Investigating the Effect of Light Intensity on the Rate of Photosynthesis

Aim

To investigate the effect of light intensity on the rate of photosynthesis in an aquatic plant, such as Hydrilla verticillata, by measuring the rate at which oxygen bubbles are released.

Variables

• Manipulated variable: light intensity, varied by changing the distance of the lamp from the plant.
• Responding variable: the rate of photosynthesis, measured as the number of oxygen bubbles released per minute (or the volume of gas collected in a fixed time).
• Controlled variables: the temperature of the water, the concentration of dissolved carbon dioxide (kept constant using a fixed concentration of sodium hydrogen carbonate solution), the same plant and piece size used, and the time interval for each reading.

Materials and apparatus

• A shoot of Hydrilla verticillata (or Cabomba)
• A beaker
• Sodium hydrogen carbonate solution, as a source of carbon dioxide
• A lamp
• A ruler, to set and measure the distance of the lamp
• A stopwatch
• A funnel
• A test tube, for collecting gas
• A wooden splint and matches, to test the gas collected

Procedure

1. Place a freshly cut Hydrilla shoot, cut end upward, under an inverted funnel in a beaker of sodium hydrogen carbonate solution.
2. Place a water-filled test tube over the stem of the funnel to collect the gas released.
3. Position the lamp at a set distance, for example 10 cm, from the plant.
4. Count the number of bubbles released per minute.
5. Move the lamp to increasing distances, for example 20 cm, 30 cm, and 40 cm, and repeat the bubble count at each distance, keeping all other conditions constant.
6. At the end of the experiment, collect a sample of the gas produced and test it with a glowing splint to confirm that it relights, showing that the gas is oxygen.

Expected results

The rate of bubble production, which represents the rate of photosynthesis, is highest when the lamp is closest to the plant, where light intensity is greatest. As the lamp is moved further away and light intensity decreases, the number of bubbles released per minute falls, levelling off at very low light intensity when the lamp is far from the plant.

Conclusion

Light intensity affects the rate of photosynthesis. As light intensity decreases, shown by moving the lamp further from the plant, the rate of oxygen bubble production falls.

This shows that light intensity is a limiting factor for the rate of photosynthesis under these conditions.

Paper 3-style questions

Context. A student set up Hydrilla under an inverted funnel in sodium hydrogen carbonate solution and counted the oxygen bubbles released per minute with a lamp placed at 10, 20, 30 and 40 cm from the plant. The counts were 48, 30, 16 and 6 bubbles per minute.

Question 1 (hypothesis). State a suitable hypothesis for this investigation.

Model answer. As the light intensity increases (the lamp is moved closer to the plant), the rate of photosynthesis increases, so more oxygen bubbles are released per minute.

Question 2 (variables). State the manipulated variable, the responding variable, and two variables that must be kept constant.

Model answer. The manipulated variable is the light intensity, changed by altering the distance of the lamp from the plant. The responding variable is the rate of photosynthesis, measured as the number of bubbles released per minute.

Variables kept constant include the temperature of the water and the carbon dioxide concentration, kept steady with the same sodium hydrogen carbonate solution; the same shoot and time interval are also fixed.

Question 3 (tabulation and graph). Describe how the results should be recorded and displayed.

Model answer. Record the distance of the lamp and the number of bubbles per minute in a table with clear headings and units. Because light intensity falls as distance increases, plot a line graph of the number of bubbles per minute (vertical axis) against the distance of the lamp (horizontal axis), then draw a smooth curve of best fit.

The curve falls as distance increases, showing the rate drops as light intensity decreases.

Question 4 (inference). Using the readings, state and explain the conclusion.

Model answer. The number of bubbles per minute falls from 48 to 6 as the lamp is moved from 10 cm to 40 cm. This shows that a lower light intensity gives a lower rate of photosynthesis, so under these conditions light intensity is a limiting factor for the rate of photosynthesis.

Safety

• The lamp becomes hot in use, so avoid touching the bulb and do not let a hot lamp touch the wet beaker, which could crack the glass.
• Keep the lamp, its plug and all electrical connections away from the water to reduce the risk of electric shock; dry your hands before touching the switch.
• Handle the beaker, funnel and test tube with care, as glass apparatus breaks easily and can cut the skin.
• When confirming the gas with a glowing splint, keep the flame away from hair, clothing and any flammable material.

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions