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

Question

Sodium alginate beads were prepared containing a suspension of Chlorella algae. These beads were placed in nine small jars containing hydrogencarbonate indicator solution. Hydrogencarbonate indicator is commonly used to detect changes in pH.

The jars were placed at different distances from a lamp, which was the only light source. The distances ranged from 40 to 130 centimetres. Jars were placed one at a time to avoid the jars from shading each other. Colour changes were observed.

[Source: © International Baccalaureate Organization 2020.]

State the name of the process which occurs in the chloroplasts of Chlorella, that uses CO2.

[1]
a.

The colours of the indicator in the jars ranged from yellow to purple after five hours.

Predict the distribution of indicator colours in jars 1 to 9.

[1]
b.i.

Explain the scientific reasoning for your hypothesis.

[3]
b.ii.

Suggest one other factor that has to be kept constant in all the jars during the experiment.

[1]
c.

Markscheme

photosynthesis/light independent reaction (of photosynthesis)

a.

the jars closer to the light had more purple colours
OR
the jar the furthest from the light was yellow
OR
purple to yellow

Allow for answers indicating colours for each jar within this range.

b.i.

a. high light (intensity) increases photosynthesis;

b. photosynthesis consumes CO2;

c. (more photosynthesis/less CO2) increases pH/decreases acidity;

d. less light means more respiration (than photosynthesis);

e. respiration produces CO2 AND lowers pH/increases acidity;

Allow inverse for all answers.

Do not accept reference to colour only instead of pH.

b.ii.

temperature/volume of indicator/identical jars/number of beads/size of beads / density of Chlorella / other reasonable answer

Do not accept light/pH/humidity.

c.

Examiners report

Candidates generally stated photosynthesis, or light independent reaction of photosynthesis.

a.

Most candidates predicted the colour range accurately from purple to yellow; weaker candidates either inverted the range or did not include the extremes (e.g. purple to magenta), which were stated in the question stem.

b.i.

Most candidates realised that increased light intensity leads to an increase rate of photosynthesis, which consumes CO2, or vice versa. Most related the change in colour to a change in CO2 concentration, but didn't link it to a change in pH, for which the indicator is specific. Stronger candidates understood the experiment and were able to analyse its results effectively, adding that the effect of respiration increased over that of photosynthesis as light intensity decreased, thus a lower pH.

b.ii.

A very large number of candidates suggested a valid factor that should be kept constant.

c.

Syllabus sections

Additional higher level » Topic 8: Metabolism, cell respiration and photosynthesis » 8.3 Photosynthesis
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