A-Level Biology Investigation Write-Up
Investigating the Effect of Light Intensity on the Rate of Photosynthesis in
Algal Balls
1. Aim/Prediction
Aim: To investigate the effect of light intensity (independent variable,
measured as 1/distance² from lamp in arbitrary units) on the rate of
photosynthesis (dependent variable, measured as colour change/absorbance
of hydrogencarbonate indicator using a colorimeter at 550 nm).
Prediction: As light intensity increases, the rate of photosynthesis will
increase, shown by a greater colour change in the indicator (lower
absorbance values). At higher intensities, the rate may plateau because
another factor (e.g. CO₂ concentration, enzyme activity, temperature)
becomes limiting.
2. Experimental Design
Independent variable: Distance of tubes from lamp (cm), converted into light
intensity using formula intensity ∝ 1/d².
Range of values: Distances: 20 cm, 30 cm, 40 cm, 50 cm, 70 cm (at least 5
values).
Dependent variable: Absorbance of indicator at 550 nm (A.U.).
Controlled variables:
1. Volume of indicator: 10 ml per tube.
2. Number of algal balls: equal in each tube (e.g. 10 algal balls).
3. Temperature: controlled by water-filled heat filter placed between lamp
and tubes.
4. Time of exposure: 30 minutes for all tubes.
Repeatability: Each distance will be tested with at least 2 tubes. A mean
absorbance will be calculated to improve reliability.
Control experiment: Tubes wrapped in foil will act as controls. These will not
photosynthesise (no light), but will still respire, showing baseline CO₂
release.
Safety risk assessment:
- Hot lamp (150W): burn hazard → keep heatproof mats in place, avoid
touching lamp directly.
- Glassware: risk of breakage → use tube racks, handle carefully.
- Indicator solution (mild irritant): avoid skin contact, wear goggles.
