HL Paper 2
Boreal forests stretch across Canada, Russia and Scandinavia. This northern ecosystem accounts for 29 % of the world’s forest areas. The long, cold winters favour tall evergreen trees with either needles or scale-like leaves. These trees are wind-pollinated and their seeds are not enclosed in a fruit. The photograph shows a typical boreal forest in winter.
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.
Suggest one advantage for the evergreen trees of the boreal forest being pollinated by wind.
Discuss the advantages of the production of seeds enclosed in fruit.
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.
In some boreal species, Rubisco is down-regulated during the winter months. Describe the role of Rubisco in photosynthesis.
Markscheme
coniferophyta/conifer/coniferous/gymnosperms/pinophyta ✔
a. waterlogged soils/poor drainage
OR
acidic soil
OR
anaerobic conditions/soil ✔
b. organic matter 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/change their distribution due to warmer conditions ✔
f. trees die so 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 ✔
animals/insects/mutualistic «relationships» not needed «for pollination»
OR
pollen not eaten by animals/insects ✔
a. seeds are protected «inside the fruit» ✔
b. seed dispersal by fruits ✔
c. example of a strategy for seed dispersal by a fruit ✔
d. dispersal reduces competition/spreads seeds away from parent plant/to colonize new areas ✔
For mpc suitable strategies are dispersal by wind, by animals ingesting/carrying away succulent fruits, by animals being attracted to colourful/sweet/tasty fruits, by animals burying nuts, by burrs or other hooked fruits sticking to animals and by self-explosion.
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 ✔
a. carbon fixation/fixes carbon dioxide/carboxylation
OR
rubisco is used in the Calvin cycle/light independent stage ✔
b. carbon dioxide linked to RuBP/ribulose bisphosphate «by rubisco» ✔
c. glycerate 3-phosphate/glycerate phosphate produced «by rubisco» ✔
Examiners report
Arabidopsis is a small flowering plant in the mustard family (Brassicaceae) that is widely used in basic research. It has a short life cycle, flowers quickly producing a large number of seeds and is easy to cultivate. It forms a circle of leaves known as a rosette that lies close to the soil. Flowers form at the end of short stems.
A study was carried out of differences in development between Arabidopsis plants grown in long days (16 hours light, 8 hours dark) or short days (8 hours light, 16 hours dark). The sixth leaf (L6) to emerge in the rosette of each plant was used in all investigations.
New leaves are initiated by the meristem and go through four stages as they develop.
• Stage 1 (S1) – rapid cell division
• Stage 2 (S2) – cell division has ceased, cell expansion continues
• Stage 3 (S3) – decreasing cell expansion rate
• Stage 4 (S4) – leaf growth complete
The start of each stage of leaf development for plants grown in long days and short days is shown above the first graph.
Leaves were removed from Arabidopsis plants that had been grown in long day and short day conditions and the concentration of starch within them was measured. This was done both at the end of the day (D) and at the end of the night (N) in each of the four stages of development (S1, S2, S3, S4).
To account for the observed phenotypic and metabolic differences, researchers analysed mRNA transcript data. They found certain transcripts over-represented in Arabidopsis plants grown in long days (dark grey) compared with the amount expected due to chance.
Other types of transcripts were over-represented in Arabidopsis plants grown in short days (light grey).
Calculate the difference (in mm2) in the mean leaf area of L6 at the start of stage 4 between the leaves of plants grown in long days and short days.
Distinguish between plants grown in long days and short days in the timing of the four stages of leaf development.
Distinguish between plants grown in long days and short days in the mean number of leaves per rosette during the experimental period.
Discuss the evidence provided in the bar chart for the hypothesis that plant leaves use up starch reserves for cell respiration during the night.
For each of the stages, identify whether the starch concentration at the end of the day is higher in the leaves grown in long day or short day conditions.
Suggest reasons for the difference in end of day starch concentrations in stage 2 (S2) for the plants grown in long days and short days.
Using the data in the bar chart, discuss the evidence for Arabidopsis plants adapting to different daylight regimes by changing the pattern of gene expression.
