5 Practical 3
Investigating the effect of concentration of carbon dioxide on the
rate of photosynthesis
Safety
The normal safety precautions associated with the use of chemicals apply.
Students should wash their hands after handling biological material.
Apparatus and materials
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fresh, washed Canadian pondweed
scalpel
tile
large beaker of aerated distilled water
10 cm3 syringe
20 cm length of glass capillary tubing,
internal diameter 1 mm
rubber tubing to connect syringe to capillary
tubing
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retort stand, boss and clamp
lamp with 60 W bulb
stopwatch
25 cm3 of 0.05 mol dm–3 sodium
hydrogencarbonate solution
25 cm3 of 0.10 mol dm–3 sodium
hydrogencarbonate solution
25 cm3 of 0.15 mol dm–3 sodium
hydrogencarbonate solution
Introduction
In this practical, you will:
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investigate the effect of different concentrations of carbon dioxide on the rate of photosynthesis of
a plant.
The plant you will use is Canadian pondweed, so the carbon dioxide is supplied as hydrogencarbonate
ions in solution.
Procedure
1
Collect some pieces of Canadian pondweed (Elodea
canadiensis) and cut them across internodes into
approximately 5 cm lengths. Transfer them to a
well-illuminated beaker of distilled water for a few
minutes and select two pieces that are bubbling
rapidly and regularly.
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Transfer the pieces into the barrel of a 10 cm3 syringe.
Fill the syringe with aerated distilled water and replace
the plunger. Invert the syringe and expel any air.
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Using a short length of rubber tubing, fix a length of
capillary tubing to the nozzle of the syringe and
clamp the apparatus vertically (see Figure 5.1).
Figure 5.1
COAS Biology 2 Teacher Resources
Original material © Cambridge University Press 2009
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5 Practical 3
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Gently push down the plunger to force the water into the top of the capillary tubing. Place a lamp
about 10 cm from the syringe. Allow the pondweed to equilibrate for a few minutes.
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Find the rate of movement of the meniscus over a suitable period of time. Assuming that the
movement is due to oxygen production by the plant, this will be proportional to the rate of
photosynthesis. The volume of oxygen produced will be equal to the distance moved by the
meniscus multiplied by the internal cross-sectional area of the tubing. The area can be calculated
if the internal radius of the tubing (r) is known, using the formula πr2.
rate of oxygen production = rate of movement of meniscus × πr2
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Repeat this procedure, using:
a 0.05 mol dm–3 sodium hydrogencarbonate solution instead of the aerated distilled water
b 0.10 mol dm–3 sodium hydrogencarbonate solution
c 0.15 mol dm–3 sodium hydrogencarbonate solution.
Each time, allow the pondweed to equilibrate to the new conditions before taking readings.
You should use the same pieces of weed in each solution.
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Present your results in a suitable form.
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a
What effect did an increase in concentration of hydrogencarbonate ions have on the rate of
photosynthesis?
b As the molarity of the hydrogencarbonate solution was increased by equal increments did the
rate of photosynthesis increase by equal amounts too? If not, why not?
c If you continued to increase the molarity of the hydrogencarbonate solution, would you expect
the rate of photosynthesis to continue increasing?
d What other conditions might affect the rate of photosynthesis?
Extension work
You can modify this investigation to find out the effects of other factors on the rate of photosynthesis
of the pondweed.
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To vary the light intensity, the rate of gas production can be measured with the lamp at different
distances from the syringe.
To vary the light wavelength, different coloured filters can be placed between the lamp and the
syringe, with the lamp a fixed distance from the pondweed. Red, blue and green filters should be
tried.
You could design experiments to investigate the effects of these factors. You must consider how to
control other variables in each investigation.
COAS Biology 2 Teacher Resources
Original material © Cambridge University Press 2009
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