Modelling absorption

We can model the process occurring in the small intestine using a piece of apparatus called Visking tubing. Visking tubing is a selectively permeable membrane, this means it has pores in it which allow small molecules through but not larger molecules.

The apparatus used to model absorption in the small intestine can be set up like this.

Diagram of Visking tubing apparatus to show absorption. Visking tubing filled with starch and amylase solution is suspended in a beaker filled with distilled water.

At regular intervals the liquid inside and outside of the Visking tubing is sampled and tested for starch (using iodine) and glucose (using Benedict’s reagent). The results are shown in this table.

Table of results showing how the liquid inside and outside of the Visking tubing is sampled and tested for starch (using iodine) and glucose (using Benedict’s solution) at regular intervals.
Question

Describe and explain the contents of the Visking tubing and water after two minutes.

The tubing still contains all the starch after two minutes, as starch molecules are too big to pass through the holes in the Visking tubing. This is the reason that even after two minutes there is no starch found in the water.

Glucose appears in the water because starch is digested (broken down) by the enzyme amylase to glucose. Glucose molecules are small enough to pass through the holes in the Visking tubing to the liquid in the beaker.

This model of the absorption of digested food in the small intestine has limitations.

ModelStructureSimilaritiesDifferences
Visking tubingWall of the small intestineSelectively permeableSmaller surface area due to lack of villi
Distilled waterBloodInitially low in solutes to allow high concentration gradientDoes not flow so concentration gradient is not maintained