Paper 2, Section B examples of past papers with mark schemes
Up to two additional marks are available for the construction of your answers. (2)
(a) Living organisms at every trophic level are part of the carbon cycle. Draw a labelled diagram of the
carbon cycle to show the processes involved.
(9)
(b)
Explain, using an example of a food chain, how trophic levels can be deduced.
(4)
(c)
Explain methods that can be used to measure the rate of photosynthesis.
(5)
MARKSCHEME
a)
Award [1] for each of the following shown on a diagram of the carbon cycle.
Award [5 max] for points not shown on a diagram.
The following show carbon which is static within the cycle at this point in time.
carbon dioxide in air/water;
(sugars/carbon compounds in) plants/producers;
(carbon compounds in) animals/consumers;
(carbon trapped in) coal/oil/gas/fossil fuels;
The following should show arrows in direction of carbon flow.
carbon dioxide absorbed by plants/producers and used in photosynthesis;
carbon dioxide released by (cell) respiration in plants/producers;
plants/producers eaten by animals/primary consumers/herbivores;
primary consumers eaten by secondary consumers;
carbon dioxide released by (cell) respiration in animals/consumers;
plants/animals die and are decomposed by (saprotrophic) bacteria/fungi;
carbon dioxide released by combustion of coal/oil/gas/fossil fuels;
carbon dioxide released by (cell) respiration in bacteria/fungi/decomposers;
forest fires/combustion releases carbon dioxide from trees/plants;
carbon dioxide emitted by volcanoes;
(b)
9 max
diagram of food chain showing at least three organisms and two linkages with arrows showing direction of energy
flow;
trophic level is a step/position in the movement/flow of energythrough an ecosystem;
(in a field situation) observe which organisms eat each other;
producer/name from example (first trophic level) does not eat other organisms/captures energy through
photosynthesis;
primary consumer/name from example (second trophic level) feeds on producers;
secondary consumer/name from example (third trophic level) feeds on primary consumers; 4 max
Since the command term is explain, the answer must be explicit. Named examples for producer and consumers in
diagram or explained example must represent a coherent food chain. Reject chains using general names such as fish or
tree or grass. But, accept sardine or oak.
(c)
measure production of oxygen;
because oxygen is a by-product of photosynthesis;
example of technique for measuring oxygen production (count bubbles/use sensors/other);
measure uptake of carbon dioxide;
because carbon dioxide is used during photosynthesis;
example of technique for measuring carbon dioxide production (sensor, aquatic pH shift);
measure biomass of (batches of) plants;
increase in biomass gives (indirect) measure of rate of photosynthesis;
Since the command term is explain, reasons must be given toreceive full marks.
(Plus up to [2] for quality)
5 max
Up to two additional marks are available for the construction of your answers.
(2)
(a)
Explain why DNA must be replicated before mitosis and the role of helicase in DNA replication.(4)
(b)
Explain how the base sequence of DNA is conserved during replication.
(5)
(c)
Describe the events that occur during mitosis.
(9)
MARKSCHEME
(a)
two genetically identical nuclei/daughter cells formed during mitosis
(so hereditary information in DNA can be passed on);
two copies of each chromosome/DNA molecule/chromatid needed;
helicase unwinds the DNA/double helix;
to allow the strands to be separated;
helicase separates the two (complementary) strands of DNA;
by breaking hydrogen bonds between bases;
4 max
(b)
DNA replication is semi-conservative;
DNA is split into two single/template strands;
nucleotides are assembled on/attached to each single/template strand;
by complementary base pairing;
adenine with thymine and cytosine with guanine / A with T and C with G;
strand newly formed on each template strand is identical to other template strand;
DNA polymerase used;
Marks may be awarded for any of the above points if clearly presented in a well-annotated diagram.
5 max
(c)
sequence of stages is prophase → metaphase → anaphase → telophase;
chromosomes condense/supercoil/become shorter and fatter in prophase;
spindle microtubules grow (from poles to equator) in prophase/metaphase;
nuclear membrane breaks down in prophase/metaphase;
spindle microtubules attach to the centromeres/chromosomes in metaphase;
chromosomes line up at equator in metaphase;
centromeres divide / (paired) chromatids separate / chromosomes separate into two chromatids in
metaphase/anaphase;
(sister) chromatids/chromosomes pulled to opposite poles in anaphase;
spindle microtubules disappear in telophase;
nuclear membrane reforms around chromosomes/chromatids in telophase;
chromosomes/chromatids decondense in telophase;
9 max
(Plus up to [2] for quality)
Up to two additional marks are available for the construction of your answers.
(2)
(a)
Describe karyotyping and one application of its use.
(4)
(b)
Describe a technique used for gene transfer.
(5)
(c)
Using a named example, discuss the benefits and harmful effects of genetic modification.
(9)
MARKSCHEME
(a)
cells undergoing mitosis are used for karyotyping;
process of mitosis is stopped at (mitotic) metaphase;
chromosomes (cut from photographs) are arranged in pairs of similar structure/homologous
chromosomes;
allows abnormalities in the chromosome number/appearance to be seen;
any valid example (e.g. in Down syndrome / gender of fetus);
detected by identifying unique feature (e.g. trisomy 21 / one extra chromosome / 47
chromosomes);
Award [3 max] for an example with no description of karyotyping.
4 max
(b)
(c)
restriction enzymes/endonucleases cut a small fragment of DNA from
an organism;
same restriction enzymes used to cut DNA of plasmid / e.g. E. coli;
sticky ends are the same in both cases;
fragment of DNA is inserted into the plasmid;
spliced together by ligase;
to make recombinant DNA/plasmids;
recombinants can be inserted into host cell and cloned;
5 max
genetic modification is when the DNA/genotype of an organism is artificially changed;
genetic modification alters some characteristic/phenotype of the organism;
named example with modification (e.g. salt tolerance in tomato plants);
benefits: [5 max]
allows crops to be grown where they would not grow naturally;
provides more food;
economic benefits;
expands world’s productive farmland;
reduces the need to clear rainforests to grow crops;
lowers cost of production;
less pesticides/fertilizers/chemicals needed so better for environment;
Award marks for any valid benefit consistent with a named example.
harmful effects: [5 max]
may be released into natural environment;
may affect food chains / unintended effects on other organisms;
may affect consumers e.g. allergies/health risks;
unfair to smaller farmers who cannot compete;
long-term effects are unknown;
risk of cross-pollination;
risk of long-term contamination of soil;
Award marks for any harmful effect consistent with the named example.
(Plus up to [2] for quality)
9 max