Using all relevant data in this question, deduce with reasons whether Arabidopsis is a long day plant or a short day plant in terms of flowering.
Markscheme
«130 − 85» = 45 «mm2»
Allow answers in the range of 40 to 50 «mm2»
a. S1/S2 is longer in short day plants
OR
the stages in long day plants are more variable in length. Accept vice versa.
b. leaves of plants grown in long day reach S2 / S3 /S4 stages sooner
OR
S1/S2/S3 completed earlier in plants grown in long days
c. leaves of plants grown in long day reach S1 later than plants grown in short days
a. rosette of plant grown in long day has fewer leaves. Accept vice versa.
b. rosette leaf number of plant grown in long day plateaus/stays constant while the number continues to increase for plants grown in short days. OWTTE.
a. lower starch levels at end of night in all stages
b. lower starch levels at end of night in both plants grown in short day and long days
c. no evidence that starch is being used for respiration
OR
starch may have been exported/stored in other tissues/example tissue «rather than used in respiration»
higher in plants grown in short days in S1 and higher in plants grown in long days for all other stages/S2, S3 and S4
Candidates must mention all stages for the mark.
a. leaves in plants grown in long day receive longer period of light / more leaf surface area so more photosynthesis occurs resulting in more starch. Accept vice versa.
b. plants in short days using starch to produce more leaves/for growth/S2 a period of rapid increase in number of leaves. Accept vice versa.
a. «mRNA» transcripts differ in plants grown in long days and short days. Accept an example of such a transcript from the bar chart
b. indicates different genes are being expressed. Accept other valid reason.
c. plants adapt to different daylight regimes by altering gene expression
d. short day length causes struggle to get enough light to photosynthesize and more «mRNA» transcripts related to photosynthesis
OR
plants produce large leaves rapidly when grown in long days which may result in more transcripts for biotic stress
a. long day plant
b. flowering hormone metabolism gene over represented in long day exposure
c. fewer leaves produced «rapidly» by plant in long day as energy shifted to flower formation. Accept other valid reasons from the data
d. plants grown in short days produce more leaves over longer period before beginning to flower/need to use light more efficiently to photosynthesize
Allow ECF if student indicates short day plant.
Examiners report
Outline how greenhouse gases interact with radiation and contribute to global warming.
Outline how plants make use of the different wavelengths of light.
Explain how organic compounds are transported within plants.
Markscheme
a. carbon dioxide is a greenhouse gas
b. methane/nitrogen oxide/water vapour is a greenhouse gas
c. sunlight/light/(solar) radiation passes through the atmosphere (to reach the Earth’s surface)
d. CO2 in atmosphere/greenhouse gases absorb/trap/reflect back some radiation/heat (emitted by the Earth’s surface)
e. CO2 in atmosphere/greenhouse gases allow short wave radiation to pass (through atmosphere) but absorb long wave/infra-red
f. solar radiation/sunlight is (mostly) short wave
g. radiation/heat emitted by the Earth is long wave/infra-red
Allow answers presented in a clearly annotated diagram.
a. light used in photosynthesis/light-dependent reactions/photolysis/photosystems/photophosphorylation/excitation of electrons/switch to flowering
b. chlorophyll absorbs red AND blue light (more)
c. chlorophyll/leaf/plant reflects/does not absorb/does not use green light
d. absorption spectrum of chlorophyll has peaks in the red and blue/sketch graph to show this
e. action spectrum shows which wavelengths plants use in photosynthesis/sketch graph of action spectrum showing peaks in the blue and red
f. accessory/other (named) photosynthetic pigments absorb different wavelengths/colours
g. violet is the shortest wavelength and red the longest
h. red light and far red/infra-red absorbed to measure length of light/dark periods
a. transported in/translocated in/loaded into phloem
b. in sieve tubes
c. by mass flow
d. from sources to sinks
e. from leaves/other example of source to roots/other example of sink
f. loading (of sugars/organic compounds) by active transport
g. cause high concentration of solutes (in phloem/sieve tubes)
h. water uptake (in phloem/sieve tubes) by osmosis/water diffuses into phloem
i. rise in (hydrostatic) pressure at source (in phloem)
j. creates a (hydrostatic) pressure gradient/higher pressure in source than sink
k. flow can be in either direction/bidirectional
Examiners report
A broad bean is the seed of a species, Vicia faba, in the Fabaceae, a family of flowering plants. This family contains many species that are used as sources of food.
