H2 Biology Hydrogencarbonate Indicator Practical: CO2, Photosynthesis and Respiration

Do not write that the indicator measures oxygen. It tracks CO2-driven pH change.

2 | Photosynthesis Setup

Aim: investigate how light intensity affects photosynthetic CO2 uptake.

Basic setup:

1. Add the same volume of hydrogencarbonate indicator to each boiling tube.
2. Add equal lengths or equal masses of the same aquatic plant, such as pondweed, if supplied.
3. Place tubes at measured distances from a lamp.
4. Keep temperature constant with a water bath or heat filter.
5. Record colour after fixed time intervals.

Expected trend:

• Close to the lamp: faster photosynthesis, lower CO2, indicator shifts towards purple.
• Far from the lamp: slower photosynthesis, less CO2 removal, indicator remains orange-red or shifts less strongly.
• In darkness: respiration dominates, CO2 rises, indicator shifts towards yellow.

Compensation point planning extension

The compensation point is where photosynthetic CO2 uptake equals respiratory CO2 release, so the net CO2 change is zero.

Planning variables:

Interpretation: below the compensation point, respiration dominates and CO2 increases. Above it, photosynthesis dominates and CO2 decreases. If leaf area differs between tubes, the apparent compensation point shifts for a reason unrelated to light intensity.

3 | Respiration Setup

Aim: compare CO2 release from living material under controlled conditions.

Possible setup:

1. Place the same mass or number of organisms in each tube.
2. Use the same volume of hydrogencarbonate indicator.
3. Keep temperature and exposure time constant.
4. Shield tubes from light if the aim is respiration only.
5. Compare the time taken for a defined colour shift.

If plants are used in the dark, the colour shift reflects respiration because photosynthesis is not occurring. If animals or germinating seeds are used, include a control tube without living material.

4 | Controls

Strong Paper 4 answers name the control and explain what it proves.

If the control tube changes colour strongly, the conclusion is weakened because the indicator system is unstable or contaminated.

5 | Data Table

For visual colour matching, keep the table simple and timestamped:

If a colorimeter is provided, record absorbance instead of subjective colour words. Plot absorbance or colour index against time and compare the initial gradients.

6 | ACE Evaluation

Use evaluation points that connect the limitation to the conclusion.

Exam-ready sentence:

The tube closest to the lamp may have changed colour faster partly because it was warmer, not only because it received higher light intensity. A water bath or heat filter should be used so that light intensity changes while temperature remains constant.

7 | Common Mistakes

• Writing that purple means more oxygen. It means lower CO2, not directly more oxygen.
• Comparing plant tubes with different leaf areas.
• Forgetting the dark control when the aim is to separate photosynthesis from respiration.
• Treating a final colour as a rate without recording the time taken.
• Using light distance as intensity directly without noting that intensity decreases approximately with $1/d^2$.

8 | Related Guides

• H2 Biology Photosynthesis and Respiration Practical
• H2 Biology Stomatal Density Practical
• DCPIP Assay in H2 Biology Practical
• H2 Biology Planning and Evaluation Guide
• H2 Biology Practical 2026 Lab Mastery Guide

Sources

1. SEAB - H2 Biology (Syllabus 9477) GCE A-Level 2026

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