Outline how acidified sea water could affect the shells of the oyster.

Outline the trends shown in the data in the graph.

Estimate how much smaller drilled oysters raised in seawater at a high CO₂ concentration were than drilled oysters raised in seawater at a normal CO₂ concentration.

Deduce from the data in the bar charts which factors were and were not correlated significantly with the number of oysters drilled by the gastropods.

Suggest reasons for the differences in the numbers of oysters drilled, as shown in the bar charts.

The radula in a gastropod is hard but not made of calcium carbonate. Outline how this statement is supported by the drilling success of the gastropods in seawater with normal or high CO₂ concentrations.

Using all the data, evaluate how CO₂ concentrations affect the development of oysters and their predation by gastropods.

Shells might dissolve/deteriorate / become smaller/thinner/weaker / OWTTE
OR
shell formation reduced / more difficult

a. positive correlation between shell thickness and shell size
OR
as shell thickness increases, shell size «also» increases 

b. (positive correlation) occurs at two different CO₂ concentrations / both high and normal concentrations 

c. trend for thickness is «slightly» lower with high CO₂

«approximately» 0.2 mm²
OR
«approximately» 40 % «smaller» 

unit required

a. significant factor: concentration of CO₂ in which oysters were raised 

b. insignificant factor: concentration of CO₂ at which oysters were presented to gastropods

a. (because) shells are thinner/smaller when the oyster is raised in high CO₂/lower pH
OR
«because» lower pH/higher acidity prevents/reduces deposition of calcium carbonate 

b. gastropods target smaller/thinner-shelled oysters more 

c. gastropods can eat/drill thin-shelled/smaller oysters at a faster rate (and move onto another) 

d. eating smaller oysters «from high CO₂ environments» means given population of gastropods require more oysters for same food intake

a. data shows that similar numbers are drilled regardless of conditions 

b. since radulas are not affected by acidification
OR
radulas not made of calcium carbonate so (remain) strong/successful at drilling

a. the data/trend lines indicate that a higher CO₂ concentration diminishes the shell thickness, making gastropod predation more successful
OR
the bar graphs suggest that oysters raised in a higher CO₂ concentration are smaller, making gastropod predation more successful 

b. CO₂ concentrations «during feeding» do not change the occurrence of drilling/predation «by gastropods» 

c. «limitation» no information about how exaggerated the CO₂ concentrations were
OR
«limitation» no information about numbers of gastropods used «in each setting»

State the organism most closely related to the lizards.

Based on the taxa shown, deduce a difficulty in gathering data to study turtle ancestry.

Molecular evidence is often used to construct a cladogram. Describe one type of molecular-based evidence to identify members of a clade.

Suggest one type of additional evidence that could provide strong support for Turtles 3 as the evolutionary route for turtles rather than Turtles 1 or Turtles 2.

Taxonomists aim to place species into genera, families and higher taxa according to their evolutionary origins. This is known as natural classification.

Explain the usefulness of natural classification in biodiversity research.

Tuatara

some «taxa» are extinct
OR
convergence «of body form» could have occurred (confusing interpretation of the data)

a. base sequences of a gene/DNA/mtDNA
OR
amino acid sequences of a protein 

b. species with the most similarities «in base sequence/amino acid sequence/genomes» have recently diverged/a common ancestor/are closely related
OR
members of a clade accumulate the fewest mutations on same base sequences/ vice versa / OWTTE

fossils / comparative anatomy / homologous structures / vestigial structures

a. «because» it allows easier identification of a species 

b. «because» it can help identify common ancestors/evolutionary paths/close relationships (showing degree of biodiversity) / OWTTE 

c. «because» it is universal/cross-cultural language that avoids problems of local names of organisms
OR
«because» it promotes international collaboration
OR
«because» it facilitates access to the history/background of the species /indexing for retrieval of relevant «taxonomic» information / OWTTE 

d. «because» it allows «biodiversity» research of larger taxa «ie examination of a family of large cats rather than one species»

State one reason that viruses are not classified as living organisms.

State the plant phylum which is characterised by the absence of vascular tissue.

C. nemoralis (pictured below) is a mollusc. Identify two external features that distinguish this snail from an arthropod.

Outline the role of plant pigments in the process of photosynthesis.

a. they do not have a metabolism/homeostasis/other specifically named life function 

b. cannot reproduce by themselves 

c. they are not cells/they need a host cell

bryophyta

a. unsegmented body (whereas arthropods are segmented) 

b. shell (versus exoskeleton in arthropods) 

c. muscular foot (which arthropods do not have) 

d. no (jointed) appendages/(jointed) legs (whereas arthropods have jointed legs/appendages) 

e. slimy/mucus-covered / arthropod is not slimy

Do not award marks for any answers after the first two given.

a. pigments/chlorophyll absorb light 

b. red and blue/violet light absorbed 

c. absorption of light energy is necessary for photolysis/use of water in photosynthesis 

d. other pigments allow for wider action spectrum than the absorption spectrum of chlorophyll

Determine the maximum percentage of yellow base colour shells found in woods.

