Visking Tubing Practical (AQA A Level Biology): Revision Note
Exam code: 7402
Visking tubing models
Visking tubing can be used to model the process of absorption that occurs in the small intestine
Visking tubing, or dialysis tubing, is a non-living, partially permeable membrane made from cellulose
Pores in the tubing are small enough to prevent the passage of large molecules, e.g. starch, but allow smaller molecules, e.g. glucose, to pass through by diffusion

Method
The procedure used to model digestion and absorption using Visking tubing is as follows:
Cut a section of Visking tubing and tie one end
Fill tubing with a starch and amylase mixture
Suspend the tubing in a beaker of water for a set period of time
Take samples from the liquid outside the tubing at regular intervals and test for the presence of starch and glucose
The results should show that glucose is present outside the tubing, while starch is absent
Starch molecules are too large to pass through the pores in the tubing
The amylase inside the tubing digests and breaks down starch into glucose molecules, which are small enough to diffuse into the surrounding liquid
The rate of absorption/diffusion can be investigated quantitatively by taking a series of samples over a period of time and measuring the concentration of glucose with the use of colorimetry
Visking tubing and temperature / pH
The Visking tubing model described above can also be used to investigate the effect of other factors on digestion and absorption, e.g.
pH: multiple visking tubes can be set up, containing solutions of starch and amylase kept at different pH levels using buffer solutions
temperature: multiple visking tubes can be set up in water baths at different temperatures
Visking tubing as a model
While both Visking tubing and the intestinal lining are partially permeable, the Visking tubing model has several limitations, e.g.:
Visking tubing does not contain biological membranes, or any features of membranes, e.g. channel proteins
active transport cannot occur across Visking tubing due to a lack of carrier proteins and energy from respiration
the surface area of Visking tubing is less than that of intestinal epithelium due to the absence of villi
the distilled water does not flow like blood, and so does not maintain the concentration gradient
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