State the specific type of plant tissue that the plastic tube is meant to model. 

Predict, with a reason, what will happen to the pressure in this tube as transpiration occurs.

Outline how this control set-up could be modified to test the effect of either humidity or temperature on the rate of transpiration.

xylem

a. pressure will decrease 

b. water volume decreases «in tube» due to evaporation transpiration 

c. «cohesion/tension of water column» causes increase in air volume «thus air pressure decreases» OWTTE

Alternative 1

humidity: 

a. outline of how independent variable is varied  
eg: cover experimental plant«s» with a plastic bag
OR
mist the experimental plant«s».

b. outline of control treatment  eg: control plant«s» is/are not covered/not misted.

c. control of other variable«s» eg: light is kept constant.

Alternative 2

temperature: 

d. outline of how independent variable is varied  eg: place set-up under/away from heat lamps at different distances.

e. outline of control treatment  eg: no heat lamp for control.

f. control of other variable«s» eg: use hygrometer to verify that heat lamp does not change humidity level.

Calculate the magnification of the image, showing your working.

. . . . . . . . . . . . . . . . . . . . x

The mean stomatal density for the lower epidermis of P. decandrum was around 600 per mm². Predict how the stomatal density for the upper epidermis would compare.

Calculation: size of bar ÷ 15 μm (1.5 cm ÷ 15 μm or 15 000 μm ÷ 15 μm);

Answer: 1000 x;

First marking point is for division by 15 μm;

Second marking point is for the correct answer; accept 930 and 1070 x.

(upper surface/epidermis usually has) fewer stomata/lower stomatal density/no stomata/OWTTE

Do not accept a numerical value only.

Most candidates managed to calculate the magnification correctly by dividing their scale bar measurement by the provided scale, others inverted the division or displayed incomprehensible calculations; some calculations were difficult to follow, with no apparent methodology or structure; there were too many unit conversion errors, sometimes because candidates did not use units within their calculation (e.g. measuring in centimetres, but calculating as millimetres); a limited number measured the complete image width but didn't adjust the scale bar proportionally.

Most predicted that the upper epidermis would have a lower density, although some stated the reverse.

Label tissues X and Y.

Calculate the actual width along line A-B of the parenchyma cell shown.

Describe the distribution of vascular tissues in the stem of dicotyledonous plants.

a. X: phloem ✔

b. Y: xylem ✔

A-B = $\frac{7\text{mm}}{400}$

OR

17.5 μm ✔

Accept answers in the range of 17 to 19 μm

a. stem vascular tissue is in bundles ✔

b. «bundles» form a ring

c. phloem is towards outside «of bundle»
OR
xylem is towards centre «of bundle» ✔

Allow answers in an annotated drawing

Using the drawing, deduce which plant phylum T. grandiflora belongs to, giving one visible recognition feature of this phylum.

Identify the structure labelled X.

Outline the relationship the bee has with the T. grandiflora flower.

After fertilization, seeds of T. grandiflora form in a small pod. If you were provided with Petri dishes, absorbent cotton balls and seeds, suggest how one variable affecting germination of these seeds could be investigated.

Angiospermophyta/Angiosperms
AND
flowers «as reproductive organs» ✔

Both required.

ovule ✔

a. mutualistic relationship
OR
bee gets nectar/pollen «as food» AND flower is pollinated/fertilized ✔ Both needed.

b. when bee enters, pollen from anther sticks to it ✔

c. pollen is picked up by stigma «of same or other flower» ✔

a. different values for the named independent variable ✔ Name of the independent variable must be included, eg temperature.

b. large / equal number of seeds in each Petri dish ✔

c. control of other variables «than seeds» ✔

d. mentions how germination will be determined eg appearance of radicle.
OR
how germination rate/percentage will be measured ✔ eg number germinated over time/in a set time. Do not accept measurement of growth of stem/number of leaves.

e. includes a control giving seeds all factors needed ✔

Possible factors include water, oxygen, temperature, pH, light, salt concentration.

