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An experiment was conducted to explore the effects of light on the pH of thylakoids isolated in a test tube. The thylakoid suspension was kept in darkness, and the pH of the test tube was monitored. After six minutes, the thylakoids were exposed to bright light. The results are shown in the graph below.

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Which of the following BEST explains the observed results?


NADP$^+$ molecules picked up electrons from the thylakoids and bonded with hydrogen ions in the solution of the test tube.


Water molecules in the lumen were split, producing additional hydrogen ions in the lumen of each thylakoid.


In an isolated suspension, thylakoids not contained within intact chloroplasts transform light energy directly into heat energy.


Proton pumps in the thylakoid membrane began pumping hydrogen ions into the lumen, changing the pH.

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