2016年6月13日 星期一

F4 2nd Term EXP03: Practical 10.X

Practical 10.X
Investigation of the difference in
transpiration rate between the two leaf
surfaces

Objective:       To measure the transpiration rate of a leafy shoot by using bubble photometer (1), and
(2 marks)         to deduct the difference in stomatal density between the upper and the lower leaf surface by comparing the transpiration rate between the two surfaces of the leaf (1)

Biological principle and experiment design:                                                                          (8 marks)
Most of the water absorbed by a plant is eventually lost in the form of water vapour (1) from
the leaf surface. This process is called transpiration (1), which creates a pulling force
to transport water and minerals (1) in the xylem vessels upwards to the leaves. This is the main transport mechanism in plants.

To measure the rate of transpiration, a (bubble) potometer (1) can be used. Since water is continuously absorbed and lost through transpiration, the air bubble inside the connected capillary tube moves towards the direction of the leafy shoot. Transpiration rate can be calculated as shown below:

                Transpiration rate = Volume of absorbed water (1)
                                                Time of measurement (1)
The abundance of the stomata (1) on both surfaces of a leaf is different. The difference can be deduced indirectly by comparing the transpiration rates between both surfaces. To measure the transpiration rate of one surface of a leaf, surface of the other side should be smeared with Vaseline that is impermeable to water and air.

To ensure a valid experimental result, we have to make an assumption that all the water uptake by the

leafy shoot is lost through transpiration (2)

Procedures:                                                                                                                            (6 marks)
1.      Smear the upper surface of a leaf from a plant with Vaseline.
2.      Cut the leafy shoot from the plant and fit it tightly into the potometer tightly. This should be done under
so as to prevent the air bubble from entering the xylem vessles (1) that may cause blockage of water
transport and absorption (1).

3.      Set up the apparatus as shown below. Seal off all connections with vaseline (1).

1.      Introduce an air bubble (1) into the capillary tube so that the change in water absorption can be observed.
2.      Allow the setup to equilibrate for 10 minutes, so that the transpiration rate would be stabilized.
3.     Record the distance (1) travelled by the bubble in (actual time) (1) minutes.
4.      Open the tap of the reservoir to re-adjustment the position of the air bubble. Repeat steps 4 to 5 to take two more readings (if time allowed).
5.      Repeat steps 1 to 7, but use another leaf with similar size and smear the lower surface with Vaseline.

Results:                                                                                                                                  (7 marks)
Leafy shoots
Upper surface smeared
Lower surface smeared
Initial position of the air bubble (cm)
1.3 ml
4.3 ml (1)
Final position of the air bubble (cm)
7.87 ml
2.7 ml (1)
Distance of the bubble movement (cm)
6.57 ml
1.6 ml (2)
Time of the bubble movement (min)
54
48 (1)
Rate of the bubble movement (cm / min)
0.085 ml min-1
0.033 ml min-1 (2)

Result Interpretation:                                                                                                           (7 marks)
1          According to the experimental result, deduce the difference in the abundance of stomata on both surfaces of a leaf.                                                                                                  (3 marks)
The transpiration rate of the leaf with lower surface smeared is much lower than that with upper surface smeared (1). This shows that the transpiration rate is much more reduced by smearing the lower leaf surface than smearing the upper surface (1).
It can be deduced that the lower leaf surface may have higher stomatal density than the upper surface (1).

2          Describe and explain the movement of the air bubble inside the capillary tube.             (2 marks)
The air bubble moves towards the leafy shoot along the capillary tube (1).
During the process of transpiration, water is absorbed into the leafy shoot by transpiration pull, leading the air bubble to moving towards the leafy shoot (1)

3          Explain why the rate of the movement of the air bubble cannot directly show the rate of transpiration?                                                                                                             (2 marks)
The rate of the movement of the air bubble can only show the rate of water absorption directly (1). Actually, not all the water absorbed is lost through transpiration (1)

Discussion:                                                                                                                              (2 marks)
4     State the limitations of this experimental setup or the source of errors of this experiment.      (1 mark)
The friction between the air bubble and the capillary wall may hinder the movement of the air bubble (OR any other reasonable answers) (1)

5     Suggest improvements for the limitations or the errors as stated in question (4).         (1 mark)
To reduce the volume of the air bubble introduced so as to reduce the friction during movement (1)

Conclusion:                                                                                                                            (2 marks)
The transpiration rates of the upper leaf surface and the lower leaf surface are 0.033 ml min-1 and 0.085 ml min-1 respectively (1).

By comparing the transpiration rates between the two leaf surfaces, it can be deduced that the lower leaf surface may have a higher stomatal density than the upper leaf surface (1).


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