Investigate the effect of light intensity on the rate of photosynthesis (LC 2026) (Leaving Cert Biology): Revision Notes

Investigate the effect of light intensity on the rate of photosynthesis

Background:

Three factors affect the rate of photosynthesis: light intensity, temperature, and carbon dioxide concentration. In this experiment, the effect of light intensity is investigated by measuring the amount of oxygen bubbles produced per minute.

To increase photosynthesis, a gardener can increase light intensity by adding more lights or increase carbon dioxide concentration by adding jars of sodium bicarbonate, a source of CO₂.

This experiment uses an aquatic plant instead of a terrestrial plant because it is easier to see and count the O₂ bubbles.

Materials needed:

• Elodea
• Water bath
• Thermometer
• Sodium bicarbonate
• Lamp
• Metre stick
• Pond water

Method:

1. Set up the experiment as shown in the diagram, ensuring the stem of the elodea is closest to the water surface and cut at an angle to ensure efficient oxygen production.
2. Allow the elodea a period of adjustment to reach the temperature of the thermostatically controlled water bath and the light intensity, approximately one minute.
3. Count the number of oxygen bubbles released by the elodea for a period of one minute.
4. Move the light source closer to the experiment (increasing the light intensity) and repeat steps two and three.
5. Repeat the experiment at distances of 1m, 80cm, 60cm, 40cm, and 20cm from the experiment, recording the results as the light intensity increases.

Results:

Distance from experiment	100 cm	80 cm	60 cm	40 cm	20 cm
Number of oxygen bubbles produced per minute	0	5	10	15	15

As light intensity increases, the rate of photosynthesis also increases up to a certain point. Beyond this point, even if the light is moved closer, the number of oxygen bubbles produced does not increase. This distance, where no further increase in the number of bubbles occurs, is known as the saturation point.