4. In an insightful experiment performed in the 1960s, chloroplasts were first s

Question

4. In an insightful experiment performed in the 1960s, chloroplasts were first soaked in an acidic solution at pH 4 (high H+ concentration), so that the stroma and thylakoid space became acidified (Figure 1). They were then transferred to a basic solution (pH 8) (low H+ concentration). This rapidly increased the pH of the stroma to 8, while the thylakoid space temporarily remained at pH 4. A burst of ATP synthesis was observed, and the pH difference between the thylakoid space and the stroma quickly disappeared. INCUBATE CHLOROPLAST FOR SEVERAL HOURS CHANGE EXTERNAL pH AND ADD ADP AND P Figure 1: Soaking chloroplasts in acidic and basic solutions. White areas are at pH 4. pH 4 PH4 pH 8 A. Explain why these conditions lead to ATP synthesis. (4 points) B. Is light needed for the experiment to work? Why or why not? (4 points)

Explanation / Answer

Answer:

4. (A) The switch in solutions creates a pH gradient across the thylakoid membrane. The flow of H+ ions down its electrochemical potential drives ATP synthase, which converts ADP to ATP.

(B) No light is needed, because the H+ gradient is established artificially without a need for the light- driven electron-transport chain.

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