Required Practical 8: Chloroplast Dehydrogenase Activity

DCPIP is a redox indicator that is blue when oxidised and becomes colourless when reduced in the relevant assay. In a suitable chloroplast extract it can act as an artificial electron acceptor, so its changing absorbance reports electron-transfer-related dehydrogenase activity.

This is not a direct measurement of net glucose formation or the complete intact leaf’s photosynthetic rate. Extraction changes the system, and the dye substitutes for part of the normal pathway. State the process the indicator reports rather than labelling any colour change simply “photosynthesis”.

Choose a named factor such as light intensity and keep chloroplast amount, dye concentration, total volume, temperature and measurement timing comparable. Use freshly prepared material as specified and protect or handle it consistently so preparation differences do not dominate.

Use a blank that accounts appropriately for the chloroplast suspension’s background absorbance at the chosen wavelength. A dark control and a suitable control lacking active extract help interpret light-related and background dye changes. If changing lamp distance, consider unwanted heating and measure or control it. Consistent mixing and rapid, defined initiation help make early readings comparable.

Plot absorbance against time and use an appropriate initial gradient or a defined comparable interval. If oxidised dye absorbance falls, a positive activity proxy can be the magnitude of the decrease per time. State that this is an absorbance-rate proxy unless the assay has been calibrated into amount reduced.

Very low absorbance, pigment background, settling, unequal extract activity and delayed readings can distort results. Independent extract preparations or biologically appropriate replicates improve inference beyond repeated readings of one tube. An increased rate under one condition does not establish the response of every intact plant in the field.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.