Suggest either one possible advantage or one disadvantage of having a banded shell, stating whether it is an advantage or disadvantage.

Using the data in the graph, distinguish between the distribution of C. nemoralis shells in woods and fields.

Deduce from the data in the graph which shell base colours are on average most and least frequent among adult snails.

Most frequent: 

Least frequent:

Discuss whether there is evidence in the data that colour plays a role in the survival of the snails.

Using the theory of natural selection, explain the differences shown in the graph between the three colours of snail.

78 (%) 

Accept answers ranging between 77 and 80 %.

advantage: camouflage
OR
disadvantage: visibility

a. more yellow in fields (than in woods) / vice versa 

b. more unbanded in woods (than in fields) / vice versa 

c. more overlap within banding than within yellow colour
OR
yellow colour range greater than banding range 

d. very little overlap between fields and woods / some outliers

Do not accept answers with only numerical data.

a. brown most frequent 

b. pink least frequent 

Evidence that colour plays a role:
a. few yellow adults (relative to juveniles) means that yellow juveniles do not survive into adulthood 

b. frequent brown adults (relative to juveniles) means brown juveniles survive well into adulthood 

Evidence that colour does not play a role:
c. similar numbers of adult and juvenile pink individuals means pink colour does not play a role 

d. all three colours show wide variation/considerable overlap therefore evidence is not strong 

a. natural selection requires that snails become adults/live to reproduce their variations/undergo differential predation OWTTE 

b. higher adult frequency of brown shows selection 

c. but results for pink do not show selection 

d. more brown juveniles survive into adulthood showing that brown is selected for/vice versa against yellow 

e. not enough alone to support theory of natural selection but may be added evidence to similar observations in other organisms OWTTE

State the phylum of this plant.

State two characteristics of plants from the phylum you stated in (a)(i).

Outline why the number of trophic levels is limited in a food chain.

Filicinophyta/Filicinophytes/Pteridophytes

Reject “ferns”

a. have roots, stem and leaves

All three, roots, stem and leaves required

b. pinnate leaves/leaves divided «repeatedly» into leaflets

c. have vascular tissue/xylem and phloem

d. produce spores/sporangia

    OR

    no flowers/fruits/seeds

[Max 2 Marks]

energy losses between trophic levels

OR

only part of the energy in one trophic level will become part of the next trophic level

Sketch the complementary strand to complete the section of a DNA diagram.

Define mutation.

Explain how evolution by natural selection depends on mutations.

a. correct base sequence: T, G, A;

b. strand drawn anti-parallel;

c. correct shapes used;

Award [2 max] if bonds are not from the correct carbon or if the nucleotides are not joined.

change in genetic makeup/DNA/nucleotide/base sequence

a. mutations cause variation among organisms of same species/population;

b. some variations/mutations make individual more suited to its environment/way of life;

c. individuals that are better adapted survive and produce offspring;

d. individuals pass on genetic characteristics/mutation/variation to offspring;

e. natural selection increases frequency of characteristics/alleles that make individuals better adapted;

In part (a) most could gain marks for the correct shapes and sequence but did not gain the third mark as it was not shown as antiparallel. Some lost marks as a 2MAX was awarded if the bonds were not from the correct carbons. In part (b) the definition of mutation was quite well known, and most gained some marks in the explanation. Weaker students just wrote about evolution without relating it to mutations as asked. They also tended to give unnecessary examples, involving finches and moths etc.

Compare and contrast the mode of nutrition of detritivores and saprotrophs.

The image shows an example of a soil food web.

[Source: Anon., n.d. The Soil Food Web. [image online] Available at: https://www.nrcs.usda.gov/wps/portal/nrcs/
photogallery/soils/health/biology/gallery/?cid=1788&position=Promo [Accessed 11 March 2020].]

Draw a food chain from this food web, showing at least three organisms.

Explain the reasons for food chains rarely containing more than four or five trophic levels.

The amount of food passing into food chains can be affected by the rate of photosynthesis. Explain the effect of one limiting factor on photosynthesis.

All of the leaves in the image are from Solanum, a wild genus of tomato.

[Source: Courtesy: National Science Foundation, Credit Leonie Moyle.]

State one cause of variation in a plant such as the tomato.

Accept not autotrophic/not photosynthetic instead of heterotrophic.

