Practical work.........47
19. Investigation of the effects of environmental factors on the
rate of photosynthesis
The rate of photosynthesis of a photosynthetic plant is
affected by factors such as light quality, light intensity, carbon
dioxide concentration and temperature.
A convenient way of investigating the effects of these factors
on the rate of photosynthesis is to measure the volume of
oxygen given off by a plant in a given time.
When a particular factor is being investigated, it is essential
that other factors are kept constant and, if possible, at
optimum levels so that no other factors are limiting.
Pondweeds, like Hydrilla and Ceratophyllum, are suitable
plant materials for this investigation. It is advisable to use
plant materials that have been well illuminated for three or
four hours before the experiment.
Fig.1 Experimental set up
48.........Practical work
A. To investigate the effect of light quality on the rate of photosynthesis
Procedure
1.
Cut the stem of a well-illuminated piece of Hydrilla to
about 10 cm long.
2.
Place it with cut surface upwards, in a boiling tube filled
with 0.25% sodium hydrogencarbonate solution.
3.
Wrap the tube with red cellophane.
4.
Set up the apparatus as shown in Fig.1.
5.
Stand the tube in a beaker of water at room temperature.
Record the temperature of the water, which acts as a heat
shield, and check it at intervals throughout the
experiment. It should remain constant and the water
should be renewed if necessary.
6.
Put a 100-watt table lamp at 20 cm from Hydrilla. Turn
on the lamp.
7.
Wait for about 5 minutes for Hydrilla to equilibrate.
8.
Suck the sodium hydrogencarbonate solution from the
tube into the pipette. Close the clip and record the initial
reading. Make sure that the clip is tightened to prevent
the solution in the pipette from falling.
9.
After a fixed period of time (e.g. 5 minutes), record the
final reading in the pipette and calculate the volume of
gas collected.
10. Repeat steps (2) to (9) using transparent and green
cellophane respectively.
Practical work.........49
B.
To investigate the effect of light intensity on the rate of photosynthesis
Procedure
1.
Cut the stem of a well-illuminated piece of Hydrilla to
about 10 cm long.
2.
Place it with cut surface upwards, in a boiling tube filled
with 0.25% sodium hydrogencarbonate solution.
3.
Set up the apparatus as shown in Fig.1.
4.
Stand the tube in a beaker of water at room temperature.
Record the temperature of the water, which acts as a heat
shield, and check it at intervals throughout the
experiment. It should remain constant and the water
should be renewed if necessary.
5.
Put a 100-watt table lamp at 10 cm from Hydrilla. Turn
on the lamp.
6.
Wait for about 5 minutes for Hydrilla to equilibrate.
7.
Suck the sodium hydrogencarbonate solution from the
tube into the pipette. Close the clip and record the initial
reading. Make sure that the clip is tightened to prevent
the solution in the pipette from falling.
8.
After a fixed period of time (e.g. 5 minutes), record the
final reading in the pipette and calculate the volume of
gas collected.
9.
Repeat steps (2) to (8) respectively at other distances (d)
of the table lamp from Hydrilla: 20 cm, 30 cm, 40 cm
and 50 cm.
10. Plot a graph of the rate of photosynthesis (volume of gas
collected per until time) against light intensity (1/d2).
50.........Practical work
C. To investigate the effect of carbon dioxide concentration on the rate of
photosynthesis
Procedure
1.
Cut the stem of a well-illuminated piece of Hydrilla to
about 10 cm long.
2.
Place it with cut surface upwards, in a boiling tube filled
with 0.1% sodium hydrogencarbonate solution.
3.
Set up the apparatus as shown in Fig.1.
4.
Stand the tube in a beaker of water at room temperature.
Record the temperature of the water, which acts as a heat
shield, and check it at intervals throughout the
experiment. It should remain constant and the water
should be renewed if necessary.
5.
Put a 100-watt table lamp at 10 cm from Hydrilla. Turn
on the lamp.
6.
Wait for about 5 minutes for Hydrilla to equilibrate.
7.
Suck the sodium hydrogencarbonate solution from the
tube into the pipette. Close the clip and record the initial
reading. Make sure that the clip is tightened to prevent
the solution in the pipette from falling.
8.
After a fixed period of time (e.g. 5 minutes), record the
final reading in the pipette and calculate the volume of
gas collected.
9.
Repeat steps (2) to (8) respectively at other
concentrations of sodium hydrogencarbonate solution:
0.15%, 0.2%, 0.25%, 0.3% and 0.4%.
10. Plot a graph of the rate of photosynthesis (volume of gas
collected per until time) against concentration of sodium
hydrogencarbonate solution.
11. From the graph, determine the concentration of the
sodium hydrogencarbonate solution at which the volume
of gas collected begins to level off.
12. Repeat steps (2) to (11) using a higher light intensity by
putting the table lamp at a closer distance.
Practical work.........51
D. To investigate the effect of temperature on the rate of photosynthesis
Procedure
1.
Cut the stem of a well-illuminated piece of Hydrilla to
about 10 cm long.
2.
Place it with cut surface upwards, in a boiling tube filled
with 0.25% sodium hydrogencarbonate solution.
3.
Set up the apparatus as shown in Fig.1.
4.
Stand the tube in a beaker of water at room temperature.
Record the temperature of the water, which acts as a heat
shield, and check it at intervals throughout the
experiment. It should remain constant and the water
should be renewed if necessary.
5.
Put a 100-watt table lamp at 20 cm from Hydrilla. Turn
on the lamp.
6.
Wait for about 5 minutes for Hydrilla to equilibrate.
7.
Suck the sodium hydrogencarbonate solution from the
tube into the pipette. Close the clip and record the initial
reading. Make sure that the clip is tightened to prevent
the solution in the pipette from falling.
8.
After a fixed period of time (e.g. 5 minutes), record the
final reading in the pipette and calculate the volume of
gas collected.
9.
Repeat steps (2) to (8) respectively at other temperatures:
5°C, 10°C, 15°C, 25°C, 30°C, 35°C and 40°C.
10. Plot a graph of the rate of photosynthesis (volume of gas
collected per until time) against temperature.
Note
1.
For any change in the experimental conditions of the
above investigations, it is necessary to allow the plant to
equilibrate before taking any reading.