Modelling Absorption (AQA AS Biology): Revision Note

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:

  1. Cut a section of Visking tubing and tie one end

  2. Fill tubing with a starch and amylase mixture

  3. Suspend the tubing in a beaker of water for a set period of time

  4. 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