Question

The pH of the thylakoid lumen of chloroplasts dropped into a solution of pH 3.5 reaches pH 3.5 within a few minutes.

A) Explain why there is a burst of ATP synthesis when the pH of the external solution is quickly raised to 8.5 and ADP and P_i are added.

B) If ample ADP and P_i are present, why does ATP synthesis cease after a few seconds?

Answer 1

Ans-A- If the lumen pH is 3.5 and it is exposed to the solution pH 8.5 it develops a proton gradient with a higher concentration of protons in the lumen and a lower concentration of protons in the solution. Now the protons from the lumen move in the solution along the concentration gradient through the ATP synthase complex embedded in the thylakoid lumen. When protons pass from ATP synthase ATP is synthesized by binding change mechanism.

Ans-B- Since the protons are moving from lumen to the solution the electrochemical gradient of the protons is decreasing constantly. When enough of the protons are transported from lumen to the solution, it demolished the electrochemical gradient of protons that was the driving force for the ATP synthesis. Now since the concentration of protons is equal in lumen and solution there is no net movement of protons from lumen to solution and this stops the ATP synthesis even though the ADP and Pi are present in enough amount.