A student isolated chloroplasts from the leaves of Arabidopsis thaliana to investigate the rate of the light-dependent reaction.
During the extraction stage, the leaves were homogenised in an ice-cold buffer containing 0.35 mol dm-3 sorbitol, which has the same water potential as the stroma of the chloroplasts.
After centrifugation, the supernatant was removed. The pellet containing the chloroplasts was then resuspended in a buffer solution containing no sorbitol (a hypotonic medium) to which the redox indicator DCPIP was added.
Explain:
why it is essential that the sorbitol solution used during the initial homogenisation has the same water potential as the chloroplast stroma.
why the pellet is resuspended in a hypotonic medium (containing no sorbitol) before carrying out the Hill reaction.
16 exam-style questions on OCR A A Level Biology Photosynthesis. Each one has a worked solution and a mark scheme showing where the marks go.