Evaluating Visking Tubing As a Model for a Gut

Evaluating Visking tubing as a model for a gut

In this practical you can:

observe the properties of Visking tubing
compare the properties of Visking tubing with the properties of a gut

Procedure

SAFETY: Wear eye protection when handling chemicals.

Visking tubing is a fine smooth membrane with small holes in it. These holes are large enough to allow water and other small molecules through. They are too small to allow large molecules through.

Investigation

aSet up a boiling tube and four test tubes in a rack.

bSet out a dimple tile, with dropper bottles of iodine solution and Benedict’s reagent in your work area.

cCollect a model gut made of Visking tubing.

dUse syringes to put 5 cm3 of starch suspension and 5cm3 of glucose solution into your model gut.

eRinse the outside of the Visking tubing under the tap then suspend it in your boiling tube as shown in the diagram.

fUse a teat pipette to remove about 1 cm3 of the “gut” contents. Put one drop on the dimple tile, and the rest in a test tube. Then put the teat pipette back into the Visking tubing.

gWith a second pipette, put water into the boiling tube until its level is the same as the gut contents.

hStart a stopclock.

iImmediately use the second teat pipette to remove about 1 cm3 of the water. Put one drop on the dimple tile, and the rest in a test tube. Then put the teat pipette back in the water outside the Visking tubing.

jTest the drops of liquid in the dimple tile by adding one drop of iodine solution from the dropper bottle. If they turn blue-black, the liquid contains starch.

kTest the liquids in the test tubes by adding an equal volume of Benedict’s reagent and then place the test tubes in a beaker of boiling water for 2 to 3 minutes. If they turn orange (or greeny-yellow), the liquid contains glucose.

lAfter 15 minutes, sample the liquids inside and outside the tubing again. Ensure that you have a fresh sample by squeezing the pipette a couple of times to expel the remnants of any earlier sample and to mix the liquids well before sampling.

mTest a drop of each liquid with iodine solution and 1 cm3 with Benedict’s reagent as in j and k.

nRecord your results in the table below.

Result of test with iodine / Result of test with Benedict’s reagent / Does the liquid contain starch? / Does the liquid contain glucose?
Gut contents at beginning
Water around gut at beginning
Gut contents after 15 minutes
Water around gut after 15 minutes

QUESTIONS

1What was the purpose of taking samples from the Visking tubing and surrounding water immediately after water was poured around the tubing?

2From your results, do you think that starch can get through the holes in the Visking tubing membrane?

3From your results, do you think that glucose can get through the holes in the membrane?

4What does this tell you about starch and glucose molecules?

5How is this length of Visking tubing containing starch and glucose similar to the gut of a multicellular organism?

6What features of a real gut are missing from this model?

ANSWERS

1To check whether the rinsing had been adequate and the tube was not leaking its contents.

2There is no starch outside the Visking tubing, but starch on both occasions inside. It looks like starch cannot get through the Visking membrane.

3There is no glucose outside the Visking at the beginning, but there is after 15 minutes. It looks like glucose does move through the Visking membrane.

4Starch molecules are too big to pass through the holes in the Visking membrane. Glucose molecules are small enough to pass through the holes in the Visking tubing.

5The similarities between Visking tubing and a real gut are:

the food molecules are contained in a tube whose walls are permeable only to small molecules

the food in the tube is a mixture of large and small molecules
the tube is surrounded by liquid which contains a low concentration of food molecules.

6The differences between Visking tubing and a real gut are:

the Visking tubing membrane is smooth, a gut lining is folded and folded again with a larger surface area for its length
a real gut is surrounded by blood flowing in vessels that take away the soluble products of digestion and maintain a concentration gradient between the gut contents and the surroundings
there are active transport mechanisms that will draw food molecules through the wall of a real gut.