DP Biology (first assessment 2025)

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Question 20N.1B.SL.TZ0.3b.ii

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Date November 2020 Marks available [Maximum mark: 3] Reference code 20N.1B.SL.TZ0.3b.ii
Level SL Paper 1B Time zone TZ0
Command term Explain Question number b.ii Adapted from N/A
b.ii.
[Maximum mark: 3]
20N.1B.SL.TZ0.3b.ii
(b.ii)

Explain the scientific reasoning for your hypothesis.

[3]

Markscheme

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.

Examiners report

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.

Syllabus sections