Photosynthesis Rate

Linked to B4: Bioenergetics

Aim

Investigate the effect of light intensity on the rate of photosynthesis using pondweed.

Practical Guide

Method & analysis

Exam Questions

Past-paper style

Mark Scheme

Model answers

Independent variable

Distance of lamp from pondweed (5, 10, 15, 20, 25, 30 cm)

Dependent variable

Number of oxygen bubbles per minute

Controlled variables

Temperature (water bath), CO₂ concentration (NaHCO₃), same pondweed, lamp wattage

✓Oxygen is produced as a by-product of photosynthesis — can count bubbles

✓Light intensity is a limiting factor for photosynthesis

✓NaHCO₃ ensures CO₂ is not limiting during the experiment

✓Light intensity is proportional to 1/distance² (inverse square law)

✓Rate plateaus when another factor (CO₂ or temperature) becomes limiting

✗Forgetting to add NaHCO₃ — without it CO₂ may become a limiting factor
✗Not controlling temperature — heat from lamp can affect enzyme activity
✗Saying 'light makes photosynthesis faster' without explaining why (more light energy for chlorophyll)

★For Higher: calculate relative light intensity as 1/d² and plot rate vs 1/d²
★Counting bubbles is less accurate than measuring volume of O₂ — mention as a limitation
★If rate plateaus, always suggest which other factor is limiting