On the diagram, label the testa and the radicle.
An experiment was done to test the hypothesis that temperature affects the rate of germination of the broad bean. Outline two factors apart from temperature that should be controlled in this experiment.
State the genus of the broad bean.
Broad beans are rich in starch and cellulose. Compare and contrast the structure of starch and cellulose.
Once the germinated bean grows above the ground, state the process used by the bean in the production of starch.
Markscheme
a. testa labelled ✔
b. radicle labelled ✔ (must point to the bottom half of the embryo or the tip).
a. same amount/type of soil/substrate / pH of soil ✔ e.g.: heater
b. same amount of water / humidity ✔
c. oxygen/aeration ✔
d. same measurement of germination / time ✔ e.g.: emergence of radicle
e. same number/source/age of seeds ✔
Accept discussion of light only in as much as it relates to temperature.
Vicia ✔
No mark if the species name is included.
Mark can be awarded if the genus is not capitalized.
a. both polymers of glucose molecules / polysaccharides
OR
both form 1,4 glycosidic bond ✔
b. starch is formed by alpha glucose while cellulose is formed by beta glucose
OR
in starch C1 hydroxyl groups are found in same plane while in cellulose on different planes
OR
in cellulose, alternatively the beta glucose needs to be placed upside-down in order to have C1 hydroxyl groups on the same plane
OR
two types of starch (amylose and amylopectin) but one type of cellulose ✔
One similarity and one difference.
Comparison to cellulose needed.
photosynthesis
Do not accept condensation or polymerization; if list of processes given, mark the first answer.
Examiners report
Many candidates did not correctly label the testa and in many cases the radicle was labelled too high up, pointing at the hypocotyl.
Several answers considered how light is needed for germination without any reference to its relationship to temperature.
This question presented some confusion to Spanish speaking candidates since the word "género" is the same for "genus" and "gender", so some candidates write "male/female" in their answers.
Candidates seemed to find it difficult to find similarities and differences in structure between starch and cellulose, and many wrote about their functions, or simply mentioned that both were made of "glucose" without referencing that they are both polymers. There was limited recognition that there are two forms of starch: amylose and amylopectin that differ in their degree of branching.
Plants have developed efficient methods for transport and for synthesis of foods.
Outline how the properties of water make it an ideal transport medium in plants.
Distinguish between the xylem and phloem of plants.
Explain how the light-independent reactions of photosynthesis rely on the light-dependent reactions.
Markscheme
a. polarity of water;
b. hydrogen bonds between water molecules;
c. cohesion between water molecules/water molecules stick together;
d. cohesion allows tensions/low pressures/transpiration pull/movement upward/against gravity;
e. adhesion to cellulose/cell walls generates tensions/pull (in xylem)
OR
adhesion to xylem walls/vessel walls causes capillary rise/upward movement;
f. solvent for many substances / many substances dissolve;
g. liquid at most temperatures experienced by plants / liquid so can flow;
Polarity of water and/or hydrogen bonding can be shown in an annotated diagram.
a. light-dependent reactions produce ATP/reduced NADP;
b. ATP generated by chemiosmosis/by photophosphorylation/by ATP synthase;
c. reduced NADP produced by/using electrons from Photosystem I;
d. RuBP + CO2 to glycerate 3-phosphate (in light independent reactions);
e. glycerate 3-phosphate reduced to triose phosphate (in light independent reactions);
f. ATP/reduced NADP used in the light-independent reactions;
g. reduced NADP provides electrons/hydrogen / to reduce (glycerate 3-phosphate)
OR
reduced NADP used to convert glycerate 3-phosphate to triose phosphate;
h. ATP provides energy (for reduction of glycerate 3-phosphate);
i. ATP needed to regenerate RuBP
j. ATP/reduced NADP run out in darkness
k. Calvin cycle only possible with light/in the day/is indirectly dependent on light;
Examiners report
Most candidates knew at least some properties of water that make it useful as a medium for transport in plants. As in previous papers, cohesion and adhesion are often treated as the same process or were confused. Another common error is to refer to hydrogen bonds as strong – their effects are strong because so many hydrogen bonds are formed in water but, thinking of them individually, they are weak interactions.
