4.7 (b) To investigate the influence of carbon dioxide
concentration on the rate of photosynthesis
Teacher Notes
Apparatus required per class group of 24 students
Fresh elodea
Sodium hydrogencarbonate
solutions of various concentrations
Boiling tubes
60
Large beaker of water @ 25 C
12
Funnel
12
Strong light source
12
Meter stick
12
Timer
12
Thermometers
12
Scissors
12
Forceps
12
Test-tube rack
12
Advance preparation

Collect Elodea (and pondwater if necessary) from a pond or canal or purchase from
a ‘fish tank’ suppliers or garden centre.

Make up sodium hydrogencarbonate solutions of various concentrations.

Try out the experiment beforehand to determine the concentration of sodium
hydrogencarbonate to get a steady (but not too fast) stream of bubbles.

Set the water bath at 25 °C and check the temperature with a thermometer.
Biology SLSS 2009
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Advance chemical preparation
a) Various concentrations of Sodium hydrogencarbonate (0.2% w/v - 1%w/v):

Dissolve 0.1 g sodium hydrogencarbonate in distilled water and make up to 100 ml
with water.

Dilute with distilled water as appropriate to get other concentrations.
Expected outcome of experiment

Provided the other factors are available in sufficient quantities, increasing the
concentration of carbon dioxide will increase the rate of photosynthesis until the
plant is photosynthesising at its maximum rate.
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.
Biology SLSS 2009
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4.7 (b) To investigate the influence of carbon dioxide
concentration on the rate of photosynthesis
Student Notes
Apparatus required per group

Fresh Elodea

Metre stick

Sodium hydrogencarbonate solutions

Timer
of various concentrations e.g. 0.02% - 1%

5 Boiling tubes

2 Thermometers

Large beaker of water at 25 °C

Scissors

Funnel

Forceps

Strong light source

Test-tube rack
Assembled apparatus
Biology SLSS 2009
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Method
1. Fill each boiling tube with a different concentration of sodium hydrogencarbonate,
label and place in the water bath. Leave to warm to 25 °C.
2. Obtain a fresh shoot of Elodea.
3. Cut the stem at an angle. Remove several leaves from around the cut end of the
stem.
4. Switch on the light source.
5. Put the plant into the boiling tube with the lowest concentration of sodium
hydrogencarbonate e.g. 0.02%, cut end pointing upwards and stand this boiling tube
in the beaker as shown in the diagram.
6. Place this boiling tube at a measured distance from the light source e.g. 15 cm.
7. Allow the plant to adjust for at least 5 minutes and observe bubbles being released
from the cut end of the stem.
8. Count and record the number of bubbles released per minute. Repeat twice.
9. Calculate and record the average number of bubbles released per minute.
10. Using the same piece of pondweed repeat steps 7 to 11 with the other
concentrations of sodium hydrogencarbonate.
11. A graph should be drawn of the rate of bubble production against sodium
hydrogencarbonate concentration. Put the sodium hydrogencarbonate concentration
on the horizontal axis.
12. At the end of the experiment, clean all of the equipment and replace it in its correct
place.
Results
NaHCO3
Light
Trial 1
Trial 2
Trial 3
Average
(No. of
(No. of
(No. of
Concentration Intensity (No. of
bubbles/min) bubbles/min) bubbles/min) bubbles/min)
or 1/d2
Conclusion/Comment
Biology SLSS 2009
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