Do not accept that both groups are decomposers or consumers for the similarity.

food chain of three or more organisms starting with plants;

a. energy is lost between the trophic levels;
b. transfer between levels is only usually 10% efficient
OR
energy transformations take place in living organisms / the process is never 100% efficient;
c. energy is lost by the organism/used in respiration / released as heat/movement;
d. energy is lost as waste/feces/urine/undigested food/uneaten parts;
e. as energy is lost between trophic levels and so (higher ones) have less biomass / less biomass available for next level;

a. the rate of reaction will be limited by the limiting factor that is nearest to its minimum value;

temperature:
b. enzymes that control photosynthesis are influenced by temperature;
c. as temperature increases, reaction rate will increase;
d. above a certain temperature, the rate of photosynthesis will decrease;
e. (where temperature is limiting) essential enzymes begin to denature/not working to optimum;

light intensity:
f. light is source of energy / converted into chemical energy;
g. as light intensity increases reaction rate will increase;
h. at a certain light intensity, rate of photosynthesis will plateau;
i. another factor becomes limiting;

CO₂ concentration:
j. CO₂ is fixed to form organic molecules;
k. as CO₂ concentration increases, reaction rate will increase;
l. at a certain concentration of CO₂, rate of photosynthesis will plateau;
m. another factor becomes limiting;

Accept answers using an annotated graph to explain

Only accept the first factor described

Do not accept pH as a limiting factor

a. mutations;
b. meiosis/crossing over/random assortment of homologous pairs;
c. sexual reproduction/recombination/random fertilisation;

Knowledge of detritivores and saprotrophs was good in 3a, with many of the poorer answers describing them as feeding on inorganic material.

Many took the word ‘draw’ literally in the food chain, and the drawings of the nematodes and fungi amused the examiners. A food chain must start with a plant. The stem did say “three organisms”. Detritus was not taken as an organism, so negated the mark.

Most were able to gain at least two marks for the explanation of energy loss through the trophic levels.

In 3c, most knew that temperature/CO₂ concentration/light affect photosynthesis but could not explain the idea of acting as a limiting factor. Only the better students gained all 3 marks.

In 3d there was a great deal of variety in the leaf shape, so an answer in terms of sexual reproduction was looked for.

Identify the dominant plant phylum in the boreal forest.

In some areas there are gaps in the boreal forest where trees fail to grow and peat tends to accumulate. Suggest reasons for this.

An increase in global temperatures poses a critical threat to boreal forests. Explain the consequences of climate change to this northern ecosystem.

The boreal forests are situated close to the north pole and even in summer the intensity of sunlight is lower than at the equator. Sketch a graph showing the effect of light on the rate of photosynthesis, labelling the axes.

coniferophyta/conifer/coniferous/gymnosperms/pinophyta ✔

a. waterlogged soil/poor drainage
OR
acidic soil
OR
anaerobic conditions/soil ✔

b. organic matter is not «fully» decomposed «leading to peat formation»
OR
decomposers/saprotrophs less active/fewer in cold «temperatures» ✔

a. higher temperatures so more transpiration/droughts/dehydration/water shortage ✔

b. more forest fires ✔

c. more/new pests/diseases because of the changed conditions ✔

d. competition from trees/plants «that colonize/spread to boreal forests» ✔

e. trees/«named» organisms «of boreal forests» not adapted to warmer conditions
OR
trees/«named» organisms migrate/extend range due to the warmer conditions ✔

f. trees die so there is loss of habitat for animals ✔

g. faster decomposition/nutrient cycling «so conditions in the ecosystem change» ✔

h. standing water/floods due to more snow/permafrost melting ✔

a. x-axis labelled as light intensity/amount of light AND y-axis labelled as rate of photosynthesis/rate of oxygen release/rate of carbon dioxide uptake ✔

b. curve starting at/slightly to the right of the x-axis origin and rising rapidly and then more slowly and plateauing but never dropping ✔

State one feature that characterizes these species as mammals.

Identify the two species most closely related.

1.

2.

Identify two species from the list that are classified in different genera.

1.

2.

fur/hair/mammary glands/feed young with milk/three inner-ear bones/lungs ventilated by diaphragm/lungs contain alveoli ✔

Canis aureus/golden jackal AND Canis lupus/grey wolf ✔

Both needed

Hapalemur aureus/Golden bamboo lemur AND Canis aureus/golden jackel/Canis lupus/grey wolf/Vulpes vulpes/red fox
OR
Vulpes vulpes/red fox AND Canis aureus/golden jackal/Canis lupus/grey wolf/Hapalemur aureus/golden bamboo lemur ✔

The most common correct answers were having fur/hair or providing milk. Warm-blood or live birth were often given but were not accepted as other classes of animals exhibit these characteristics.

Well done but some thought the species aureus meant they were similar rather than choosing the genus name.

Draw a labelled diagram to show the structure of a single nucleotide of RNA.

Distinguish between the processes of meiosis and mitosis.

Explain the development of antibiotic resistance in terms of natural selection.

1. ribose drawn as a pentagon and labelled;
2. base linked correctly (to C1) of ribose and labelled;
3. phosphate linked correctly (to C5) of ribose and labelled;

Award [2 max] if more than one nucleotide drawn.

“Sugar” alone is insufficient.

