A student isolated chloroplasts from spinach leaves into an ice-cold sorbitol buffer solution to prepare a suspension of active chloroplasts. She used this suspension and the redox indicator DCPIP (which is blue when oxidised and colourless when reduced) to investigate the light-dependent reactions of photosynthesis.
The student set up three test tubes as follows:
At the start, the contents of all three tubes were blue. After 20 minutes at 20 ∘C20\text{ }^\circ\text{C}20 ∘C, the contents of Tube A and Tube B remained blue, while the contents of Tube C turned colourless.
The isolation buffer contained sorbitol at a concentration that matches the water potential of the chloroplast stroma. Explain why isolating the chloroplasts in a solution with a higher water potential than the stroma would prevent them from carrying out the light-dependent reaction.
Explain what the results of Tube A and Tube C show about the reduction of DCPIP in this investigation.
The herbicide DCMU blocks the transfer of electrons from photosystem II to the electron transport chain, preventing the reduction of DCPIP. Explain how DCMU would decrease the rate of carbon dioxide fixation in the light-independent reaction.
