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EXPERIMENT 3.1

Activity: Movement of substances across a semi-permeable membrane

Aim : To study the movement of substances across a semi-permeable membrane.

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ProblemStatement : What substances are able to move across a semi-permeable membrane?

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Hypothesis : Molecules which are smaller than the pores of the Visking tubing are able to moveacross the plasma membrane.

Variables (a) Manipulated : Size of solute molecules

(b) Responding: Colour of solution

(c) Fixed: Temperature and time

Materials : Benedicts solution, 1% starch suspension, iodine solution, 30% glucose solution,Visking tubing (25 cm), distilled water and thread.

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Apparatus : Two test tubes, three beakers, Bunsen burner, wire gauze and tripod stand

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Technique : Carry out tests for the presence of starch and simple sugar using iodine solution andBenedicts solution respectively and record the results.

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Procedure : 1. A knot was tied firmly at one end of a Visking tubing after wetting it.

2. 10 ml of glucose solution and 10 ml of starch suspension were poured into theVisking tubing. A knot was tied at the top of the Visking tubing. The colour of themixture was recorded.3. The outside of the Visking tubing was rinsed using distilled water.

4. 300 ml of distilled water was poured into a beaker. 10 ml of iodine solution wasadded in. the colour of the solution was recorded.

5. The Visking tubing was immersed in the beaker of solution for 30 minutes.

6. After 30 minutes, the Visking tubing was removed and transferred to a dry beaker.

7. The colour of the contents in the Visking tubing and the solution in the beaker wererecorded.8. The tests for the presence of glucose in the content of the Visking tubing and thesolution in the beaker were carried out using following steps:

(a) 2 ml ofthe solution was poured into a test tube and 1 ml of Benedicts solutionwas added.

(b) The mixture was heated in a beaker of water bath and the change in the colourwas recorded.

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Results :Solution in Content

Iodine testBenedicts test

Initial colour Final colour

Visking tubingGlucose solution +starch suspension

Colourless Dark blue Red precipitate

BeakerDistilled water +

iodine solution

Golden

yellow

Light yellow Red precipitate

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Discussion : 1. Iodine molecules are smaller than starch molecules. This explains why the iodinemolecules are able to move into the Visking tubing while starch molecules cannotmove out from the Visking tubing.

2. Glucose molecules are smaller than starch molecules.3. Starch needs to be hydrolysed to glucose in the alimentary canal so that the

glucose molecules are small enough to diffuse into the epithelium cells of the villusand into the blood capillaries.

E4 PI 2Conclusion : The Visking tubing acts as a semi-permeable membrane, allowing smaller molecules

like glucose molecules to pass through and preventing larger molecules like starchfrom passing through. The hypothesis is accepted.

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Prepared by Mr. Pang Lung Chong 2011