All design choices and example data
What should be varied?
- Use more chloroplast extract at the brighter light settings. Extract concentration would change with light intensity and provide an alternative explanation for the signal.
- Vary incident light intensity between otherwise comparable chloroplast assays. Light is the intended independent variable in this assay question. This is the suitable option for the stated question.
- Increase assay duration at the dimmer light settings. Duration would covary with light intensity. The final signal would reflect unequal reaction periods.
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. This is the suitable option for the stated question.
- Record final indicator absorbance after different periods at each light intensity. Unequal periods change how much indicator can be reduced. Compare changes over the same stated interval.
- Record incident light intensity as the electron-transfer rate. Light intensity is the input being varied, not the assay response. Measure the indicator absorbance change.
Which control approach is useful?
- Keep extract concentration and duration fixed, but allow brighter lamps to warm the mixtures. Light and temperature would change together. Keep assay temperature comparable to isolate light intensity.
- Keep duration and temperature fixed, but vary chloroplast extract concentration with light intensity. Different extract concentrations change the amount of active material, confounding the light comparison.
- Keep extract concentration, temperature and duration comparable. These reduce changes unrelated to incident light. This is the suitable option for the stated question.
Matched example
Extract concentration, starting absorbance and temperature are comparable.
Calculation questions and worked method
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.
Extract concentration also increases
More extract is used at higher light settings. The stronger decline in these example values cannot isolate light intensity.
Calculation questions and worked method
Calculate the mean absorbance decline at light intensity 100.
Mean = (0.44 + 0.45 + 0.46) ÷ 3 = 0.45. Absorbance decline = 0.8 − 0.45 = 0.35.
Calculate that decline per second, in s⁻¹ (absorbance is dimensionless).
Decline rate = 0.35 ÷ 120 = 0.0029167 s⁻¹. This is an assay proxy, not direct carbon fixation.
Evaluation prompt
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.
Example explanation and self-check points
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.
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Original fixed datasets and reasoning tasks. Full requirements: AQA practical assessment · OCR A specification. These activities do not replace laboratory work or assessment of practical competence.
