Starch is a large molecule consisting of between 300 to 500 glucose molecules joined together.  A glucose molecule is only very small, consisting of 24 atoms.  Cellophane is similar to the membrane that surrounds your cells.  This activity uses a model to demonstrate how small molecules, that are the products of the digestive process, can pass through the cells lining the small intestine.  The effect of pH on the activity of an enzyme responsible for the digestion of starch into glucose will be investigated.

Materials        

          3 pieces cellophane tubing, 15 cm long                                    3 x 250 mL beakers

                        starch suspension                                                        glucose solution

                        iodine                                                                          Benedict's reagent

                        10 mL measuring cylinder                                          3 sticky labels

                        3 rubber bands                                                          apron and safety glasses

                        Distilled water                                                            dilute hydrochloric acid

                        Amylase solution                                                        thermometer

                        Incubator set at 37oC                                                 tile

                        Pipette, washed and rinsed                                         test tubes and rack

                        Water bath                                                                  heating apparatus

                        80 mL beaker                                                              electric kettle

Method          

1. Collect equipment.  Put safety gear on.  Wash and rinse all beakers and measuring cylinder.

2. Hold one end of the tubing under water until it is soft, then tie a knot in that end.  Handle the tubing firmly but with care to avoid tearing it.  

3. Hold the other end of the tubing under water to soften it.  Rub fingers back and forth on the tubing to open it.

4. Collect a quantity of starch suspension in the 80 mL beaker.  Three-quarters fill the

tubing with starch suspension.  Use measuring cylinder to add 2 mL distilled water to the starch suspension in the tubing.

5 Rinse the tubing under the tap to remove any starch from the outside, then rest in a beaker.  Secure using a rubber band, as shown in diagram.  Take care not to tear tubing.   Label the beaker Starch.

6 Repeat steps 2-5, adding 1 mL amylase solution and 1 mL distilled water to the starch suspension in the cellophane tubing, rather than 2 mL distilled water.  Label the beaker Starch and amylase.

7 Repeat steps 2-5, this time adding 1 mL amylase solution and 1 mL dilute hydrochloric acid to the starch suspension in the cellophane tubing.  Label the beaker Starch, amylase and acid.

8 Fill each of the beakers with enough water (at 37°C) to cover the tubes and leave for 24 hours in the incubator.

9 For each beaker test for the presence of starch and glucose in the water inside and outside of the tubing, Record results.

10 Rinse and return equipment.  Wipe down work area.  Wash hands.