1. antibiotics can (generally) kill/destroy bacteria;
2. some bacteria show variation/antibiotic resistance;
3. variation/resistance is due to a random mutation;
4. resistant bacteria are not killed/destroyed by the antibiotic
   OR
   bacteria without the mutation die;
5. (resistant) bacteria have a selective advantage / unequal success;
6. the bacteria with this variation/resistance reproduces/multiplies;
7. mutation/gene is passed on to the offspring / the offspring will be resistant to the antibiotic;
8. resistant bacteria become more common;
9. bacteria have evolved to be resistant to the antibiotic;

Award [6 max] if pathogen is used instead of bacteria throughout the answer with no mention of bacteria.

Occasionally, deoxyribose was seen rather ribose; sometimes more than one nucleotide was shown limiting maximum marks.

The best answers had clear comparative statements or were in a comparison table. The worst answers were continuous prose descriptions of the processes, as these all too often failed to be able to be pieced together to make full marking points.

About half of the candidates did not mention cell divisions, but simply mentioned "two cycles" which was too vague and instead of writing haploid/diploid, many said 23/46, which only applies to human or certain organisms.

This natural selection question had better responses than in previous years. There seems to be a widespread misconception that bacteria can mutate after exposure to an antibiotic rather than surviving because of a genetic variation. Some confusion was demonstrated concerning the use of antibiotics: some candidates did not understand that antibiotics are used against bacteria (not viruses). There was lots of confusion in the use of different but similar words — antibiotics / antibody / antigen

The images show parts of plants belonging to two different phyla.

State the phylum of plant X and of plant Y.

X:

Y:

Explain one cause of mutation.

Outline the types of evidence that can be used to place a species in a particular clade.

The cladogram includes four marsupial (non-placental mammal) families.

[Source: Koala image: Quartl, https://commons.wikimedia.org/wiki/Phascolarctos_cinereus#/media/
File:Friendly_Female_Koala.JPG; Wombat image: JJ Harrison, https://en.wikipedia.org/wiki/Wombat#/
media/File:Vombatus_ursinus_-Maria_Island_National_Park.jpg; Marsupial lion: Nobu Tamura,
https://en.wikipedia.org/wiki/Marsupial_lion#/media/File:Thylacoleo_BW.jpg;
Diprotodontoidea image: Anne Musser]

Deduce the family that is most closely related to the Diprotodontoidea.

X: Filicinophyta ✔

Y: Coniferophyta/Conifera/Gymnosperms ✔

Not chromosomal.

a. a clade is a group of organisms that have evolved from a common ancestor ✔

b. identify the base sequences of a gene ✔

c. identify amino acid sequence of a protein ✔

d. comparing homologous structures ✔

e. the fewer the differences, the closer they diverged in time from a common ancestor ✔ Accept vice versa.

Vombatidae/wombats ✔

Those who had been taught it could identify the filicinophyte and the coniferophyte, albeit with a variety of spellings.

Many were confused by a relatively simple question, but a large proportion gained both marks by linking radiation or carcinogens to changes in the DNA base sequence.

Those who had been taught it gained at least two marks for evidence and clades, with comparing base sequences of genes and amino acid sequences of proteins amongst the most common correct answers.

Nearly everyone spotted the wombat correctly.

Based on their structure, the insect and bat wings are analogous. Outline what is meant by an analogous trait.

The bird and bat wings share homologous bone structures whereas the insect wing does not. Outline the conclusion that can be drawn about the evolution of these wings, based on homologous structures.

Explain how cladistics can be used to investigate evolutionary relationships.

Cladistics and other evolutionary evidence suggest that mammals and birds have a more recent common ancestor than mammals and amphibians. Draw a cladogram to show the relationships between mammals, birds and amphibians.

a. analogous traits have a different evolutionary history/ancestry ✔

b. different structures are adaptations for flight ✔

c. selective pressure leads to a similar solution to the problem of flying ✔

a. bird and bat share a more recent common ancestor (than the insect) ✔

b. bird and bat are more closely related than insect and bat / insect and bird ✔

c. bird and bat wings evolved from a common ancestor (by natural selection) ✔

a. (cladistics) shows evolutionary relationships through a common ancestor ✔

b. cladistics uses DNA/protein/derived/shared anatomical characteristics/traits ✔

c. time of divergence is related to the number of differences in DNA (base sequence) / protein (sequence of amino acids) ✔

d. homologous (versus analogous) traits are used to place an organism in a clade ✔

e. more shared characteristics mean a more recent common ancestor ✔

What is important is that the proper relationships are shown not the style used.

Candidates found it hard to express themselves without contradiction.

In both parts the key to successful answers was in using the organisms given in the example. Contradictions were thus avoided.

Candidates found it hard to express themselves without contradiction.

In both parts the key to successful answers was in using the organisms given in the example. Contradictions were thus avoided.

Candidates found it hard to express themselves without contradiction.

Carrying examples into part (b) made for the most successful answers. Very few gained marks for providing answers that included molecular level cladistic evidence, e.g. DNA, protein sequencing, etc.

Candidates found it hard to express themselves without contradiction.

Examiners accepted a wide variety of diagrams and orientations as long as the relationships among amphibians, birds and mammals were appropriate. Nevertheless, there was poor performance on this question. It was the question most often left blank in Section A.

