Solution to PS 5 #4

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a) When the external pH rises to 8.0, the stromal pH also rises quickly, but the luminal pH remains low initially because the thylacoid membrane is relatively impermeable to protons.  The resulting pH gradient across the thylacoid membrane drives ATP synthesis as protons flow through the chloroplast ATP synthase.
b)  Initially ATP is synthesized as protons flow from the lumen to the stroma through  ATP synthase.  Eventually the  pH gradient across the thylacoid membrane collapses and the synthesis of ATP stops.

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