F4 EXP03: Practical 3.5

Practical 3.5

Study of osmosis in living plant tissue

Objective:       To study osmosis in potato tissues in different concentrations of sucrose solutions

Biological principle and experiment design:                                                                          (9 marks)

The cell membrane in living plant tissues is differentially permeable (1) that it allows small molecules (e.g. water molecules) to pass through freely but not for large molecules (e.g. sucrose molecules).

Water molecules enter or leave the cell according to the water potential gradient (1), leading to the change in volume and weight or the cell. As a result, the volume and the weight of the whole tissues changes due to the gain or loss of water by osmosis.

In this experiment, potato tissues are placed in sucrose solutions of different concentrations. The change in volume and the weight are then measured, so that osmosis of living plant tissues in solutions of different concentrations can be studied.

The independent variable is the concentrations of sucrose solution (1).

The dependent variable is the extent of osmosis, which can be measured and expressed as percentage change in length(1) and weight(1) of the potato tissues (2).

To ensure a fair test, controlled variables such as volume of the sucrose solution / time of immersion of

the potato tissues / temperature of the sucrose solution should be kept constant throughout the process of experiment.

To ensure the results are valid and significant, this experiment assumes that all cells in the potato tissues

should have similar water potential(1).

Predicted results:

The potato tissues in the distilled water will have increase in length and mass (1), while those in 20% sucrose solution will have decrease in length and mass (1).

There is no significant change in length and mass for the potato tissues in 10% sucrose solution (1).

Procedures:                                                                                                                            (8 marks)

1            According to the following dilution table (稀釋表), different sample solutions were prepared:

Concentration of sample solution	0% (distilled water)	10%	20%
Volume of distilled water (cm3)	80	40(1)	0
Volume of 20% sucrose solution (cm3)	0	40(1)	80(1)
Final volume of sample solution (cm3)	80

2           9 boiling tubes were divided into 3 groups and filled with different sample solutions:

-          A1 to A3: 20 cm³ distilled water

-          B1 to B3: 20 cm³ 10% sucrose solution

-          C1 to C3: 20 cm³ 20% sucrose solution

3           9 potato strips were prepared with a cork borer. All of them were cut into the length of 2 cm.

1           Any peels left on the potato strips were removed because water cannot pass through the peels by osmosis (1) due to the impermeability of the peels (1). This may affect the experimental result (2).

2           The initial weight of each potato strips was measured by electronic balance and recorded: (1)

Strip	A1	A2	A3	B1	B2	B3	C1	C2	C3
weight (g)	?	?	?	?	?	?	?	?	?

3           The strips were put in the boiling tubes with corresponding labels as shown below:

Boiling tubes	A1	A2	A3	B1	B2	B3	C1	C2	C3
Sample solutions	20cm3 Distilled water  (0% sucrose solution)			20cm3 10% sucrose solution			20cm3 20% sucrose solution

4           The boiling tubes were sealed with plastic wrap and then were left for 30 minutes.

5           After 30 minutes, all potato tubes were taken out with care. The surplus liquid on their surface was soaked up with tissue paper because the surplus liquid may increase the overall weight of the potato strip (1), leading to errors in measurement of the weight (1) (2).

6           The final lengths and weights of each potato strips were measured and recorded.

7           The % change in length and % change in weight was calculated according to the following equation:

                                                       Final length / weight – Initial length / weight

      % change in     = -------------------------------------------------------------------- X 100%

          length / weight (%)                             Initial length / weight

Results:                                                                                                                                  (6 marks)

Table showing the effect of solution concentration on the weight of potato strips

Sample solution	Initial weight of potato strip (g)			Final weight of potato strip (g)			Average % change in weight (%)
	Tubes		Average	Tubes		Average
Distilled water	A1			A1
	A2			A2
	A3			A3
10% sucrose solution	B1			B1
	B2			B2
	B3			B3
20% sucrose solution	C1			C1
	C2			C2
	C3			C3

Table showing the effect of solution concentration on the length of potato strips

Sample solution	Initial length of potato strip (cm)			Final length of potato strip (cm)			Average % change in length (%)
	Tubes		Average	Tubes		Average
Distilled water	A1			A1
	A2			A2
	A3			A3
10% sucrose solution	B1			B1
	B2			B2
	B3			B3
20% sucrose solution	C1			C1
	C2			C2
	C3			C3

[Detailed raw data (1); average of data (1); correct calculation of % change (1)] X 2 = 6

Result Interpretation:                                                                                                           (9 marks)

1          Describe and explain the change in weight and length of the potato strips placed in distilled water. (3)

The weight and the length increase (1).

As the water potential of distilled water is higher than that of potato cells (1), there is a net movement of water molecules into the potato cells by osmosis (1).

2          Describe and explain the change in weight and length of the potato strips placed in 10% sucrose solution. (3)

The change in weight and length is not signficiant (1).

This is because the water potential of 10% sucrose solution is similar to that of potato cells (1). Thus, no net movement of water molecules into or out of potato cells by osmosis (1).

3          Describe and explain the change in weight and length of the potato strips placed in 20% sucrose solution. (3)

The weight and the length decrease (1).

The water potential of 20% sucrose solution is lower than that of potato cells (1), leading to a net movement of water molecules away from the potato cells by osmosis (1).

Discussion:                                                                                                                             (8 marks)

4     Explain why it is necessary to peel off the potato strips. (2)

The peels are impermeable to water (1) and may block the movement of water molecule by osmosis (1).

5     Explain the purpose of soaking up the surplus liquid on the surface of the potato strips before weighing. (2)

The surplus liquid may increase the weight of the potato strips (1), leading to a measurement error (1).

6      Explain the purpose of sealing the openings of boiling tubes with plastic wraps. (2)

This can prevent evaporation of water (1) which may affect the concentration / water potential of the sample liquid (1)

7     Explain why it is necessary to measure the weight and the length of the potato strips immediately after soaking up of the surplus liquid. (2)

This can shorten the time of exposure of the potato strips to air and reduce the water loss by evaporation (1). Loss of water may affect the weight and length of the potato strips (1)

Conclusion:                                                                                                                            (2 marks)

Living tissues are differentially permeable in nature (1). Osmosis occurs when the living tissues are placed in solutions of different water potential (1).