Final answer:
The quick disappearance of the blue color when ADP and phosphate are added to lit chloroplasts is due to the acceleration of the light-dependent reactions of photosynthesis, where these substrates aid in the production of ATP.
Step-by-step explanation:
When a suspension of chloroplasts is exposed to light and small quantities of ADP and phosphate ions are added, the observed quick disappearance of a blue color can be explained by the activation of the light-dependent reactions of photosynthesis. In these reactions, which occur in the thylakoid membranes of chloroplasts, photon energy is absorbed by photosystem II (PS II) and used to extract electrons from water. These electrons travel through the electron transport chain to photosystem I (PS I), where they reduce NADP+ to NADPH. Meanwhile, ATP synthase uses the electrochemical gradient established by the proton movement across the thylakoid membrane to combine ADP and phosphate ions to form ATP. Therefore, the blue color disappearing more quickly suggests the reagent that is indicating the blue color is being reduced as a consequence of the photosynthetic activity being stimulated by the added ADP and phosphate, which together are substrates that facilitate the production of ATP, thereby enhancing the light-dependent reactions.