3.3.5 Visking Tubing Practical

Investigating the absorption of the products of digestion using Visking tubing

• Visking tubing (sometimes referred to as dialysis tubing) is a non-living partially permeable membrane made from cellulose
• It is sometimes used to model the process of absorption that occurs in the small intestine
• Pores in the membrane are small enough to prevent the passage of large molecules (such as starch and sucrose) but allow smaller molecules (such as glucose) to pass through by diffusion

Image showing how a visking tubing membrane can be used to mimic the walls of the small intestine

Method

• Fill a section of Visking tubing with a mixture of starch and amylase solutions
• Suspend the tubing in a beaker of water for a set period of time
• Take samples from the liquid outside of the visking tubing at regular intervals and test for the presence of starch and glucose
  • Starch is tested for using iodine. A blue-black colour is produced in the presence of starch
  • Glucose is tested for using Benedict's reagent. An orange-red precipitate is formed in the presence of glucose

• The amylase present inside the visking tube digests and breaks down starch into glucose
  • Glucose is small enough to diffuse across the partially permeable membrane

• Over time the concentration of glucose in the liquid outside the visking tube should increase as more starch (substrate) has been digested
  • As a result, the amount of precipitate produced from the Benedict's reagent test will increase over time

• The rate of absorption/diffusion can be investigated more quantitatively by:
  • Estimating the concentration of glucose that has diffused into the liquid surrounding the Visking tubing at each time interval (separate beakers are set up for each time interval) using the semi-quantitative Benedict’s test
    • Comparisons between the time intervals can be made with a set of colour standards (known glucose concentrations) or a colorimeter to give a more quantitative set of results
    • A graph could be drawn showing how the rate of absorption changes with the concentration gradient between the inside and outside of the tubing

Limitations

• Using visking tubing membrane as a substitute for the membrane of the small intestine:
  • Both are selectively/partially permeable
  • The small intestine has a much larger surface area due to the presence of villi

• Using distilled water as a substitute for blood:
  • Both have an initially low solute concentration
  • The distilled water does not flow and so does not maintain the concentration gradient the way blood does

Investigating factors affecting digestive enzyme activity

• Visking tubing can also be used to study the effects of different factors on the rate of digestive enzyme activity
• Investigating the effect of pH
  • Eg. multiple visking tubings are set up containing solutions of starch and amylase kept at different pH levels using buffer solutions

• Investigating the effect of temperature
  • Eg. multiple visking tubings are set up in water baths of different temperatures