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Date May 2021 Marks available 1 Reference code 21M.1.HL.TZ1.10
Level Higher level Paper Paper 1 Time zone Time zone 1
Command term Question number 10 Adapted from N/A

Question

The graph shows atmospheric oxygen levels over time.

About 2.5 billion years ago, a significant rise in atmospheric oxygen occurred. What was the cause of this rise?

A. Photosynthesis by non-vascular land plants

B. Photosynthesis by vascular land plants

C. Oxygen produced by photosynthetic bacteria being released from the ocean into the atmosphere

D. Volcanic activity

Markscheme

C

Examiners report

This question was based on the first Application in 2.9 (Photosynthesis) which requires a study of changes to the Earth's atmosphere due to photosynthesis. The graph shows the oxygenation of the atmosphere happening between 2.5 and 2 billion years ago. To answer the question correctly candidates had to choose which group of photosynthesizers were most likely to have caused this. Over 55 % correctly chose photosynthetic bacteria in the oceans, but large numbers also chose vascular land plants, including some of the stronger candidates. This group appeared less than 0.5 billion years ago. Detailed knowledge of the timings of plant evolution are not expected but students are expected to appreciate that the Great Oxygenation Event was due to prokaryotes photosynthesizing in the oceans.

Syllabus sections

Core » Topic 4: Ecology » 4.4 Climate change
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