The average mark was 2/4 for the differences between xylem and phloem. As in 6(c) marks were often lost because only one side of a distinction was given in the answer. A common misconception is that transport in xylem is unidirectional whereas in phloem it is bidirectional. Simultaneous bidirectional transport in individual sieve tubes was hypothesized at one time but this has been falsified. Sap can move in either direction at different times in both phloem sieve tubes and xylem vessels and recent research shows that xylem sap drops back down to the roots as often as every night in some herbaceous plants, making the ascent of sap in air-filled vessels a daily task. Adhesion to cellulose (not lignin) in xylem walls and capillary action is therefore more important for water transport in plants than previously realized.
This was a fair but challenging question and it yielded the highest correlation coefficient on the paper. Answers covered the whole gamut from the thoroughly confused to the masterly. Full names of intermediates in the Calvin cycle are preferred because abbreviations such as GP are often ambiguous. Weak points in some answers were the need for reduced NADP in the reduction of glycerate 3-phosphate to triose phosphate and the need also for energy from ATP both for this reduction reaction and for phosphorylation reactions in the regeneration of ribulose bisphosphate.
Most of the surface of the Earth is covered with a wide diversity of ecosystems. Outline two general characteristics of all ecosystems.
Vascular plants can be found in a wide variety of ecosystems.
Outline active transport in phloem tissue.
Vascular plants can be found in a wide variety of ecosystems.
Explain how a plant replaces the water it loses in transpiration.
Markscheme
a. organisms/community plus the environment / biotic and abiotic «components»
b. interactions
c. ecosystems show sustainability
d. nutrients are recycled in ecosystems
e. energy flows through ecosystems
f. producers «are part of all ecosystems»
g. decomposers/saprotrophs «are part of all ecosystems»
a. active transport/pumps used to load sugars/sucrose into phloem/companion cells/sieve tubes
b. loading in sources/unloading in sinks
OR
sucrose/sugars moved from source to sink
c. active transport moves H+ out of phloem/sieve tubes «to make H+ gradient in the leaf/source»
d. H+ gradient used for co-transport of sucrose into phloem/sieve tubes/companion cells
Accept protons or hydrogen ions instead of H+ ions.
Accept the equivalent of mpc and mpd for unloading in the sink.
a. transpiration/evaporation of water causes suction/tension
b. water sucked/drawn out of xylem «in leaf»
c. water moves up in xylem
d. due to suction/tension/pulling forces
e. cohesion of water/hydrogen bonds between water molecules
f. movement from roots to leaves
g. water enters root by osmosis/due to higher solute concentration inside root
Examiners report
Compare and contrast the mode of nutrition of detritivores and saprotrophs.
Explain how some plant species are able to respond to changes in their abiotic environment and flower at a precise time of the year.
Outline the extension of the stem in plants.
Markscheme
Accept not autotrophic/not photosynthetic instead of heterotrophic.
Do not accept that both groups are decomposers or consumers for the similarity.
a. genes for flowering are activated/gene activation/changes to gene expression;
b. shoot apex changes from producing leaves/stem to producing flowers;
c. daylength/duration of the day/night length/photoperiod measured/detected/responded to;
d. short day plants flower when they have a long night/period of darkness
OR
long day plants only flower when they have a short night/period of darkness;
e. so short day plants/SDPs flower in late summer/fall/autumn/winter
OR
so long day plants/LDPs flower in spring/(early) summer;
a. apical meristem (of shoot/stem) produces cells/elongates the stem
OR
cell division/mitosis in tip/apex of shoot/stem;
b. auxin stimulates cell/stem growth/extension/enlargement;
c. elongation of cells causes stem to grow (in length);
Examiners report
About half of candidates answered correctly and there were some well-informed answers, but also many that showed a lack of familiarity with nutrition in detritivores and saprotrophs.
