What should be varied?
- Use more chloroplast extract at the brighter light settings.
- Vary incident light intensity between otherwise comparable chloroplast assays.
- Increase assay duration at the dimmer light settings.
AQA RP8 context · practical reasoning
Chloroplasts: separate light from another variable
Investigate light intensity and a redox-indicator absorbance change in a supplied chloroplast assay.
Every assay starts at absorbance 0.80. Final absorbance is recorded after 120 s. A decline is used as an indicator of electron-transfer activity under the assay conditions, not a direct measurement of whole-leaf carbon fixation. Values are authored examples.
Questions and answers · Matched example · 3 readings per condition.
Plan the comparison
Which recording method supports the stated light comparison?
- Record initial and final indicator absorbance over the same 120 s.
- Record final indicator absorbance after different periods at each light intensity.
- Record incident light intensity as the electron-transfer rate.
Which control approach is useful?
- Keep extract concentration and duration fixed, but allow brighter lamps to warm the mixtures.
- Keep duration and temperature fixed, but vary chloroplast extract concentration with light intensity.
- Keep extract concentration, temperature and duration comparable.
Calculate from evidence
Extract concentration, starting absorbance and temperature are comparable.
| Light intensity / μmol photons m⁻² s⁻¹ | Reading 1 | Reading 2 | Reading 3 |
|---|---|---|---|
| 0 | 0.79 | 0.8 | 0.81 |
| 50 | 0.67 | 0.68 | 0.69 |
| 100 | 0.54 | 0.55 | 0.56 |
| 150 | 0.44 | 0.45 | 0.46 |
| 200 | 0.39 | 0.4 | 0.41 |
Absorbance decline = 0.80 − mean final absorbance. Decline rate = decline ÷ 120 s. Give numerical answers to about three significant figures.
Calculate the mean absorbance decline at light intensity 100.
Calculate that decline per second, in s⁻¹ (absorbance is dimensionless).
Evaluate the comparison
Use the two example patterns to explain why changing extract concentration weakens a light-intensity conclusion. State a useful improvement and distinguish the measured assay response from whole-plant photosynthesis.
Use the alternate example below alongside the data above.
| Light intensity / μmol photons m⁻² s⁻¹ | Reading 1 | Reading 2 | Reading 3 |
|---|---|---|---|
| 0 | 0.79 | 0.8 | 0.81 |
| 50 | 0.63 | 0.64 | 0.65 |
| 100 | 0.44 | 0.45 | 0.46 |
| 150 | 0.24 | 0.25 | 0.26 |
| 200 | 0.14 | 0.15 | 0.16 |
Answers and reasoning
What should be varied?
Vary incident light intensity between otherwise comparable chloroplast assays. Light is the intended independent variable in this assay question.
Which recording method supports the stated light comparison?
Record initial and final indicator absorbance over the same 120 s. A change over a known duration supports an assay-rate comparison.
Which control approach is useful?
Keep extract concentration, temperature and duration comparable. These reduce changes unrelated to incident light.
Calculate the mean absorbance decline at light intensity 100.
Mean = (0.54 + 0.55 + 0.56) ÷ 3 = 0.55. Absorbance decline = 0.8 − 0.55 = 0.25.
Calculate that decline per second, in s⁻¹ (absorbance is dimensionless).
Decline rate = 0.25 ÷ 120 = 0.0020833 s⁻¹. This is an assay proxy, not direct carbon fixation.
Evaluation example
The confounded example has a larger indicator decline at the higher light settings, but extract concentration also increases. A light-only effect cannot be isolated from that design. Keep extract concentration, starting indicator conditions, temperature and duration comparable while varying light. Indicator reduction supports an electron-transfer comparison in this assay; it does not directly measure whole-plant carbon fixation or establish that one light level is optimal in every plant.
- I connect the stronger decline to the second changing factor, extract concentration.
- I propose comparable extract, indicator, temperature and timing conditions.
- I distinguish the assay proxy from a direct whole-plant carbon-fixation measurement.
Original authored examples, not real experimental predictions, official marks or a laboratory protocol.