| Lesson Plan |
| Grade: |
Date: 25/02/2026 |
| Subject: Biology |
| Lesson Topic: describe and carry out investigations using redox indicators, including DCPIP and methylene blue, and a suspension of chloroplasts to determine the effects of light intensity and light wavelength on the rate of photosynthesis |
Learning Objective/s:
- Describe how redox indicators such as DCPIP and methylene blue report photosynthetic electron transport.
- Explain the influence of light intensity and wavelength on the rate of photosynthesis in isolated chloroplasts.
- Design and carry out a quantitative assay using chloroplast suspensions and redox indicators to measure photosynthetic rate.
- Analyse data to construct an action spectrum and identify light‑saturation points.
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Materials Needed:
- Fresh spinach leaves and homogenisation buffer
- Centrifuge, tubes and cuvettes
- DCPIP (0.5 mM) and methylene blue (0.1 mM) solutions
- Adjustable LED light source with colour filters (450 nm, 550 nm, 650 nm)
- Thermostated water bath (25 °C) and stopwatch
- Projector or screen for data tables and safety briefing
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Introduction:
Begin with a quick demonstration of a blue DCPIP solution turning colourless under a lamp, prompting students to predict why this happens. Review prior knowledge of photosynthetic electron transport and the role of light as a limiting factor. Explain that today they will investigate how intensity and wavelength affect the rate using redox indicators, and that success will be measured by accurate data collection and correct interpretation of action spectra.
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Lesson Structure:
- Do‑now (5'): Students answer a short question on redox reactions in photosynthesis on the board.
- Mini‑lecture (10'): Review redox indicators, experimental principle, and show a schematic diagram.
- Practical set‑up (15'): Students isolate chloroplasts, prepare indicator mixtures, and calibrate the light source.
- Data collection (20'): Groups run assays with DCPIP and methylene blue under three wavelengths and two intensities, recording time for colour loss.
- Data analysis (10'): Calculate mean times, rates, and plot graphs on laptops/tablets.
- Whole‑class discussion (5'): Interpret action spectra, identify limiting factors, and answer reflection questions.
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Conclusion:
Summarise how the colour change of the indicators reflected photosynthetic electron flow and how light variables altered the rate. Ask each group to write one key finding on an exit ticket. Assign homework to research how temperature influences the same assay and prepare a brief report.
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