A student prepared a suspension of active chloroplasts from spinach leaves (Spinacia oleracea) using a cold, buffered sorbitol solution. They set up three cuvettes to investigate the light-dependent stage of photosynthesis using the electron acceptor DCPIP, which is blue in its oxidised state and absorbs light strongly at 600 nm600\text{ nm}600 nm, but becomes colourless when reduced.
The three cuvettes were prepared as follows:
The absorbance of each cuvette at 600 nm600\text{ nm}600 nm was monitored using a colorimeter. Over a 15-minute period, the absorbance of Cuvette A and Cuvette B remained high and stable, whereas the absorbance of Cuvette C decreased rapidly.
The sorbitol buffer used during the isolation process was prepared to be isotonic to the chloroplast stroma. Explain why it was essential that the water potential of the isolation medium matched that of the chloroplasts.
Explain the purpose of preparing and monitoring Cuvette B in this experiment.
Some commercial herbicides, such as DCMU, act as inhibitors that block electron transport at photosystem II, which prevents the reduction of DCPIP. Explain how such an herbicide would lead to a reduction in weed growth.
5 exam-style questions on AQA A Level Biology Photosynthesis (A-level only). Each one has a worked solution and a mark scheme showing where the marks go.