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.