Required practical: Investigate the effect of light intensity on the rate of photosynthesis

Aim

To measure how changing light intensity affects the rate of oxygen production from a photosynthesising plant (often Elodea / Cabomba), using a valid method with controlled variables.

Method (step-by-step)

1. Set up a boiling tube or beaker with hydrogencarbonate indicator solution or water + sodium hydrogencarbonate to supply CO₂.
2. Place a lamp at a measured distance from the plant; count bubbles per minute (or collect gas volume over time) as a proxy for rate.
3. Repeat at several distances (e.g. 10–60 cm) or use neutral density filters to vary intensity without moving the lamp.
4. Keep temperature stable (use a water bath if needed) and allow 2–3 minutes equilibration after each change.
5. Repeat each distance and calculate a mean rate; plot rate vs 1/distance² or use a light meter for intensity.

Equipment

• Aquatic plant (e.g. Elodea)
• Boiling tube or beaker, water + sodium hydrogencarbonate
• Bench lamp, ruler, stopclock
• Optional: thermometers, water bath, light meter

Variables

• Independent: Light intensity (distance from lamp, or measured µmol m⁻² s⁻¹).
• Dependent: Rate of photosynthesis (bubbles min⁻¹ or volume of O₂ per unit time).
• Control: CO₂ concentration (same hydrogencarbonate concentration); Temperature; Same plant mass / leaf area where possible; Same wavelength of light (lamp type)

Results & analysis

You should see rate increase with intensity up to a plateau if another factor becomes limiting (CO₂ or temperature). Explain anomalies (e.g. heating from the lamp) and justify bubble counting as a proxy only if volume is proportional to moles of O₂ under fixed conditions.

Graphs & interpretation

Plot rate vs intensity (or vs 1/d² if using distance). A curve that levels off supports limiting factors. Quote units and refer to at least two data points when describing the trend.

Exam-style question prompts

• Describe how you would investigate the effect of light intensity on the rate of photosynthesis.
• Explain why temperature must be controlled in this practical.
• Suggest one improvement to reduce random error when counting bubbles.

Exam tips

• Name the gas as oxygen from photosynthesis and link to balanced word/symbol equations when asked.
• Use ‘rate’ language: per second / per minute, not just ‘more bubbles’.
• Link to limiting factors: light, CO₂, temperature — say which is most likely limiting at the plateau.

Related topic revision

• Photosynthesis — topic revision
• Limiting factors of photosynthesis