| Date | May 2010 | Marks available | 3 | Reference code | 10M.3.HL.TZ1.10 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Describe | Question number | 10 | Adapted from | N/A |
Question
The element arsenic (As) is not needed for plant growth and development. The accumulation of arsenic in the Chinese brake fern (Pteris vittata) was studied. Young ferns with five or six leaves were transplanted to soil contaminated with arsenic and were grown for 20 weeks in a greenhouse.
The graph below shows the arsenic concentrations in leaves and roots of the Chinese brake fern during the 20 weeks after transplanting. Arsenic concentration is expressed as mg As kg−1 dry weight of plant.
The table below shows the total amount of arsenic accumulated by the Chinese brake fern, expressed as a concentration in the plant tissue and as a percentage of the arsenic originally in the soil.
Using the data in the graph, describe the accumulation of arsenic in the Chinese brake fern.
Assuming the mean rate of arsenic accumulation over the first 20 weeks continued, calculate how long it would take to remove all the arsenic from the soil.
Using the data in the table, discuss the potential of using Chinese brake fern to remove arsenic from contaminated soil.
Suggest one possible consequence of arsenic accumulation in plants for other organisms in the community.
Markscheme
arsenic accumulates in leaves;
arsenic concentration (in leaves) increases rapidly in the first 7 weeks;
arsenic (in leaves) increases from 0 to approximately 6000 mg kg–1 at 7 weeks;
maximum arsenic level (in leaves) is about 7500 mg kg–1 at end of experiment / valid numerical example;
after week 7 arsenic concentration in plant increases more slowly/begins to plateau;
arsenic concentration in roots remains relatively unchanged throughout the experiment;
(Accept range of 7 – 10 weeks in all of above points)
77 weeks (Allow answers in range 70 – 80 weeks)
Chinese brake fern could be used to remove arsenic from soil;
after 20 weeks about 25 % of soil arsenic removed by fern;
arsenic concentration increases (rapidly) in fern (tissue) in 20 weeks;
eventually/longer periods may reach toxic levels for plant;
plants may become toxic for consumers;
arsenic may accumulate in the food chain / biomagnification;
Examiners report
Almost all candidates correctly indicated that arsenic accumulates in the leaves and better candidates were able to expand on this and get two or three marks.
(i) While the mean rate of arsenic accumulation was thought to be a difficult concept by respondents on G2 forms, many candidates were able to get this correct.
In (ii) most candidates indicated that the Chinese brake fern could be used to remove arsenic from contaminated soil but only a few received a second marking point.
The majority of candidates were able to get the mark here by either indicating that the plants would become toxic to consumers or that biomagnification may occur.