State one type of environmental factor that may increase the mutation rate of a gene.

Identify one type of gene mutation.

State two characteristics that identify lice as members of the arthropoda.

1.

2.

Some lice live in human hair and feed on blood. Shampoos that kill lice have been available for many years but some lice are now resistant to those shampoos. Two possible hypotheses are:

Discuss which hypothesis is a better explanation of the theory of evolution by natural selection.

a. radiation 

b. chemical mutagens/carcinogens/papilloma virus/cigarette smoke

base substitution/insertion/deletion/frameshift

a. jointed appendages 

b. «chitinous» exoskeleton 

c. segmented body
OR
bilateral symmetry
OR
mouth AND anus
OR
paired appendages

a. «scientists would accept» hypothesis A as the better one as mutations are random 

b. scientists would reject hypothesis B because characteristics acquired during the lifetime of the individual being inherited is Lamarckian/not part of the evolution by natural selection theory/not all mutations are heritable 

c. «the resistance» mutation would be present in the population initially and not caused by the shampoo «as hypothesis B states» 

d. both hypotheses include variation in the population of lice «resistant and non-resistant» 

e. variation is necessary for natural selection to occur 

f. frequency of the best adapted increases and these individuals reproduce/pass on resistance to their offspring, so the resistant population increases «so hypothesis A is better»

OWTTE can be used for any of the answers in this part.

Isolated communities in rural Finland, Hungary and some of the Scottish islands have a high incidence of red-green colour blindness. Describe the inheritance of red-green colour blindness.

The human hand is an example of adaptive radiation. Outline adaptive radiation.

Explain how the human body defends itself against pathogens.

a. sex linked/gene is on the X chromosome ✔

b. allele «for red-green colour blindness» is recessive/colour blindness is recessive trait/disorder ✔

c. heterozygous females are unaffected/carriers ✔

d. X^B denotes normal allele and X^b denotes colour blindness allele ✔

e. more frequent in males because they only have one X chromosome ✔

f. 50 % chance of colour blindness in sons whose mother who is heterozygote/X^BX^b ✔

Accept any other letter for the alleles.

Award mpb, mpc, mpd and mpf if these points are clearly made on a Punnett grid.

a. «happens in a group of species that» evolve from a common ancestor ✔

b. evolution «of a structure» in different ways ✔

c. for different functions ✔

d. common features remain «despite the differences» ✔

e. homologous structures are evidence «of adaptive radiation»  

Must see “homologous” ✔

f. an example of adaptive radiation ✔

g. example of specific adaptation ✔

h. second description of a specific adaptation ✔

example 1:

f. pentadactyl limb

g. human hand is adapted for grasping/climbing/manipulation 

h. front limb of mole is adapted for digging»

example 2:

f. «Darwin’s» finches’/birds’ beaks

g. nectar feeding has a long/thin beak

h. seed feeding has a short/stout beak

a. skin/mucous membranes primary/first defence against pathogens ✔

b. tears/mucus contain enzymes/lysozymes which destroy pathogens ✔ 

c. stomach/skin/some mucus produces acid which kills pathogens ✔ 

d. phagocytic white blood cells/phagocytes/macrophages ingest pathogens 
OR
lysosomes in phagocytes/macrophages release enzymes that digest pathogens ✔ 

e. phagocytes/macrophages give non-specific immunity «to diseases» ✔ 

f. specific immunity provided by lymphocytes ✔ 

g. lymphocytes divide to produce clones «of plasma cells» ✔

h. plasma cells/lymphocytes produce antibodies ✔

i. antibodies are specific to a pathogen/antigen ✔

j. memory cells provide immunity against future attacks by same pathogen ✔

k. blood clotting/fibrin closes opening in the body so pathogens cannot enter✔

Accept leukocytes instead of white blood cells

Outline energy flow through a community in a natural ecosystem.

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

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

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

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

      Benefits:

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

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

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.

Living organisms have been placed in three domains: archaea, eubacteria and eukaryote. Distinguish archaea from eubacteria.

List two types of evidence used to determine which species belong in the same clade.

Award [1] for each correct row.
Award reasonable distinctions even if not strictly contrasted.

a. DNA/base sequences (of a gene/genes)

b. amino acid sequences (in a protein/proteins)

Do not credit references to morphology.

State the genus of this organism.

State the domain in which it is classified.

Outline the method of nutrition carried out by P. caudatum.

Outline one aspect of how P. caudatum carries out homeostasis.

Apart from the ribosomes, explain the evidence for the endosymbiotic theory of the origin of eukaryotic cells.

