2008 Biology Paper 3-Question 2

A semi permeable membrane in defined as a membrane that allows certain molecules to diffuse through it but does not allow the diffusion of other molecules. The diffusion of molecules through a semi permeable membrane depends on the size of the molecules.

Based on the above information, plan a laboratory experiment to study the size of molecules that can diffuse through a semi permeable membrane.

The planning of your experiment must include the following aspects:

§ Problem statement

§ Objective of investigation

§ Hypothesis

§ Variables

§ List of apparatus and materials

§ Technique used

§ Experimental procedure or method

§ Presentation of data

§ Conclusion

ANSWER

1. Aim:

To study the size of molecules that can diffuse through a semi permeable membrane.

2. Problem Statement:

What size molecules can diffuse through a semi permeable membrane/Visking tubing?

3. Hypothesis:

The smaller the molecules, the easier to pass through a semi-permeable membrane. //Water/glucose molecules can diffuse through the Visking tubing but not the starch/sucrose molecules

4. Variables:

Manipulated: Size of molecules /Type of substances

Responding: Diffusion of molecules through semi permeable membrane/ Visking tubing/ Change in water level in capillary tube (osmometer)

Fixed: Volume of solution in the Visking tubing/ Temperature/ Time

5. Apparatus:

Beaker, test tube, Bunsen burner, measuring cylinder, syringe, stopwatch.

Materials

Benedict's solution, iodine solution, Visking tubing, starch suspension, glucose solution, thread

6. Technique:

Carrying out food test on glucose/starch // Using Benedict’s solution/Iodine solution on the liquid outside the Visking tubing in the beaker and record the result// Measure and record the change in level of liquid in capillary tube using a ruler.

7. Procedure:

1. A Visking tubing is soaked in water to soften it.

2. The Visking tubing is tied at one end of the tube tightly with a piece of thread.

3. Visking tubing is filled with 10 ml glucose and 10 ml starch solution using a syringe.

4. The other end of the tube is tied tightly with a piece of thread.

5. The outer surface of the Visking tubing is rinsed with distilled water.

6. The Visking tubing is immersed into a beaker filled with distilled water.

7. After 20 minutes, a Benedict's test and iodine test carried out on the liquid outside the Visking tubing/ in the beaker.

8. The results of the both test are recorded in a table.

Precautions:

1. Rinse the outside surface of the Visking tubing.

2. Tie the Visking tubing tightly

8. Result: Draw a Table

9. Conclusion:

The smaller the molecules, the easier to pass through a semi-permeable membrane.Hypothesis is accepted.