Light intensity and the rate of photosynthesis

Objectives

• Measure the rate of photosynthesis by counting bubbles.
• Investigate the effect of light intensity.
• Recognise a limiting factor from a graph.

Theory

Photosynthesis: carbon dioxide + water → glucose + oxygen, using light energy absorbed by chlorophyll. A water plant releases the oxygen as bubbles, so bubbles per minute measure the rate. Light intensity falls with the square of the distance from the lamp (intensity ∝ 1/d²). At low intensity, light is the limiting factor and the rate rises as the lamp comes closer. At high intensity the curve levels off, because something else — carbon dioxide or temperature — now limits the rate.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light, chlorophyll)

Procedure

1. Count the bubbles released in one minute.
2. Repeat at five or more different lamp distances, keeping the temperature and carbon dioxide the same.
3. Reduce the sodium hydrogen carbonate (less carbon dioxide) and count again.
4. Plot bubbles per minute against light intensity.

Precautions

• Let the plant settle for a few minutes at each distance.
• Keep the water temperature the same so that only the light changes.
• Repeat each count and average.

Subject

Biology

Grade

9–11

Chapter

Photosynthesis (Nepal National Curriculum (CDC), grade 10)

Difficulty

Medium

Duration

25 min

Safety level

Low

Version

1.0

Apparatus

• Pondweed (Hydrilla) in a boiling tube
• Sodium hydrogen carbonate solution
• Bench lamp
• Metre rule
• Water bath
• Thermometer
• Stopwatch