- Specification route
- RP8
- Question bank
- 14 questions
- Course stage
- Year 13 / A-level only
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14 standard questions. No sign-in required.
Practise Biology maths and data skills
Sessions use up to the requested number of available questions. Standard practice uses levels 1–5.
Sample questions
In the experiment investigating the rate of dehydrogenase activity in chloroplasts, what is the role of the blue dye DCPIP?
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It takes the place of NADP in the electron transfer chain.
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Answer: It acts as an artificial electron acceptor, picking up electrons from the light-dependent reaction and turning colourless as it is reduced.
Required practical 8. Dehydrogenase enzymes in chloroplasts normally reduce NADP. DCPIP acts as an alternative electron acceptor. When reduced, it changes from blue to colourless, providing a measurable endpoint.
When extracting chloroplasts from leaves, the extraction medium must be cold, isotonic, and buffered. Why must it be isotonic?
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Isotonic means having the same water potential.
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Answer: To prevent water from moving into or out of the chloroplasts by osmosis, which could cause them to burst or shrivel.
Required practical 8. An isotonic solution has the same water potential as the organelles. This prevents osmotic lysis or shrinkage. (Cold reduces enzyme activity, buffered maintains pH).
In the experiment measuring dehydrogenase activity using DCPIP, one test tube containing chloroplasts and DCPIP is completely wrapped in aluminium foil. What is its purpose?
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Without light, the light-dependent reaction cannot happen.
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Answer: It acts as a dark control, showing what happens to DCPIP when the chloroplast preparation is kept without light.
The foil-wrapped tube is a negative control for light. Comparing it with an illuminated tube tests whether DCPIP reduction depends on the light-dependent reactions of the chloroplast preparation.
A student forgets to add a buffer to the chloroplast extraction medium. Why might the DCPIP fail to turn colourless during the experiment?
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Dehydrogenase is an enzyme. Enzymes are highly sensitive to their environment.
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Answer: The pH may have fluctuated, denaturing the dehydrogenase enzymes in the chloroplasts and halting the electron transfer chain.
Required practical 8. A buffer maintains pH. If pH changes significantly, the active sites of dehydrogenase enzymes denature. They can no longer remove electrons/hydrogen, so DCPIP cannot be reduced.
In the experiment investigating dehydrogenase activity in chloroplasts, how does DCPIP act as an indicator?
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It 'steals' the electrons that are moving down the electron transfer chain in the light-dependent reaction.
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Answer: It is a redox indicator that takes the place of NADP; it turns from blue to colourless when it accepts electrons released from the photoionisation of chlorophyll.
Required practical 8. Dehydrogenase enzymes usually reduce NADP. DCPIP acts as an artificial electron acceptor. When it gains electrons (is reduced) during the light-dependent reaction, it changes from blue to colourless.
During the preparation of the chloroplast suspension, the leaf blending process is carried out in a cold, isotonic, buffered solution. Why must the solution be ice-cold?
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When you destroy the cell, you release dangerous digestive chemicals from other organelles like lysosomes.
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Answer: To reduce the kinetic energy and activity of hydrolytic enzymes that could digest and damage the chloroplasts once the cells are broken open.
Required practical 8. Homogenisation releases lysosomal enzymes. Keeping the medium ice-cold lowers the kinetic energy, severely reducing enzyme activity and preventing the auto-digestion of the extracted chloroplasts.
A student sets up a control tube containing chloroplast suspension and DCPIP, but wraps it completely in aluminium foil. What result is expected, and what does this control test?
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Foil blocks out a specific abiotic factor necessary for the first stage of photosynthesis.
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Answer: The DCPIP should remain blue if it is not reduced in darkness; comparison with an illuminated tube tests whether the observed DCPIP reduction depends on light.
The foil-wrapped tube is a dark control. If DCPIP is reduced in the illuminated preparation but remains blue in the dark control, the comparison supports the conclusion that the observed reduction depends on the light-dependent activity of the chloroplast preparation.
Another control tube is set up containing DCPIP and an extraction medium that lacks any chloroplasts. What is the purpose of this specific control?
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This tube keeps DCPIP and light but removes the chloroplasts.
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Answer: To test whether DCPIP changes colour in the light when chloroplasts are absent, providing a control for non-biological bleaching or reduction of the dye.
A tube containing DCPIP but no chloroplasts controls for colour changes that could occur without chloroplast activity. If it remains blue while the chloroplast-containing illuminated tube loses colour, that supports attributing the reduction of DCPIP to the chloroplast preparation.
Practise this topic
14 standard questions are available (levels 1–5). No sign-in required.