Paramecium

eukaryotes

heterotroph/consumer as it feeds on bacteria/algae/yeast/smaller single celled organisms
OR
heterotroph/consumer as it does not have chloroplasts

Heterotrophic must be qualified.

a. lives in fresh water so water enters cell (by osmosis);

b. contractile vacuoles collect and expel water;

c. homeostasis is keeping internal conditions within limit/constant / involves osmoregulation/regulating water content/potential;

a. mitochondria/chloroplasts show features in common with prokaryotes/similar size;

b. multiply by binary fission/in same manner;

c. have naked loop of DNA/circular DNA/own DNA;

d. surrounded by a double membrane;

In a about half of the candidates named the genus correctly (Paramaecium), but significantly fewer could state the correct domain (eukaryota). The most common wrong answers were prokaryote and eubacteria. Only the better candidates were able to outline the method of nutrition (heterotrophy) in b and describe how homeostasis is carried out, in spite of being specifically stated in the syllabus. The endosymbiotic theory was quite well known.

Outline the structures in M. tuberculosis that are not present in a human cell.

Explain the production of antibodies when a patient is infected with the TB bacterium.

Describe the risk to the human population of indiscriminate use of antibiotics.

a. cell wall ✔

b. pili/flagella ✔

c. 70S ribosomes ✔

d. nucleoid / circular DNA
OR
naked DNA ✔

e. plasmids ✔

As candidates do not need to know the structure of Mycobacterium tuberculosis, all prokaryotic structures are accepted.

Ignore references to membrane bound organelles.

a. phagocytes/lymphocytes are white blood cells ✔

b. TB bacterium has a specific antigen ✔

c. this antigen is recognised by white blood cells ✔

d. a clone of lymphocytes/plasma cells/B cells are produced ✔

e. antibodies are produced by lymphocytes ✔

f. each lymphocyte produces just one type of antibody ✔

g. (this is) specific immunity ✔

h. (part of the) antibody/immunoglobulin binds to the antigen / specific antibody binds to the specific antigen ✔

i. antibodies are proteins/immunoglobulins ✔

j. (some) plasma cells become memory cells ✔

k. memory cells reproduce quickly ✔

l. memory cells prevent infection in the future ✔

Allow annotated diagrams to explain the process.

a. antibiotics block bacterial processes ✔

b. example of bacterial process ✔ e.g. cell wall formation

c. variations exist naturally in a population / some are naturally resistant to the antibiotic ✔

d. bacteria that are not resistant to this antibiotic will die / only resistant will survive (when antibiotic given) ✔

e. this characteristic could be passed to next generation ✔

f. (natural selection) leads to changes in the proportions/frequency in the population ✔

g. human population will be exposed to antibiotic resistant bacteria and will not have antibiotic to kill them ✔

h. (antibiotic resistant bacteria) may pass resistance to other bacteria species/types by means of plasmids (so other bacteria species turn resistant too) ✔

(attempted by about 60 % of the cohort)

Part (a) was a departure from the 'draw a prokaryote' type of question that has appeared numerous times. In general, it was well answered. There were some good diagrams as answers which clearly showed only the prokaryotic characteristics. Part (b) asked for an explanation of the production of antibodies. Many weaker students wrote at length about non-specific phagocytosis which was not credited. Weaker students confused antibodies and antigens and even talked about antibiotics. This question was designed to be answered by the Standard Level students using Standard Level material. Some tried to include HL material and became very confused. Part (c) was really a question about Darwin's theory of evolution set in context. Many of the answers were very Lamarckian with the antibiotic causing the mutation, instead of some bacteria already having resistance. Better students opened with comments and an explanation that antibiotics only kill bacteria.

(attempted by about 60 % of the cohort)

Part (a) was a departure from the 'draw a prokaryote' type of question that has appeared numerous times. In general, it was well answered. There were some good diagrams as answers which clearly showed only the prokaryotic characteristics. Part (b) asked for an explanation of the production of antibodies. Many weaker students wrote at length about non-specific phagocytosis which was not credited. Weaker students confused antibodies and antigens and even talked about antibiotics. This question was designed to be answered by the Standard Level students using Standard Level material. Some tried to include HL material and became very confused. Part (c) was really a question about Darwin's theory of evolution set in context. Many of the answers were very Lamarckian with the antibiotic causing the mutation, instead of some bacteria already having resistance. Better students opened with comments and an explanation that antibiotics only kill bacteria.

(attempted by about 60 % of the cohort)

Part (a) was a departure from the 'draw a prokaryote' type of question that has appeared numerous times. In general, it was well answered. There were some good diagrams as answers which clearly showed only the prokaryotic characteristics. Part (b) asked for an explanation of the production of antibodies. Many weaker students wrote at length about non-specific phagocytosis which was not credited. Weaker students confused antibodies and antigens and even talked about antibiotics. This question was designed to be answered by the Standard Level students using Standard Level material. Some tried to include HL material and became very confused. Part (c) was really a question about Darwin's theory of evolution set in context. Many of the answers were very Lamarckian with the antibiotic causing the mutation, instead of some bacteria already having resistance. Better students opened with comments and an explanation that antibiotics only kill bacteria.

Outline the stages in the production of mRNA by transcription.

Describe the functions of proteins in cell membranes.

