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 only · 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 |
Original authored examples, not real experimental predictions, official marks or a laboratory protocol.