Core practical 6: Light intensity and photosynthesis

Aim

To investigate how the distance of a lamp from an aquatic plant affects the rate of photosynthesis, measured by the number of oxygen bubbles released per minute.

Background

Plants make glucose by photosynthesis: carbon dioxide + water → glucose + oxygen, using light energy absorbed by chlorophyll. Oxygen is a waste product. In water plants such as Elodea (pondweed), the oxygen forms bubbles that escape from the cut end of the stem, so counting bubbles gives an estimate of the rate of photosynthesis.

The rate of photosynthesis can be limited by a shortage of light or carbon dioxide, or by a low temperature. A limiting factor is the factor that is in shortest supply, so it stops the rate going any higher. If light is the limiting factor, increasing the light intensity increases the rate.

Light intensity falls as you move away from a lamp, so moving the lamp further from the plant is a simple way to change the light intensity. Higher tier: light intensity is inversely proportional to the square of the distance from the lamp (the inverse square law), so doubling the distance makes the light intensity one quarter of its original value.

Hypothesis

As the lamp is moved further from the pondweed, the light intensity falls, so the rate of photosynthesis (bubbles per minute) will decrease, because less light energy is available to make glucose.

Variables

Equipment

• Fresh pondweed (Elodea or Cabomba), about 10 cm long
• Beaker (250 cm³ or 500 cm³)
• Sodium hydrogencarbonate solution (about 1 g in 100 cm³ of water) as a source of carbon dioxide
• LED lamp or bench lamp
• Clear heat shield (a tank or beaker of water), if using a filament lamp
• Metre rule
• Stopwatch
• Scalpel or scissors and a tile (to cut the stem)
• Thermometer
• Paper clip or small weight to hold the pondweed down
• Eye protection

Risk assessment

Method

1. Put on eye protection. Half-fill a beaker with water at room temperature, add the sodium hydrogencarbonate and record the temperature.
2. Cut about 1 cm off the end of the pondweed stem, at an angle. Attach a small paper clip near the end and place the weed in the beaker with the cut end upwards.
3. Place the heat shield between the lamp and the beaker (not needed for an LED lamp). Keep the lamp, plug and cable away from the water.
4. Place the lamp so that it is 10 cm from the pondweed, measured with the metre rule. Switch off or shut out other sources of light.
5. Switch on the lamp and leave the pondweed for 2 minutes to adjust to the light.
6. Count the number of bubbles released from the cut end in 1 minute and record it.
7. Repeat step 6 twice more at the same distance and record all three counts.
8. Move the lamp to 20 cm and leave the pondweed for 2 minutes before counting.
9. Count the bubbles in 1 minute, three times, and record the counts.
10. Repeat steps 8 and 9 with the lamp at 30 cm, 40 cm and 50 cm.
11. Record the temperature of the water at the end. If it has risen by more than about 1 °C, repeat with fresh water and a better heat shield.
12. Calculate the mean number of bubbles per minute at each distance and plot a graph of mean rate against distance.

Results

Fill this table in as you go. Print the PDF for a copy to write on.

Drawing the graph

Draw a line graph with distance from the lamp (cm) on the x-axis and mean number of bubbles per minute on the y-axis. Plot the mean at each distance with a small cross and draw a smooth curve of best fit. (Higher tier: also calculate 1 ÷ distance² for each distance and plot mean bubbles per minute against 1 ÷ distance². While light is the only limiting factor the points lie close to a straight line; at the highest light intensities the line may level off because another factor, such as carbon dioxide, starts to limit the rate.)