Q9 - Counting Electrons Through the Oxygen-Evolving Complex

Theoretical B Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Joseph Priestley (1733-1804) discovered that plants consume CO . In this process they produce elemental 2 oxygen, which he also discovered. In chloroplasts, the oxygen-evolving complex loses single electrons when exposed to light. After a specific number of electrons are lost, the complex regains electrons from water to produce oxygen, in a cycle. Consecutive pulses of light were flashed at a solution of chloroplasts, and the amount of oxygen produced from each flash was recorded.

Figure 1 for Q9: Counting Electrons Through the Oxygen-Evolving Complex

Using the information and data, determine which of the statements are true or which are false.

A. 2 light pulses are sufficient to complete the oxygen-evolving cycle.

B. A maximum of 4 electrons is lost by the complex.

C. At the beginning of the experiment, most complexes had already lost 1 electron.

D. More oxygen is evolved for the same light exposure at the end of the experiment.

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Question reproduced from IBO 2017, Theoretical Exam B, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Official answer key & worked solutions