Explain how natural selection can lead to speciation.

a. DNA is unwound/strands are separated «by RNA polymerase» ✔

b. new nucleotides attached to template strand «by RNA polymerase» ✔

c. complementary base pairing/base pairing with an example
OR
adenine with thymine/uracil with adenine/cytosine with guanine/guanine with cytosine ✔

d. mRNA detaches from template ✔

e. DNA rewinds ✔

a. facilitated diffusion by channel proteins ✔

b. active transport by protein pumps
OR
protein pumps eg sodium-potassium ✔

c. cell recognition by glycoproteins/protein receptors ✔

d. communication/receptors for hormones/signal molecules ✔

e. cell adhesion ✔

f. allow up to one additional mark for AHL material ✔

a. natural selection is caused by selection pressures in the environment
OR
example of a selection pressure ✔

b. natural selection requires that variation exists within a species ✔

c. variation arises randomly due to mutation
OR
variation is enhanced by meiosis/sexual reproduction ✔

d. over-production of offspring promotes selection
OR
natural selection occurs when there is competition/overpopulation/predators/environmental changes/changes in selection pressures ✔

e. well adapted individuals/individuals with best variations survive to reproduce/survival of fittest ✔

f. «frequency of» genes/alleles conferring an advantage are selected for
OR
genes/alleles conferring a disadvantage are selected against ✔

g. genetic divergence/difference increases
OR
natural selection «genetically» isolates members of a species so eventually they can no longer produce fertile offspring ✔

h. genetic divergence» leads to reproductive isolation ✔

i. geographical/behavioural/ecological factors may lead to «reproductive» isolation ✔

j. prolonged «reproductive» isolation leads to speciation ✔

k. up to one additional mark for AHL information ✔

Almost all students had a clear idea of complementary base pairing and that A pairs with U in RNA. However, this question revealed weakness in the understanding of transcription and the ability to distinguish transcription from DNA replication and RNA translation. Often times candidates had hybrid answers containing information from at least two, but sometimes all three processes. Many candidates were able to gain a couple of the easier marks in 7a on transcription despite having a poor understanding of the process.

A wide variety of fairly well-developed answers. Often, they did not couple a type of protein with its function or missed marks by being too general. Saying “transport proteins” was not enough. Often the terms integral and peripheral proteins were mentioned without going beyond that. Channel proteins and facilitated diffusion was most commonly given. The mistaken idea that proteins offer structural support or strengthen the cell membrane was seen repeatedly.

There were some excellent explanations given in well-structured responses. Many candidates avoided talking about natural selection in genetics terms, losing marks in the process. Darwin’s finches, industrial melanism and the giraffe were common examples. Some students invented their own creative examples.

The idea that natural selection limits biodiversity appeared a few times. The flawed case of the peppered moth was given to exemplify speciation, however Darwin's finches were the most frequent example.

State the relationship between sloth body temperature and ambient temperature.

Explain how this relationship differs from that in humans.

Describe the trend in body posture as ambient temperature rises from 22 to 34 °C.

Suggest reasons for this trend.

The mean daily food intake fluctuated from day to day. State the month that contains the day on which the mean intake of food was highest.

Outline the relationship between ambient daily temperature and food intake in March.

Suggest, with a reason, how the activity of the sloth varies with ambient temperature.

State one feature of the sloth that would indicate it is a mammal.

body temperature increases with ambient temperature / positive correlation;

Since direct can be either –/+, no credit for direct correlation alone.

humans maintain/regulate a constant body temperature at different ambient temperatures/maintained by homeostasis;

As ambient temperature increases, the sloth spends more time with limbs spread
OR
as ambient temperature increases the sloth spends less time curled in a ball
OR
as ambient temperature rises, the posture changes from 1 to 6;

1. less surface area is exposed when curled up
   OR
   more surface area is exposed when all limbs spread;
2. curled position prevents heat loss/provides warmth
   OR
   stretched out position allows more heat loss/body cooled;

May;

1. food intake rises as daily temperature increases / positive correlation;
2. the lowest food intake corresponds to the lowest temperature;

1. the sloth will be more active at higher temperatures as it takes in more food for energy;
2. as temperature rises, the sloth uncurls to dissipate/lose heat;

Reason required.

1. mammals have mammary glands;
2. produce milk for their offspring;
3. bodies covered in hair/fur;

Characteristic must be exclusive to mammals.

If more than one answer, use the first one given.

Often steady or constant body temperatures for humans was given rather than homeostasis. Some candidates erroneously described humans as warming up in response to cold ambient temperatures.

There were a few beautiful responses where surface area was specifically incorporated into the answer achieving maximum marks.

Good reasoning was not seen often, best answers needed some of the elements in this linkage: higher temperature led to uncurling which led to movement which led to eating because of energy needs; "sloth prefers staying curled up so cold does not affect them" or "sloth enjoys staying curled up" were not acceptable

Have hair/fur or feed young with milk were the best answers. Many students wrote "give birth to live young;" While this is true of most mammals, monotremes lay eggs. Furthermore, some other animals give birth to live young (ovoviviparous) such as some reptiles and some groups of fish. We credited only characteristics exclusive to all mammals.

