4.7 (a) To investigate the influence of light intensity on the
rate of photosynthesis
Teacher Notes
Apparatus required per class group of 24 students
Fresh Elodea
Boiling tube
12
Large beaker of water @25 C
12
Strong light source
12
Meter stick
12
Light meter
12
Timer
12
Thermometer
12
Scissors
12
Forceps
12
Pond water or 1% w/v sodium
hydrogencarbonate
Advance preparation

Collect Elodea (and pondwater if necessary) from a pond or canal or purchase from
a ‘fish tank’ suppliers or garden centre.

Try out the experiment beforehand to determine the distance(s) suited to the light
source

Set the water bath at 25 °C and check the temperature with a thermometer.
Advance chemical preparation
a) Sodium hydrogencarbonate (1% w/v):

Dissolve 1 g sodium hydrogencarbonate in distilled water and make up to 100 ml
with water.

Dilute with distilled water as appropriate to get other concentrations.
Biology SLSS 2009
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Expected outcome of experiment

Increasing the light intensity will increase the rate of photosynthesis provided there is
sufficient carbon dioxide available and provided the temperature is warm enough to
allow the reactions to occur.
Disposal and post-experiment work
o Excess sodium hydrogencarbonate solution can be stored.
o If sodium hydrogencarbonate is to be disposed, it should be flushed in excess water
to the foul water drain.
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4.7 (a) To investigate the influence of light intensity on the
rate of photosynthesis
Student Notes
Apparatus required per group

Fresh Elodea

Thermometer

Boiling tube

Scissors

Large beaker of water at 25 °C

Forceps

Strong light source

Light meter (optional)

Metre stick

Timer

Pond water or or add 1% w/v sodium hydrogencarbonate
Assembled apparatus
Method
1. Obtain a fresh shoot of Elodea.
2. Cut the stem at an angle. Remove several leaves from the cut end of the stem.
3. Fill a boiling tube with pond water or add 1% w/v sodium hydrogencarbonate to the
boiling tube.
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4. Place the plant into the boiling tube, cut end pointing upwards.
5. Place this tube into the water bath.
6. Switch on the light source.
7. Place the water bath containing the elodea at a measured distance from the light
source e.g. 15 cm.
8. Allow the plant to adjust for at least 5 minutes and observe bubbles being released
from the cut end of the stem.
9. Count and record the number of bubbles released per minute. Repeat twice.
10. Calculate and record the average number of bubbles released per minute.
11. Measure the light intensity at this distance using the light meter or calculate the
light intensity by using the formula: light intensity = 1/d2, where ‘d’ represents the
distance.from the light source. Record result.
12. Repeat the procedure from step 9 at other measured distances e.g. at 30 cm, 45 cm,
60 cm, 75 cm.
13. A graph should be drawn of rate of bubble production against light intensity.
Put light intensity on the horizontal axis.
14. At the end of the experiment, clean all of the equipment and replace it in its correct
place.
Result
Distance
Light
Trial 1
Trial 2
Trial 3
Average
From
Intensity (No. of
(No. of
(No. of
(No. of
Light Source(cm) or 1/d2
bubbles/min) bubbles/min) bubbles/min) bubbles/min)
Conclusion/Comment
Biology SLSS 2009
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