a. sustainable communities/ecosystems allow continued survival of organisms/OWTTE ✔

b. natural ecosystems can be sustainable over long periods of time/OWTTE ✔

c. natural ecosystems/rainforest more sustainable than agricultural areas/plantations ✔

d. diverse community/high biodiversity/higher biodiversity in natural ecosystems/rainforest
OR
less/low biodiversity in agricultural areas/agricultural soils ✔

e. agricultural areas/monocultures more affected by pests/diseases ✔

f. nutrient recycling «efficient» in natural ecosystems/rainforest ✔

g. nutrients removed with crops/nutrients removed when crops are harvested
OR
less formation of humus/less organic matter in agricultural soils ✔

h. more water recycling/more rainfall/more transpiration in natural ecosystems/rainforest ✔

i. larger biomass/more carbon stored «in biomass» in natural ecosystems/rainforest ✔

j. shallower soils/less soil erosion/degraded soils/infertile soils in agricultural areas ✔

a.

a. shoot apex is an «apical» meristem/has undifferentiated cells ✔

b. mitosis «in shoot apex» ✔

c. cell division/cytokinesis/cells produced «in shoot apex» ✔

d. cell elongation «in shoot apex» ✔

e. stem/shoot growth «due to the cell division and elongation in the shoot apex» ✔

f. produces auxin ✔

g. auxin stimulates growth/cell elongation ✔

h. growth towards light ✔

i. differentiation of cells «produced by the shoot apex» ✔

j. leaf initiation/leaf development begins/leaf «primordia» formation «at shoot apex» ✔

k. flowers produced «by shoot apex» ✔

b.

a. polyploidy is having more than two sets of «homologous» chromosomes ✔

b. triploid has three sets/is 3n ✔

c. tetraploid has four sets/is 4n ✔

d. Allium/vizcacha rats/other named example» ✔

e. details of chromosome numbers in diploid and polyploid species in the example ✔

f. non-disjunction/failure of chromosome pairs to separate during meiosis ✔

g. diploid gamete «can lead to polyploidy» ✔

h. fusion of diploid and haploid gamete produces triploid cells ✔

i. DNA replication but no subsequent mitosis doubles the chromosome number/produces tetraploid «from diploid»
OR
fusion of two diploid gametes produces tetraploid/4n ✔

j. polyploid/tetraploid «crossed» with diploid/non-polyploid produces infertile offspring ✔

k. meiosis fails in triploids because «homologous» chromosomes cannot pair up ✔

l. polyploid individuals are reproductively isolated
OR
polyploidy causes instant/immediate speciation
OR
tetraploids can form a new species because they can cross with each other
OR
polyploids cannot cross/produce fertile offspring with diploids ✔

m. speciation by polyploidy is common in plants/commoner in plants than animals ✔

n. polyploid individuals tend to be larger ✔

c.

Examiners report

[N/A]
a.
[N/A]
b.
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c.

Syllabus sections

Additional higher level » Topic 10: Genetics and evolution » 10.3 Gene pools and speciation
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Additional higher level » Topic 10: Genetics and evolution
Additional higher level

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