By the end of this lesson
- Plan the light and leaf-disc investigation.
- Interpret indicator colour as net gas exchange.
- Evaluate control experiments, safety and semi-quantitative evidence.
BIG QUESTION
How can indicator colour reveal whether photosynthesis or respiration has the greater gas effect?
Hydrogen-carbonate indicator reports carbon-dioxide concentration, so matched light treatments reveal net—not gross—exchange.
Create matched light treatments
Place equal-area leaf discs from the same healthy leaf into sealed tubes containing equal volumes of hydrogen-carbonate indicatorA pH indicator used to infer carbon dioxide concentration: red/orange at atmospheric levels, yellow when carbon dioxide rises and purple when it falls., keeping tissue above the liquid if the method requires.
Expose tubes to darkness, low light and bright light for the same time. Include an indicator-only control experiment and use identical tubes with equal air volume.
Keep temperature constant while varying light
Changing lamp distance changes light intensity but can also change temperature. Use LED lighting, a water bath or heat shield and measure temperature.
Keep leaf species, disc area, tissue age, indicator volume and concentration, exposure time and starting colour constant. Repeat each treatment.
Interpret colour as net carbon-dioxide exchange
In darkness, respiration raises carbon dioxide and shifts indicator toward yellow. In bright light, photosynthesis may remove carbon dioxide faster than respiration releases it, shifting toward purple.
Red/orange may indicate little net change or a compensation condition; it does not show that both processes stopped.
Improve measurement carefully
Colour comparison is subjective and semi-quantitative. Photograph against a standard background or use a calibrated colorimeter to obtain more objective values.
Indicator reports carbon dioxide indirectly through pH. Leaf damage, contamination or unequal air volume can also alter results.
Work safely
Wear eye protection, use small indicator volumes and avoid skin or eye contact. Handle hole borers and cutting tools on a tile and away from fingers.
Do not heat sealed tubes, and follow local risk assessment for lamp, glassware and biological material.
Graph skills: SLAPUK and choosing a scaleNet CO₂ change can be positive or negative.
Predicted indicator outcomes
Predictions assume healthy leaf tissue and sufficient exposure time.
| Treatment | Likely colour direction | Interpretation |
|---|---|---|
| Dark | Toward yellow. | Respiration raises CO₂. |
| Low light | Yellow/red depending on balance. | Respiration may equal or exceed photosynthesis. |
| Compensation intensity | Remains near red/orange. | No net CO₂ exchange. |
| Bright light | Toward purple. | Photosynthesis removes CO₂ faster than respiration releases it. |
| No-leaf control experiment | Little change. | Checks indicator stability and environmental effects. |
Language that earns precision
- hydrogen-carbonate indicator
- A pH indicator used to infer carbon-dioxide concentration.
- leaf disc
- A standardised circular sample of leaf tissue.
- net photosynthesis
- Photosynthetic gas effect remaining after respiration is accounted for.
- indicator-only control
- A control experiment: a matched tube without leaf tissue.
Common misconceptions
Claim: Purple indicator proves oxygen was released.
Correction: It indicates reduced carbon dioxide concentration, not oxygen directly.
Claim: A lamp-distance experiment changes only light.
Correction: The lamp may also change temperature.
Claim: Red indicator means no respiration.
Correction: Respiration and photosynthesis may have equal net carbon-dioxide effects.
Claim: More leaf discs always improve validity.
Correction: Unequal tissue area changes respiration and photosynthesis capacity between treatments.
Match the answer to the command word
State light treatment, measured colour/CO₂ response, control experiments, repeats and safety.
Interpret every colour through the relative rates of photosynthesis and respiration.
Identify temperature confounding and subjective colour judgement.
Recall, apply, then justify
Describe a valid investigation of light intensity and net gas exchange from leaf discs.
Hint
Include treatments, control experiments, measurements and repeats.
Suggested marking points
- Use equal-area discs from the same leaf/species.
- Place them in identical tubes with equal indicator and air volumes.
- Expose tubes to several measured light intensities for equal time.
- Keep temperature and other conditions constant.
- Include dark and indicator-only control experiments.
- Repeat and compare colour objectively or with a colorimeter.
Explain expected dark and bright-light results.
Hint
Use net carbon dioxide.
Suggested marking points
- In darkness only respiration affects net exchange.
- Carbon dioxide rises and indicator moves toward yellow.
- In bright light photosynthesis may exceed respiration.
- Carbon dioxide falls and indicator moves toward purple.
Explain why a water bath may be used when lamp distance changes.
Hint
Prevent a confounding variable.
Suggested marking points
- Lamp distance can alter temperature as well as light intensity.
- A water bath helps keep temperature constant so light is the intended cause.
Design a method to estimate the compensation light intensity rather than merely classify three colours.
Planning hint
Use many intensities and an objective zero-change criterion.
What a strong design includes
- Use a range of closely spaced measured light intensities.
- Standardise leaf area, indicator, air volume, time and temperature.
- Measure indicator absorbance or calibrated CO₂ change.
- Include initial readings and no-leaf control experiments.
- Repeat each intensity and plot net CO₂ change against light intensity.
- Estimate compensation intensity where change crosses zero.
- Refine with additional intensities around the crossing point.