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
a Set up a boiling tube and four test tubes in a rack.
b Set out a dimple tile, with dropper bottles of iodine solution and Benedict’s reagent
in your work area.
c Collect a model gut made of Visking tubing.
d
e
Use syringes to put 5 cm3 of starch suspension and 5cm3 of glucose solution into
your model gut.
Rinse the outside of the Visking tubing under the tap then suspend it in your boiling
tube as shown in the diagram.
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Use 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.
g With a second pipette, put water into the boiling tube until its level is the same as
the gut contents.
h Start a stopclock.
i Immediately 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.
j Test 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.
k Test 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.
l After 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.
m Test a drop of each liquid with iodine solution and 1 cm3 with Benedict’s reagent as
in j and k.
n Record your results in the table below.
f
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
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QUESTIONS
1
What was the purpose of taking samples from the Visking tubing and
surrounding water immediately after water was poured around the
tubing?
2
From your results, do you think that starch can get through the holes in the
Visking tubing membrane?
3
From your results, do you think that glucose can get through the holes in
the membrane?
4
What does this tell you about starch and glucose molecules?
5
How is this length of Visking tubing containing starch and glucose similar
to the gut of a multicellular organism?
6
What features of a real gut are missing from this model?
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ANSWERS
1
To check whether the rinsing had been adequate and the tube was not
leaking its contents.
2
There is no starch outside the Visking tubing, but starch on both occasions
inside. It looks like starch cannot get through the Visking membrane.
3
There 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.
4
Starch 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.
5
The 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.
6
The 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.
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