Describe the endosymbiotic theory.

Explain the need for halving the chromosome number during a sexual life cycle and how this is done.

Outline the binomial system of classification.

a. theory that eukaryotic cells evolved from prokaryotes
OR
origin of eukaryotic organisms based on some organisms living inside/engulfed by other organisms
OR
prokaryotic cell engulfed another prokaryote including it in cytoplasm without digesting it;
b. mitochondria/chloroplasts have double membranes;
c. mitochondria/chloroplasts have their own DNA/loop of DNA/naked DNA;
d. mitochondria/chloroplasts have similar size to prokaryotes;
e. mitochondria/chloroplasts can reproduce by binary fission;
f. mitochondria/chloroplasts have 70S ribosomes (same as prokaryotes);

a. chromosome number is halved so the zygote/offspring has same number as the parent / so that chromosome number is not doubled;
b. process is meiosis;
c. DNA/chromosomes replicate (so each chromosome consists of two chromatids);
d. homologous chromosomes pair in prophase I;
e. (these) separate in anaphase I into two cells;
f. (after meiosis I) cells are haploid;
g. in meiosis II chromatids are separated;
h. result is four haploid cells/gametes;
i. each gamete is genetically unique;
j. (uniqueness) is due to crossing over/independent assortment/random alignment of chromosomes;
k. fertilization results in the formation of a diploid zygote;
l. (fertilization) results in variation in a population

a. the binomial system of names for species is universal among biologists
OR
named according to a globally recognized scheme;
b. allows to classify organisms into groups based on similar characteristics/common ancestry/DNA;
c. every species is given a binomial name;
d. members of the same species can mate and reproduce fertile offspring
e. genus is written first, followed by species;
f. genus is capitalized, (followed by) species is lower case
OR
an underlined correct example/stated that it must be underlined or italicized;

The theory of endosymbiosis was quite well known. Common errors included the original engulfing process being carried out by a eukaryote, rather than a prokaryote.

This provided a departure from the usual questions requiring a straight recall of meiosis. In general, it was well attempted, with all but a few students failing to state that meiosis was at the heart of the question. Despite the question stating halving of the chromosome number, many insisted on writing about chromosomal errors such as Down syndrome, which was not pertinent to the question.

This part was about the use of the binomial system for naming organisms. Weaker students insisted on describing the complete classification starting from kingdom, which was not required and put themselves in danger of losing the clarity mark for including superfluous material. Most were able to gain 2 marks for saying that the genus was written before species and about the use of capital/lower case or underlining or italicising.

Outline the roles bacteria play in the carbon cycle.

Describe the evolution of antibiotic resistance in bacteria.

Explain the process of genetically modifying bacteria.

a. decomposition of dead organic material «by saprotrophic bacteria» 

b. «decomposition» leads to CO₂ formation/regeneration due to respiration 

c. «saprotrophic bacteria only» partially decompose dead organic matter in acidic/anaerobic conditions in waterlogged soil 

d. results in peat formation in bogs/swamps 

e. photosynthetic bacteria/cyanobacteria fix CO₂ in photosynthesis

a. problem results from excessive use of antibiotics by doctors/veterinarians/in livestock
OR
low antibiotic doses taken by patients (not finishing treatment) 

b. natural variation exists in any population of bacteria making some resistant to a specific antibiotic 

c. variation arises from mutation
OR
antibiotic resistance can be transferred between bacteria by plasmids 

d. antibiotic kills all bacteria except those that are resistant 

e. resistant bacteria survive, reproduce and pass on resistance to offspring 

f. soon population is made of mainly antibiotic resistant bacteria 

g. this is an example of natural selection «increasing frequency of characteristics that make individuals better adapted to environment»

a. genetic modification carried out by gene transfer between species 

b. genes transferred from one organism to another produce the same protein/amino acid sequence 

c. due to universality of genetic code
OR
organisms use same codons of mRNA to code for specific amino acids 

d. mRNA for required gene extracted/identified 

e. DNA copies of mRNA made using reverse transcriptase 

f. PCR used (to amplify DNA to be transferred) 

g. genes/DNA transferred from one species to another using a vector 

h. plasmid acts as vector to transfer genes to bacteria/E. coli 

i. plasmid cut open at specific base sequences using restriction endonuclease
OR
plasmid cut to produce blunt ends then extra cytosine/C nucleotides added
OR
sticky ends made by adding extra guanine/G nucleotides
OR
mention of sticky ends if not gained already 

j. cut plasmids mixed with DNA copies stick together (due to complementary base pairing) 

k. DNA ligase makes sugar-phosphate bonds to link nucleotides of gene with those of plasmid 

l. bacteria that take up plasmid are identified 

m. (genetically modified) bacteria will reproduce carrying the transferred gene 

n. example – eg: as production of human insulin using E. coli bacteria

Accept any of the points clearly explained in an annotated diagram.