Most could use the flower as a recognition feature, but it was more difficult for some to deduce the phylum.

This question was derived from the skill of drawing half-views of animal pollinated flowers and the understanding of mutualistic relationships involved in plant reproduction. A large number could identify the ovule, but many incorrect answers, such as ovary, were also seen, as well as totally irrelevant answers.

Although some could not write "mutualistic/mutualist", the majority could outline the relationship, but some confused pollination, fertilization and seed dispersal; it was more difficult for most to name either the anther or the stigma.

This question was an open opportunity for candidates to display their understanding of experimental processes. Although a certain number of well-structured answers were seen, this was probably the most poorly handled question of this paper. Although most had understood that only one variable could be changed at a time, it was expected they should have the basic knowledge that water is essential for germination and would go beyond experiments involving one seed with water and one with no water, as too many suggested. The majority could not understand that, to draw valid conclusions, many seeds should be used in many repeats and that controls should be used. They also had much difficulty at suggesting how results could be gathered, often confusing germination with seedling growth. One can therefore wonder about how the practical programme is delivered in many schools.

Describe briefly how scientists obtained leaf phloem sap from the potato plants.

Suggest reasons for different amounts of sucrose in the leaf phloem sap of the potato plants.

a. aphids insert stylet in «potato» plants/feed from «potato» plants

“Aphids” is essential for the mark.

b. phloem exudates/sap obtained from severed stylets

“Stylets” is essential for the mark.

a. sucrose produced by leaves during photosynthesis

b. sucrose moves/translocates from source/leaves to sink/roots/tubers
OR
sucrose carried by phloem to tuber

c. «wk 5» high sucrose with increased leaf growth/photosynthesis / OWTTE

d. «wk 5-7» more sucrose used for general plant growth / OWTTE

e. «wk 7-11» concentration sucrose increases due to greater production/photosynthesis «than usage/storage» / OWTTE

f. sucrose transformed into starch in tuber «from week 9»

g. contribution of amino acids unknown so difficult to know about different amounts of sucrose / OWTTE

Award the mark for realizing that amino acids play a role in the ratio

h. «abiotic» conditions in greenhouse may vary over time / OWTTE

Accept abiotic factors only if variation through time is explicit.

Describe the trend in the data.

Transpiration continued after the fifth leaf had been removed. Suggest what can be concluded.

State the independent variable in this investigation.

Explain how the results in the graph could have been obtained.

negative correlation between the number of leaves removed and transpiration rate Do not accept type of correlation alone, as a description is required.
OR
as more leaves are removed the transpiration rate drops ✔ OWTTE

transpiration does not only occur in the leaves
OR
transpiration through stem/shoot ✔

number of leaves «removed» ✔

a. using a potometer ✔ Accept transpirometer but not respirometer.

b. leafy shoot attached to a reservoir and a graduated «capillary» tube ✔

c. as transpiration increases water uptake «by roots» also increases ✔

d. distance/time for bubble «in capillary tube» to travel is used to measure transpiration rate ✔

State the function of the tap and reservoir.

Describe how the apparatus could be used to demonstrate that the transpiration rate is affected by air movement.

One criticism of the experiment is that it only measured the rate of transpiration indirectly. Explain how the experiment is an indirect measurement of transpiration.

Label with an R the tissue where the red dye would appear.

