Photosynthesis Limiting Factors Experiment for O-Level Biology
07 Nov 2025, 00:00 Z
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Practical course completion-record note
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TL;DR
The photosynthesis limiting factors experiment for O-Level Biology usually asks you to compare light, carbon dioxide, or temperature using Cabomba bubble counts or hydrogencarbonate indicator.
Practise one bubble-count setup and one indicator setup, then turn the results into a graph that clearly shows when the rate plateaus because another factor becomes limiting.
Students lose marks when they describe the trend but never identify the new limiting factor, or when they treat bubble count as exact oxygen volume without qualification.
Link Back to the Hub
Rotate this limiting-factor studio with other ecology and enzyme drills via the O-Level Biology Experiments hub so every Paper 3 format stays fresh.
1 | Exam focus
- The syllabus requires candidates to describe how light intensity, CO₂ concentration, and temperature affect photosynthesis (SEAB 2026 syllabus, PDF).
- Paper 3 builds on this by assessing:
- Planning: identifying the limiting factor, setting up controls, and justifying apparatus choices (e.g. data loggers, coloured filters).
- MMO: counting bubbles consistently, measuring colour changes, recording probe readings.
- PDO: tabulating rate data, plotting graphs, labelling axes with units.
- ACE: explaining trends with limiting-factor theory, evaluating reliability, proposing improvements.
2 | Cabomba bubble-count core practical
- Place a freshly cut sprig of Cabomba or Elodea upside down in a beaker filled with 0.2 % sodium hydrogencarbonate solution.
- Position a lamp at 10 cm distance; allow 5 minutes for acclimatisation.
- Count oxygen bubbles released in 1 minute for three consecutive minutes; take the mean.
- Repeat with lamp distances of 20 cm, 30 cm, 40 cm. Keep room temperature constant (cite thermometer use).
- Optional: wrap the beaker in coloured acetate sheets to compare red, blue, and green light.
Record data like this:
| Light setup | Distance / cm | Bubble count (trial 1) |
