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Here is a question form my test.

If you could measure the pH of the interior of thylakoids and the surrounding stroma during active photosynthesis what would be the relative pH of each?

The answer is C.

the stroma would have a higher pH

I understand that proton gradient is from stroma to thylakoid space, but that does not mean stroma would have less protons than thylakoid space, if stroma would have a higher pH, then the proton will never flow from stroma to thylakoid space, right?

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Energy from an electron transport chain is used to actively transport protons from the stroma to the thylakoid lumen. Thus the lumen has a relatively higher proton concentration and therefore a lower pH than the stroma.

Higher [H+] means lower pH.

Lower [H+] means higher pH.

Keep in mind that the flow of actively transported protons from the stroma to the lumen is against their concentration gradient. There wouldn't be a net flow of protons this way without active transport.

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