Practical Skill: Visking Tubing Investigating the absorption of the products of digestion using Visking tubing

Question 1

Visking tubing (sometimes referred to as dialysis tubing) is a

Answer 1

non-living partially permeable membrane made from cellulose

Question 2

It is sometimes used to model the process of

Answer 2

absorption that occurs in the small intestine

Question 3

Pores in the membrane are small enough to…. but

Answer 3

prevent the passage of large molecules (such as starch and sucrose) but allow smaller molecules (such as glucose) to pass through by diffusion

Question 4

method

Answer 4

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

Question 5

The rate of absorption/diffusion can be investigated more quantitatively by:

Answer 5

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

Question 6

limitations

Answer 6

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

Question 7

Investigating factors affecting digestive enzyme activity

Answer 7

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