Label with a C the tissue where organic compounds are transported.

to reset the potometer
OR
move the air column to the right/end
OR
to replace/replenish water in tube ✔

Accept meniscus or air bubble as alternatives to column in all parts of question

a. a fan is placed so that air blows on the leaves «and the experiment is repeated» ✔  Do not accept “place plant in wind”

b. a control with no air blowing «in still air/no fan» ✔

c. distance moved by the bubble in a given time is measured
OR
time is recorded for the bubble to move a given distance ✔

d. the bubble is reset to the beginning «with the tap» ✔

e. greater distance moved by bubble over time = higher transpiration rate ✔

f. repeat experiment at same temperature ✔

g. use different speeds of fan to determine effect of a range of air movement ✔

Accept different distances from fan for mpg

a. transpiration is water evaporating from «the leaves of» a plant ✔ 

b. what is being measured is water uptake to the plant ✔ 

c. assumes no water used for other processes/photosynthesis ✔

This is not about water loss from apparatus

[Source: Nbnidhi, https:// commons.wikimedia.org/wiki/ File:Transverse_section_of_dicot _stem.jpg]

Award [1] for correct labeling of R

[Source: Nbnidhi, https:// commons.wikimedia.org/wiki/ File:Transverse_section_of_dicot _stem.jpg]

Award [1] for correct labeling of C

All sections of this question revealed that students were unfamiliar with use and set-up of the potometer. This question was also commented on frequently by teachers in the G2 form who noted the absence of reservoirs in some of the potometers used in student labs. However, it was felt that students familiar with the working of any potometer could have used the information in the diagram to answer this question correctly. 

Few candidates defined the rate of transpiration in terms of the distance travelled by the meniscus per unit time. Common misconceptions were that the air bubble in the potometer was formed as a result of photosynthesis, or that the leaves released carbon dioxide or oxygen. Some candidates explained what the results would be, revealing a misunderstanding of a “describe” question. Weaker students did not understand that the question was asking about air movement around the plant, not the movement of the air column within the potometer.

This item was rarely answered well, with many candidates simply describing what would happen in a potometer. An ability to outline limitations, assumptions and possible errors shows the ability to evaluate, and is one of the objectives of the DP Biology course. A similar question on the limitations of the potometer last year was also answered poorly.

This was another question that was difficult for students. This question again required students to understand one of the skills from the subject guide, this time in topic 9.2 (Identification of xylem and phloem in microscope images of stem and root). Student responses to this question, however, suggested that few students could identify phloem and xylem under a microscope.

This was another question that was difficult for students. This question again required students to understand one of the skills from the subject guide, this time in topic 9.2 (Identification of xylem and phloem in microscope images of stem and root). Student responses to this question, however, suggested that few students could identify phloem and xylem under a microscope.

Suggest a possible research question that could be investigated using a potometer.

Evaluate the limitations of using a potometer in the investigation you suggested in (a).

a. the research question identifies the independent variables 

b. the research question identifies the dependent variable/derived value 

c. the research question identifies the organism tested 

If several variables are given, mark the first one only.

Examples:

a. Effect of changing/increasing wind/temperature/light/humidity/surface area/comparing different species

b. accept transpiration rate

c. accept common names.

a. not all of the water taken up by the plant is used for transpiration 

b. some water taken up might be used for photosynthesis/ cell turgidity 

c. (bubble) potometer measures rate of water uptake (not transpiration directly)
OR
bubble in potometer may expand due to changes in temperature/pressure

d. (cut) plant/twig gets damaged/may not survive 

e. conditions in a lab are not equivalent to those in nature/ difficult to change only one abiotic factor 

f. difficult to extrapolate values from a twig to whole plant

Outline how the rate of water uptake is recorded in this potometer.

State a variable that needs to be controlled in this experiment.

Explain the effect of relative humidity on the rate of water uptake.

a. measure distance «of movement» of air bubble/water in capillary tube 

b. multiply by cross section of capillary 

c. record/divide by time elapsed

Allow any other valid method.

temperature
OR
atmospheric pressure
OR
solar radiation/light
OR
wind speed
OR
leaf area
OR 
size of plant

Do not accept humidity.

a. increased «relative» humidity decreases water uptake
OR
inverse relationship 

b. increased «relative» humidity lowers transpiration «rate» 

c. diffusion gradient reduced «as humidity increases» 

d. less loss of water through stomata
OR
stomata closed

Accept inverse for dry/low humidity.