The only relevant changes in the abiotic environment were night length variation over the seasons of the year, which determines when flowering should occur. There were many complicated answers describing the interconversion of the forms of phytochrome, but according to the syllabus this level of detail is not expected and often the simpler ideas that plants can measure night length and respond by the timing of flowering in the year were omitted. Also mostly missing, were the idea of changes to gene expression in the shoot apex, so floral organs start to develop instead of leaves. The average score for this question was only slightly higher than one mark, but the correlation coefficient was high.
Again, accounts were varied, with stronger ones clearly explaining how the shoot apical meristem generates cells by mitosis and how elongation of these cells, stimulated by auxin, causes stem elongation. Some candidates were side-tracked by phototropism but were able to score some marks from among irrelevant ideas.
Every cell is surrounded by a cell surface membrane which regulates the movement of materials into and out of the cell.
Discuss alternative models of membrane structure including evidence for or against each model.
Outline the process used to load organic compounds into phloem sieve tubes.
Markscheme
a. early evidence showed membranes are partially permeable AND organic solvents penetrate faster than water
b. suggests they have non-polar regions
c. chemical analysis showed membranes consist mainly of proteins and lipids
d. layer of phospholipids spread over water, orientate themselves into monolayer with nonpolar/hydrophobic tails out of water and polar/hydrophilic heads in water surface
e. when shaken with water form micelles/particles with tails inwards away from water
f. Davson–Danielli model proposed phospholipid bilayer coated with protein molecules on both surfaces
g. evidence from electron microscopy «supported Davson–Danielli model»
h. three-layered structure/ sandwich/railway tracks/two dark bands with a light band between
i. model could not account for hydrophobic proteins / artifacts due to low resolution
j. fluorescent labelling / freeze fracturing later used to investigate membrane structure
k. led to Singer-Nicholson / fluid mosaic model of protein molecules floating in fluid lipid bilayer
l. shows particles/proteins project partially and sometimes right through lipid bilayer
m. indicates peripheral and integral proteins present
Accept any of the points clearly explained in an annotated diagram.
a. active transport/loading of sucrose/amino acids/organic metabolites
b. sucrose moves by apoplastic / symplastic routes
c. «loading» at source into companion cells «of sieve tubes»
d. movement «of sucrose» through plasmodesmata
e. high concentration of solutes in phloem leads to water movement by osmosis
Examiners report
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.
Describe the roles of the shoot apex in the growth of plants.
Research suggests that many living plant species are polyploid. Explain how polyploidy occurs and, using a named example, how polyploidy can lead to speciation.
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. 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» ✔
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 ✔
Examiners report
Auxin can be used to promote the development of roots from stem and leafy cuttings in some plants. In a study into the distribution of auxin in the development of these roots, scientists measured the amount of auxin in different leaves of a shoot tip of Petunia hybrida.
The figure indicates the numbering of leaves on the shoot, from L1 as the youngest and smallest to L6 as the largest and oldest leaf. The developmental stage of L5 and L6 was very similar, so L5 was not analysed. The stem base is the lowest part of the cutting where roots may form.
The graph shows the auxin concentration in the different leaves.
N-1-naphthylphthalamic acid (NPA) is an inhibitor used to block auxin transport. NPA was sprayed onto the leaves of a set of cuttings for 14 days. Development of the roots in control (non-treated) and NPA-treated cuttings was measured 14 days after taking the cuttings.
The table shows the influence of NPA on rooting.
The scientists also measured the changes in auxin concentration in L6 and the stem base during the early period of root formation. They recorded the concentration in the control and NPA-treated cuttings for 24 hours after taking the cuttings.
