Laboratory Investigation:

The Model Gut 


Big Molecules, Small Molecules



The small intestine absorbs nutrients.  That is to say it allows digested food molecules to pass from the digestive tube (the gut) into the bloodstream.  This experiment models that by using dialyzing tubing to represent the gut.  Dialyzing tubing looks like adhesive tape but it is in fact a flattened hollow tube.  The plastic it is made of contains microscopic holes through which certain molecules can pass. 



  1. First, the dialyzing tubing is prepared by tying one end with a string.  We want this end to be perfectly sealed.  The best way to do that is to fold it over before tying it as shown in the photos.

    •  model_gut02.jpg (9335 bytes) Figure 1
      (click to see a bigger picture)

    • model_gut04.jpg (8772 bytes) Figure 2 

  2. Once it is sealed on one end, the other end is opened and a mixture of starch and glucose (glucose is a simple sugar) is squeezed into the tube.  

    •  model_gut05.jpg (9840 bytes) Figure 3

  3. Once it is full, the top end must be closed in the same way the bottom end was sealed. Although it does not look like one, this is a model of an intestine! 

    •  model_gut01.jpg (5176 bytes) Figure 4

  4. To be sure that none of the liquid mixture got on the outside of the model gut (that would falsify all experimental results), the outside of the tube is washed carefully.

    • model_gut06.jpg (11812 bytes) Figure 5 

  5. Once it is clean, the model gut is placed inside a large test tube.  The test tube is then filled with distilled water.  

    • model_gut07.jpg (13752 bytes) Figure 6 

  6. To see if any of the sugar or the starch has seeped through the walls of the model gut into the distilled water, a small sample of the water is taken using a pipette.

    • model_gut09.jpg (13010 bytes) Figure 7 

  7. The sample taken from the test tube is put in a new test tube and tested to see if either sugar or starch are present.  Below is the 'before' and 'after' of the test for simple sugars using Fehlings solution - this would be a strong positive result.

    •  model_gut03.jpg (7577 bytes) Figure 8

  8. A sample is taken every 5 minutes and tested for both starch and glucose.




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