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Date	November 2020	Marks available	1	Reference code	20N.3.SL.TZ0.3
Level	Standard level	Paper	Paper 3	Time zone	TZ0 / no time zone
Command term	Suggest	Question number	3	Adapted from	N/A

Question

To investigate whether carbon dioxide is required for photosynthesis, a plant was irrigated using water from which carbon dioxide had been removed and was then placed in the apparatus shown in the diagram. The apparatus was left in darkness for 24 hours to destarch the leaves. Then, after several hours in light, a leaf was removed from the plant and found to contain no starch when tested. A control was performed using a second plant. A leaf from this plant tested positive for starch.

[Source: © International Baccalaureate Organization 2020.]

Describe the control for this experiment.

[2]

Outline how the carbon dioxide could be removed from the water used to irrigate the plant.

[1]

Suggest how a plastic bag placed around the plant pot prevents carbon dioxide from reaching the plant’s leaves.

[1]

A chromatograph was made of the photosynthetic pigments of a leaf of the plant.

[Source: Adapted from “Diversity of Photosynthetic Pigments” by Alexander F. Motten in Tested Studies for
Laboratory Teaching, Volume 16 of the Association for Biology Laboratory Education and used by permission of the
author.]

Outline what measurements would be taken to identify pigment X.

[2]

Markscheme

a. same apparatus with carbon dioxide present;

b. (control has) no sodium hydroxide/alkali;

c. control irrigated with untreated water/water with CO₂;

“Same apparatus” alone does not get the mark.

a. boiling (and cooling) the water;

b. expose the water to a vacuum;

Allow distillation of water.

a. to prevent CO₂ from (organisms in) the soil affecting the experiment;

b. the plastic bag is impermeable to gases;

a. the distance travelled from the origin/O to the solvent front;

b. the distance travelled by the pigment (from the origin O to X);

Accept X to solvent front if the candidate indicates that this allows O to X to be calculated.

Examiners report

Many candidates had difficulty describing the control for this experiment. The question states that the experiment was to investigate whether carbon dioxide is required for photosynthesis therefore it seems straightforward that the plant would be in similar conditions with and without carbon dioxide. Some candidates assumed that the control plant would have similar conditions and no light.

There were many suggestions on how carbon dioxide could be removed from water despite boiling the water being an assessment statement.

The plastic bag caused considerable confusion with many candidates stating that it was to prevent carbon dioxide reaching the roots and being taken up the stem to the leaves for photosynthesis.

Most candidates managed to outline what measurements would be taken to identify the pigment on the chromatogram with many identifying the pigment although this was not required.

Syllabus sections

Core » Topic 2: Molecular biology » 2.9 Photosynthesis

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