2015年12月9日 星期三

F4 EXP02: Practical 3.1


Practical 3.1
Demonstration of osmosis using
dialysis tubing (P.12)

Objective:       To demonstrate osmosis of solutions (sucrose solution蔗糖溶液) using dialysis tubing

Biological principle and experiment design:                                                                          (8 marks)
Because of the presence of numerous small pores on its wall, dialysis tubing is differentially (1) permeable that it only allows small molecules (e.g. water molecules) to pass through it freely but not for large ones (e.g. sucrose molecules). Therefore, It can be used to demonstrate osmosis.
To observe the action of osmosis, dialysis tubing is fully filled with sucrose solution and connected with a capillary tube (毛細管). If there is a net movement of water molecules into the dialysis tubing by osmosis,
the volume of the tubing will increase(1). The pressure / hydrostatic pressure(1) produced inside the tubing will press the meniscus of the solution inside the capillary tube upwards, and vice versa (反之亦然).
In order to ensure that osmosis is due to the water potential(1) difference
between the solutions inside the tubing and the surroundings, control(1) set-up is established.
This set-up is the same as the experimental set-up but distilled water(1) is used instead

of the sucrose solution inside the tubing. 

Predicted results (2):
The liquid level in the capillary tube of the experimental set-up will increase (1),
while that of the control set-up will not have significant change (1)

Procedures:                                                                                                                            (2 marks)
1           A dialysis tubing of 15 cm long was wet.
2           The tubing was opened and tied a knot at one end.
3           20% sucrose solution was filled into the tubing using a dropper.
4           Another end of the tubing was firmly tied to a capillary tube with cotton thread.
1           The outside of the tubing was rinsed with distilled water, so that the sucrose solution on the outside
of the tubing could be removed, to prevent the water potential of the surrounding solution from being affected (2).
2           The dialysis tubing with the capillary tube was immersed in a beaker filled with 400 ml distilled water.
3           The initial liquid level was marked.
4           After 30 minutes, the final liquid level was marked.
5           The change in liquid level was calculated and recorded.
6           Procedures 1 to 9 was repeated to set up a control, in which distilled water was used instead of the sucrose solution inside the tubing.

Results:                                                                                                                                  (2 marks)
Change in height of the meniscus in different set-up:
Set-up
Change in liquid level (cm)
Experimental set-up
+ ?
Control set-up
0 (insignificant change)

Result Interpretation結果詮釋:                                                                                      (9 marks)
1          Name the process which causes the change in height of the meniscus. (1)     Osmosis
2          What nature of the dialysis tubing enables the above process to take place? (1)
Differentially permeable

3          Describe and explain the change in liquid level of the experimental set-up. (5)
The liquid level increased (1)
Since the water potential of distilled water is higher than that of the sucrose solution inside the
Tubing (1), there is a net movement of water molecules into the tubing by osmosis (1)
along the water potential gradient (1).
The established hydrostatic pressure pushes the liquid level upwards (1)

4          Why does the liquid level of the sucrose solution not rise further after a period of time? (2)
As the liquid level rises, the increasing weight of the liquid inside the capillary tube cancels
out (1) the upward pressure due to the water potential gradient (1)

Discussion:                                                                                                                     (10 marks)
5     What happens if the dialysis tubing is filled with sucrose solution of higher concentration instead? Why? (3)
The liquid level will increase faster(0.5) and higher(0.5).
As the water potential gradient across the tubing is steeper(1), the rate(0.5) and extent(0.5) of
water movement by osmosis becomes higher.

6     What happens to the result if the tubing is filled with distilled water but the beaker is filled with sucrose solution? Why? (5)
The liquid level will fall (1) and the dialysis tubing will finally shrink (1).
It is because water molecules will flow out from the tubing (1) along the water potential gradient (0.5),
leading to an decrease in the pressure inside the tubing(0.5).
The weight of the liquid inside the capillary tube will push the liquid level downwards (1).

5.          What is the advantage of using a capillary tube instead of a glass tube with a larger lumen? (2)
The lumen of the capillary tube is smaller (1).
Any change in the liquid level becomes more significant (1).

Conclusion:                                                                                                                            (2 marks)
Osmosis occurs across the dialysis tubing separating distilled water and sucrose solution (1).
A net water flow from distilled water to the sucrose solution is found (1).

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