The scientists wanted to know whether the accumulation of auxin over time in the stem base of the controls affected expression of the GH3 gene, known to have a role in growth regulation in different plants. The technique that was used to quantify the level of transcription of the GH3 gene was Northern blotting. In this procedure the darkness
and thickness of the band is an indicator of the level of transcription of a particular gene. The image shows the result of the Northern blot from 2 hours to 24 hours after cutting.
Calculate the difference in the concentration of auxin found in L1 and L6.
. . . . . . . . . . . . . . . . . . pmol g–1
Identify the relationship between the concentration of auxin and the age of the different leaves.
Analyse the effect of NPA on the formation of roots.
Compare and contrast the changes in auxin concentration in the stem base over time for the control and NPA-treated cuttings.
Deduce the effect of NPA on auxin transport between L6 and the stem base.
Based on all the data presented and your knowledge of auxin, discuss the pattern of auxin production and distribution in the leaves and the possible relationship to root formation in leafy cuttings of Petunia hybrida.
State the name of the molecule which is produced by transcription.
Compare the pattern of GH3 transcription with the pattern of auxin concentration in the stem base control cuttings. You may use the table provided to help you to record the patterns before you compare them. (Please note: a simple
comparison in the table will not gain marks)
The scientists concluded that auxin activates the transcription of the GH3 gene. Using the information on the auxin concentration in the stem base in the graph and the Northern blot, evaluate whether this conclusion is supported.
Markscheme
45 «pmol g–1»
Allow answers in the range of 44 «pmol g–1» to 46 «pmol g–1».
less auxin as the leaves become older/larger Vice versa
OR
negative correlation from L1 to L4
L4 and L6 leaves have least auxin concentration
OR
L4 and L6/older leaves have about the same concentration of auxin/do not have significantly different concentrations
a. NPA decreased the «mean» number of roots per rooted cutting «by about 5» OWTTE
b. NPA decreased the «mean» length per root «by more than half»
c. NPA decreased the «mean» total root length per planted cutting «to about 2 % of control» OWTTE
d. NPA inhibited the formation of roots
OR
decreased all three measures
Accept other correct statements of overall changes in values.
The word “mean” is not required.
a. both decrease up to 6 hours/initially
b. NPA-treated decrease more/at a faster rate than control «up to 6 hours»
c. after 6 hours, control increases while NPA treated continues to fall
a. NPA «appears to have» no effect on concentrations/transport of auxin in L6 as control and NPA-treated remain at same «low» level
OWTTE
A valid reason must be given for the mark.
b. NPA «probably» inhibits the auxin efflux pumps/transport «in the leaves» as the levels drop in NPA-treated in stem base «but not in control»
OWTTE
A valid reason must be given for the mark.
c. the transport of auxin to the stem base must occur from younger leaves
OR
L6 is not the source of auxin in the stem base
d. NPA inhibits the auxin pumps/transport «in the leaves» as the levels drop in NPA-treated in stem base
a. L1 has the highest concentration of auxin so appears to be/is the main source/the producer of auxin
b. as leaves age, they «appear to» decrease the production of auxin
Vice versa
c. the stem base is an auxin sink as seen by the accumulation in the control stem base «where roots form»
OWTTE
d. high concentration of auxin «in the stem base» promotes root formation
Vice versa
mRNA/RNA
a. at 2 and 24 hours, auxin levels are similar and at 2 and 24 hours GH3 levels are similar
b. the pattern for the formation of auxin is similar to the pattern of transcription of the GH3 gene
OR
both decrease and then increase
c. «however» there is a lag between the peaks of the GH3 transcription and the peaks of auxin
A comparison must be made to award marks. Do not award marks for simple completion of the table.
a. the data «partially» supports the conclusion
OR
the relationship is not clear
b. the auxin concentration «seems to» rise before the transcription level increases
OR
there is a lag between auxin concentration changing and transcription level changing
OR
the auxin concentration falls before the transcription level falls
To award mp b, awareness of the lag should be demonstrated
c. more data is needed «before two hours/after 24 hours»
OWTTE