What is the purpose of using DCPIP in this practical?
It acts as an artificial electron acceptor, allowing the rate of chloroplast electron transfer to be measured by a colour change.
What colour change occurs when DCPIP is reduced?
Blue to colourless (or greenish if chlorophyll masks the colour).
What does the rate of DCPIP decolourisation represent?
The rate of the light-dependent reactions or dehydrogenase enzyme activity.
Why are leaves crushed in an isolation medium?
To release intact chloroplasts into a suspension for testing.
Why must the isolation medium have the same water potential as leaf cells?
To prevent osmosis that could cause chloroplasts to burst or shrink.
Why should the isolation medium be ice-cold?
To reduce enzyme activity and protect chloroplast membranes during preparation.
What is the purpose of a buffer in the isolation medium?
To maintain constant pH, ensuring enzymes remain active and stable.
Why should the experiment be carried out in a dark room?
To control light exposure, so only the experimental lamp provides light for photosynthesis.
How is light intensity varied in this experiment?
By changing the distance between the lamp and the test tube.
What must be kept constant when testing different light intensities?
The wavelength (colour) of the light.
What must be kept constant when testing different wavelengths of light?
The light intensity (keep the lamp at the same distance).
What is the purpose of the test tube wrapped in aluminium foil?
It acts as a dark control, showing that colour change depends on light.
What variable is measured to determine the rate of reaction?
The time taken for DCPIP to turn colourless, or the absorbance change over time.
What instrument can be used to measure colour change more accurately?
A colorimeter, to measure absorbance objectively.
What graph should be plotted to analyse the results?
Absorbance (y-axis) against time (x-axis).
What does a steep slope on the absorbance–time graph show?
Faster DCPIP reduction, meaning a higher rate of light-dependent reactions.
Why should the same person decide when the solution becomes colourless?
To reduce subjectivity and increase consistency between results.
Why should the experiment be repeated and a mean taken?
To increase reliability and reduce the effect of anomalies.
Why must the amount of leaf tissue and time spent grinding be controlled?
To ensure each sample contains a similar number of chloroplasts, keeping DCPIP concentration comparable.
Why is this method considered an indirect measure of dehydrogenase activity?
It measures electron transfer rate (via colour change) rather than substrate use or product formation directly.
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5.1 - Communication And Homeostasis66
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5.2 - Excretion117
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5.4 - hormonal Comunictaion71
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4.2 - Biodiversity154
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4.3 - classification And Evolution139
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4.1 - Communicable Diseases224
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Photosynthesis70
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The Nervous System87
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DCPIP practical20
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Practical Terminolgy6
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Animal Responses93
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Respiration56
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Pracical - Investigating The Rate Of Respiration13
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Practical - Repsirometer6
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Plant Response43
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Excretion - The Rest Is In Your Notes30
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Biodeiversity 233
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Statistics In Biology9
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Vaccinations0
